MFV r354257:

[FreeBSD/FreeBSD.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / dsl_scan.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
 * Copyright 2016 Gary Mills
 * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
 * Copyright 2017 Joyent, Inc.
 * Copyright (c) 2017 Datto Inc.
 */

#include <sys/dsl_scan.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_prop.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_synctask.h>
#include <sys/dnode.h>
#include <sys/dmu_tx.h>
#include <sys/dmu_objset.h>
#include <sys/arc.h>
#include <sys/zap.h>
#include <sys/zio.h>
#include <sys/zfs_context.h>
#include <sys/fs/zfs.h>
#include <sys/zfs_znode.h>
#include <sys/spa_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zil_impl.h>
#include <sys/zio_checksum.h>
#include <sys/ddt.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/zfeature.h>
#include <sys/abd.h>
#include <sys/range_tree.h>
#ifdef _KERNEL
#include <sys/zfs_vfsops.h>
#endif

/*
 * Grand theory statement on scan queue sorting
 *
 * Scanning is implemented by recursively traversing all indirection levels
 * in an object and reading all blocks referenced from said objects. This
 * results in us approximately traversing the object from lowest logical
 * offset to the highest. For best performance, we would want the logical
 * blocks to be physically contiguous. However, this is frequently not the
 * case with pools given the allocation patterns of copy-on-write filesystems.
 * So instead, we put the I/Os into a reordering queue and issue them in a
 * way that will most benefit physical disks (LBA-order).
 *
 * Queue management:
 *
 * Ideally, we would want to scan all metadata and queue up all block I/O
 * prior to starting to issue it, because that allows us to do an optimal
 * sorting job. This can however consume large amounts of memory. Therefore
 * we continuously monitor the size of the queues and constrain them to 5%
 * (zfs_scan_mem_lim_fact) of physmem. If the queues grow larger than this
 * limit, we clear out a few of the largest extents at the head of the queues
 * to make room for more scanning. Hopefully, these extents will be fairly
 * large and contiguous, allowing us to approach sequential I/O throughput
 * even without a fully sorted tree.
 *
 * Metadata scanning takes place in dsl_scan_visit(), which is called from
 * dsl_scan_sync() every spa_sync(). If we have either fully scanned all
 * metadata on the pool, or we need to make room in memory because our
 * queues are too large, dsl_scan_visit() is postponed and
 * scan_io_queues_run() is called from dsl_scan_sync() instead. This implies
 * that metadata scanning and queued I/O issuing are mutually exclusive. This
 * allows us to provide maximum sequential I/O throughput for the majority of
 * I/O's issued since sequential I/O performance is significantly negatively
 * impacted if it is interleaved with random I/O.
 *
 * Implementation Notes
 *
 * One side effect of the queued scanning algorithm is that the scanning code
 * needs to be notified whenever a block is freed. This is needed to allow
 * the scanning code to remove these I/Os from the issuing queue. Additionally,
 * we do not attempt to queue gang blocks to be issued sequentially since this
 * is very hard to do and would have an extremely limitted performance benefit.
 * Instead, we simply issue gang I/Os as soon as we find them using the legacy
 * algorithm.
 *
 * Backwards compatibility
 *
 * This new algorithm is backwards compatible with the legacy on-disk data
 * structures (and therefore does not require a new feature flag).
 * Periodically during scanning (see zfs_scan_checkpoint_intval), the scan
 * will stop scanning metadata (in logical order) and wait for all outstanding
 * sorted I/O to complete. Once this is done, we write out a checkpoint
 * bookmark, indicating that we have scanned everything logically before it.
 * If the pool is imported on a machine without the new sorting algorithm,
 * the scan simply resumes from the last checkpoint using the legacy algorithm.
 */

typedef int (scan_cb_t)(dsl_pool_t *, const blkptr_t *,
    const zbookmark_phys_t *);

static scan_cb_t dsl_scan_scrub_cb;

static int scan_ds_queue_compare(const void *a, const void *b);
static int scan_prefetch_queue_compare(const void *a, const void *b);
static void scan_ds_queue_clear(dsl_scan_t *scn);
static boolean_t scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj,
    uint64_t *txg);
static void scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg);
static void scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj);
static void scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx);

extern int zfs_vdev_async_write_active_min_dirty_percent;

/*
 * By default zfs will check to ensure it is not over the hard memory
 * limit before each txg. If finer-grained control of this is needed
 * this value can be set to 1 to enable checking before scanning each
 * block.
 */
int zfs_scan_strict_mem_lim = B_FALSE;

/*
 * Maximum number of parallelly executing I/Os per top-level vdev.
 * Tune with care. Very high settings (hundreds) are known to trigger
 * some firmware bugs and resets on certain SSDs.
 */
int zfs_top_maxinflight = 32;           /* maximum I/Os per top-level */
unsigned int zfs_resilver_delay = 2;    /* number of ticks to delay resilver -- 2 is a good number */
unsigned int zfs_scrub_delay = 4;       /* number of ticks to delay scrub -- 4 is a good number */
unsigned int zfs_scan_idle = 50;        /* idle window in clock ticks */

/*
 * Maximum number of parallelly executed bytes per leaf vdev. We attempt
 * to strike a balance here between keeping the vdev queues full of I/Os
 * at all times and not overflowing the queues to cause long latency,
 * which would cause long txg sync times. No matter what, we will not
 * overload the drives with I/O, since that is protected by
 * zfs_vdev_scrub_max_active.
 */
unsigned long zfs_scan_vdev_limit = 4 << 20;

int zfs_scan_issue_strategy = 0;
int zfs_scan_legacy = B_FALSE;  /* don't queue & sort zios, go direct */
uint64_t zfs_scan_max_ext_gap = 2 << 20;        /* in bytes */

unsigned int zfs_scan_checkpoint_intval = 7200; /* seconds */
#define ZFS_SCAN_CHECKPOINT_INTVAL      SEC_TO_TICK(zfs_scan_checkpoint_intval)

/*
 * fill_weight is non-tunable at runtime, so we copy it at module init from
 * zfs_scan_fill_weight. Runtime adjustments to zfs_scan_fill_weight would
 * break queue sorting.
 */
uint64_t zfs_scan_fill_weight = 3;
static uint64_t fill_weight;

/* See dsl_scan_should_clear() for details on the memory limit tunables */
uint64_t zfs_scan_mem_lim_min = 16 << 20;       /* bytes */
uint64_t zfs_scan_mem_lim_soft_max = 128 << 20; /* bytes */
int zfs_scan_mem_lim_fact = 20;         /* fraction of physmem */
int zfs_scan_mem_lim_soft_fact = 20;    /* fraction of mem lim above */

unsigned int zfs_scrub_min_time_ms = 1000; /* min millisecs to scrub per txg */
unsigned int zfs_free_min_time_ms = 1000; /* min millisecs to free per txg */
unsigned int zfs_obsolete_min_time_ms = 500; /* min millisecs to obsolete per txg */
unsigned int zfs_resilver_min_time_ms = 3000; /* min millisecs to resilver per txg */
boolean_t zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
boolean_t zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */

SYSCTL_DECL(_vfs_zfs);
SYSCTL_UINT(_vfs_zfs, OID_AUTO, top_maxinflight, CTLFLAG_RWTUN,
    &zfs_top_maxinflight, 0, "Maximum I/Os per top-level vdev");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, resilver_delay, CTLFLAG_RWTUN,
    &zfs_resilver_delay, 0, "Number of ticks to delay resilver");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, scrub_delay, CTLFLAG_RWTUN,
    &zfs_scrub_delay, 0, "Number of ticks to delay scrub");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, scan_idle, CTLFLAG_RWTUN,
    &zfs_scan_idle, 0, "Idle scan window in clock ticks");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, scan_min_time_ms, CTLFLAG_RWTUN,
    &zfs_scrub_min_time_ms, 0, "Min millisecs to scrub per txg");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, free_min_time_ms, CTLFLAG_RWTUN,
    &zfs_free_min_time_ms, 0, "Min millisecs to free per txg");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, resilver_min_time_ms, CTLFLAG_RWTUN,
    &zfs_resilver_min_time_ms, 0, "Min millisecs to resilver per txg");
SYSCTL_INT(_vfs_zfs, OID_AUTO, no_scrub_io, CTLFLAG_RWTUN,
    &zfs_no_scrub_io, 0, "Disable scrub I/O");
SYSCTL_INT(_vfs_zfs, OID_AUTO, no_scrub_prefetch, CTLFLAG_RWTUN,
    &zfs_no_scrub_prefetch, 0, "Disable scrub prefetching");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, zfs_scan_legacy, CTLFLAG_RWTUN,
    &zfs_scan_legacy, 0, "Scrub using legacy non-sequential method");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, zfs_scan_checkpoint_interval, CTLFLAG_RWTUN,
    &zfs_scan_checkpoint_intval, 0, "Scan progress on-disk checkpointing interval");

enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
/* max number of blocks to free in a single TXG */
uint64_t zfs_async_block_max_blocks = UINT64_MAX;
SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, free_max_blocks, CTLFLAG_RWTUN,
    &zfs_async_block_max_blocks, 0, "Maximum number of blocks to free in one TXG");

/*
 * We wait a few txgs after importing a pool to begin scanning so that
 * the import / mounting code isn't held up by scrub / resilver IO.
 * Unfortunately, it is a bit difficult to determine exactly how long
 * this will take since userspace will trigger fs mounts asynchronously
 * and the kernel will create zvol minors asynchronously. As a result,
 * the value provided here is a bit arbitrary, but represents a
 * reasonable estimate of how many txgs it will take to finish fully
 * importing a pool
 */
#define        SCAN_IMPORT_WAIT_TXGS           5


#define DSL_SCAN_IS_SCRUB_RESILVER(scn) \
        ((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
        (scn)->scn_phys.scn_func == POOL_SCAN_RESILVER)

extern int zfs_txg_timeout;

/*
 * Enable/disable the processing of the free_bpobj object.
 */
boolean_t zfs_free_bpobj_enabled = B_TRUE;

SYSCTL_INT(_vfs_zfs, OID_AUTO, free_bpobj_enabled, CTLFLAG_RWTUN,
    &zfs_free_bpobj_enabled, 0, "Enable free_bpobj processing");

/* the order has to match pool_scan_type */
static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
        NULL,
        dsl_scan_scrub_cb,      /* POOL_SCAN_SCRUB */
        dsl_scan_scrub_cb,      /* POOL_SCAN_RESILVER */
};

/* In core node for the scn->scn_queue. Represents a dataset to be scanned */
typedef struct {
        uint64_t        sds_dsobj;
        uint64_t        sds_txg;
        avl_node_t      sds_node;
} scan_ds_t;

/*
 * This controls what conditions are placed on dsl_scan_sync_state():
 * SYNC_OPTIONAL) write out scn_phys iff scn_bytes_pending == 0
 * SYNC_MANDATORY) write out scn_phys always. scn_bytes_pending must be 0.
 * SYNC_CACHED) if scn_bytes_pending == 0, write out scn_phys. Otherwise
 *      write out the scn_phys_cached version.
 * See dsl_scan_sync_state for details.
 */
typedef enum {
        SYNC_OPTIONAL,
        SYNC_MANDATORY,
        SYNC_CACHED
} state_sync_type_t;

/*
 * This struct represents the minimum information needed to reconstruct a
 * zio for sequential scanning. This is useful because many of these will
 * accumulate in the sequential IO queues before being issued, so saving
 * memory matters here.
 */
typedef struct scan_io {
        /* fields from blkptr_t */
        uint64_t                sio_offset;
        uint64_t                sio_blk_prop;
        uint64_t                sio_phys_birth;
        uint64_t                sio_birth;
        zio_cksum_t             sio_cksum;
        uint32_t                sio_asize;

        /* fields from zio_t */
        int                     sio_flags;
        zbookmark_phys_t        sio_zb;

        /* members for queue sorting */
        union {
                avl_node_t      sio_addr_node; /* link into issueing queue */
                list_node_t     sio_list_node; /* link for issuing to disk */
        } sio_nodes;
} scan_io_t;

struct dsl_scan_io_queue {
        dsl_scan_t      *q_scn; /* associated dsl_scan_t */
        vdev_t          *q_vd; /* top-level vdev that this queue represents */

        /* trees used for sorting I/Os and extents of I/Os */
        range_tree_t    *q_exts_by_addr;
        avl_tree_t      q_exts_by_size;
        avl_tree_t      q_sios_by_addr;

        /* members for zio rate limiting */
        uint64_t        q_maxinflight_bytes;
        uint64_t        q_inflight_bytes;
        kcondvar_t      q_zio_cv; /* used under vd->vdev_scan_io_queue_lock */

        /* per txg statistics */
        uint64_t        q_total_seg_size_this_txg;
        uint64_t        q_segs_this_txg;
        uint64_t        q_total_zio_size_this_txg;
        uint64_t        q_zios_this_txg;
};

/* private data for dsl_scan_prefetch_cb() */
typedef struct scan_prefetch_ctx {
        zfs_refcount_t spc_refcnt;      /* refcount for memory management */
        dsl_scan_t *spc_scn;            /* dsl_scan_t for the pool */
        boolean_t spc_root;             /* is this prefetch for an objset? */
        uint8_t spc_indblkshift;        /* dn_indblkshift of current dnode */
        uint16_t spc_datablkszsec;      /* dn_idatablkszsec of current dnode */
} scan_prefetch_ctx_t;

/* private data for dsl_scan_prefetch() */
typedef struct scan_prefetch_issue_ctx {
        avl_node_t spic_avl_node;       /* link into scn->scn_prefetch_queue */
        scan_prefetch_ctx_t *spic_spc;  /* spc for the callback */
        blkptr_t spic_bp;               /* bp to prefetch */
        zbookmark_phys_t spic_zb;       /* bookmark to prefetch */
} scan_prefetch_issue_ctx_t;

static void scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue);
static void scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue,
    scan_io_t *sio);

static dsl_scan_io_queue_t *scan_io_queue_create(vdev_t *vd);
static void scan_io_queues_destroy(dsl_scan_t *scn);

static kmem_cache_t *sio_cache;

void
scan_init(void)
{
        /*
         * This is used in ext_size_compare() to weight segments
         * based on how sparse they are. This cannot be changed
         * mid-scan and the tree comparison functions don't currently
         * have a mechansim for passing additional context to the
         * compare functions. Thus we store this value globally and
         * we only allow it to be set at module intiailization time
         */
        fill_weight = zfs_scan_fill_weight;
        
        sio_cache = kmem_cache_create("sio_cache",
            sizeof (scan_io_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
}

void
scan_fini(void)
{
        kmem_cache_destroy(sio_cache);
}

static inline boolean_t
dsl_scan_is_running(const dsl_scan_t *scn)
{
        return (scn->scn_phys.scn_state == DSS_SCANNING);
}

boolean_t
dsl_scan_resilvering(dsl_pool_t *dp)
{
        return (dsl_scan_is_running(dp->dp_scan) &&
            dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
}

static inline void
sio2bp(const scan_io_t *sio, blkptr_t *bp, uint64_t vdev_id)
{
        bzero(bp, sizeof (*bp));
        DVA_SET_ASIZE(&bp->blk_dva[0], sio->sio_asize);
        DVA_SET_VDEV(&bp->blk_dva[0], vdev_id);
        DVA_SET_OFFSET(&bp->blk_dva[0], sio->sio_offset);
        bp->blk_prop = sio->sio_blk_prop;
        bp->blk_phys_birth = sio->sio_phys_birth;
        bp->blk_birth = sio->sio_birth;
        bp->blk_fill = 1;       /* we always only work with data pointers */
        bp->blk_cksum = sio->sio_cksum;
}

static inline void
bp2sio(const blkptr_t *bp, scan_io_t *sio, int dva_i)
{
        /* we discard the vdev id, since we can deduce it from the queue */
        sio->sio_offset = DVA_GET_OFFSET(&bp->blk_dva[dva_i]);
        sio->sio_asize = DVA_GET_ASIZE(&bp->blk_dva[dva_i]);
        sio->sio_blk_prop = bp->blk_prop;
        sio->sio_phys_birth = bp->blk_phys_birth;
        sio->sio_birth = bp->blk_birth;
        sio->sio_cksum = bp->blk_cksum;
}

void
dsl_scan_global_init(void)
{
        /*
         * This is used in ext_size_compare() to weight segments
         * based on how sparse they are. This cannot be changed
         * mid-scan and the tree comparison functions don't currently
         * have a mechansim for passing additional context to the
         * compare functions. Thus we store this value globally and
         * we only allow it to be set at module intiailization time
         */
        fill_weight = zfs_scan_fill_weight;
}

int
dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
{
        int err;
        dsl_scan_t *scn;
        spa_t *spa = dp->dp_spa;
        uint64_t f;

        scn = dp->dp_scan = kmem_zalloc(sizeof (dsl_scan_t), KM_SLEEP);
        scn->scn_dp = dp;

        /*
         * It's possible that we're resuming a scan after a reboot so
         * make sure that the scan_async_destroying flag is initialized
         * appropriately.
         */
        ASSERT(!scn->scn_async_destroying);
        scn->scn_async_destroying = spa_feature_is_active(dp->dp_spa,
            SPA_FEATURE_ASYNC_DESTROY);

        bcopy(&scn->scn_phys, &scn->scn_phys_cached, sizeof (scn->scn_phys));
        avl_create(&scn->scn_queue, scan_ds_queue_compare, sizeof (scan_ds_t),
            offsetof(scan_ds_t, sds_node));
        avl_create(&scn->scn_prefetch_queue, scan_prefetch_queue_compare,
            sizeof (scan_prefetch_issue_ctx_t),
            offsetof(scan_prefetch_issue_ctx_t, spic_avl_node));

        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            "scrub_func", sizeof (uint64_t), 1, &f);
        if (err == 0) {
                /*
                 * There was an old-style scrub in progress.  Restart a
                 * new-style scrub from the beginning.
                 */
                scn->scn_restart_txg = txg;
                zfs_dbgmsg("old-style scrub was in progress; "
                    "restarting new-style scrub in txg %llu",
                    (longlong_t)scn->scn_restart_txg);

                /*
                 * Load the queue obj from the old location so that it
                 * can be freed by dsl_scan_done().
                 */
                (void) zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                    "scrub_queue", sizeof (uint64_t), 1,
                    &scn->scn_phys.scn_queue_obj);
        } else {
                err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
                    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
                    &scn->scn_phys);
                if (err == ENOENT)
                        return (0);
                else if (err)
                        return (err);

                /*
                 * We might be restarting after a reboot, so jump the issued
                 * counter to how far we've scanned. We know we're consistent
                 * up to here.
                 */
                scn->scn_issued_before_pass = scn->scn_phys.scn_examined;

                if (dsl_scan_is_running(scn) &&
                    spa_prev_software_version(dp->dp_spa) < SPA_VERSION_SCAN) {
                        /*
                         * A new-type scrub was in progress on an old
                         * pool, and the pool was accessed by old
                         * software.  Restart from the beginning, since
                         * the old software may have changed the pool in
                         * the meantime.
                         */
                        scn->scn_restart_txg = txg;
                        zfs_dbgmsg("new-style scrub was modified "
                            "by old software; restarting in txg %llu",
                            (longlong_t)scn->scn_restart_txg);
                }
        }

        /* reload the queue into the in-core state */
        if (scn->scn_phys.scn_queue_obj != 0) {
                zap_cursor_t zc;
                zap_attribute_t za;

                for (zap_cursor_init(&zc, dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj);
                    zap_cursor_retrieve(&zc, &za) == 0;
                    (void) zap_cursor_advance(&zc)) {
                        scan_ds_queue_insert(scn,
                            zfs_strtonum(za.za_name, NULL),
                            za.za_first_integer);
                }
                zap_cursor_fini(&zc);
        }

        spa_scan_stat_init(spa);
        return (0);
}

void
dsl_scan_fini(dsl_pool_t *dp)
{
        if (dp->dp_scan != NULL) {
                dsl_scan_t *scn = dp->dp_scan;

                if (scn->scn_taskq != NULL)
                        taskq_destroy(scn->scn_taskq);
                scan_ds_queue_clear(scn);
                avl_destroy(&scn->scn_queue);
                avl_destroy(&scn->scn_prefetch_queue);

                kmem_free(dp->dp_scan, sizeof (dsl_scan_t));
                dp->dp_scan = NULL;
        }
}

static boolean_t
dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
{
        return (scn->scn_restart_txg != 0 &&
            scn->scn_restart_txg <= tx->tx_txg);
}

boolean_t
dsl_scan_scrubbing(const dsl_pool_t *dp)
{
        dsl_scan_phys_t *scn_phys = &dp->dp_scan->scn_phys;

        return (scn_phys->scn_state == DSS_SCANNING &&
            scn_phys->scn_func == POOL_SCAN_SCRUB);
}

boolean_t
dsl_scan_is_paused_scrub(const dsl_scan_t *scn)
{
        return (dsl_scan_scrubbing(scn->scn_dp) &&
            scn->scn_phys.scn_flags & DSF_SCRUB_PAUSED);
}

/*
 * Writes out a persistent dsl_scan_phys_t record to the pool directory.
 * Because we can be running in the block sorting algorithm, we do not always
 * want to write out the record, only when it is "safe" to do so. This safety
 * condition is achieved by making sure that the sorting queues are empty
 * (scn_bytes_pending == 0). When this condition is not true, the sync'd state
 * is inconsistent with how much actual scanning progress has been made. The
 * kind of sync to be performed is specified by the sync_type argument. If the
 * sync is optional, we only sync if the queues are empty. If the sync is
 * mandatory, we do a hard ASSERT to make sure that the queues are empty. The
 * third possible state is a "cached" sync. This is done in response to:
 * 1) The dataset that was in the last sync'd dsl_scan_phys_t having been
 *      destroyed, so we wouldn't be able to restart scanning from it.
 * 2) The snapshot that was in the last sync'd dsl_scan_phys_t having been
 *      superseded by a newer snapshot.
 * 3) The dataset that was in the last sync'd dsl_scan_phys_t having been
 *      swapped with its clone.
 * In all cases, a cached sync simply rewrites the last record we've written,
 * just slightly modified. For the modifications that are performed to the
 * last written dsl_scan_phys_t, see dsl_scan_ds_destroyed,
 * dsl_scan_ds_snapshotted and dsl_scan_ds_clone_swapped.
 */
static void
dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx, state_sync_type_t sync_type)
{
        int i;
        spa_t *spa = scn->scn_dp->dp_spa;

        ASSERT(sync_type != SYNC_MANDATORY || scn->scn_bytes_pending == 0);
        if (scn->scn_bytes_pending == 0) {
                for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
                        vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
                        dsl_scan_io_queue_t *q = vd->vdev_scan_io_queue;

                        if (q == NULL)
                                continue;

                        mutex_enter(&vd->vdev_scan_io_queue_lock);
                        ASSERT3P(avl_first(&q->q_sios_by_addr), ==, NULL);
                        ASSERT3P(avl_first(&q->q_exts_by_size), ==, NULL);
                        ASSERT3P(range_tree_first(q->q_exts_by_addr), ==, NULL);
                        mutex_exit(&vd->vdev_scan_io_queue_lock);
                }

                if (scn->scn_phys.scn_queue_obj != 0)
                        scan_ds_queue_sync(scn, tx);
                VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT,
                    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
                    &scn->scn_phys, tx));
                bcopy(&scn->scn_phys, &scn->scn_phys_cached,
                    sizeof (scn->scn_phys));

                if (scn->scn_checkpointing)
                        zfs_dbgmsg("finish scan checkpoint");

                scn->scn_checkpointing = B_FALSE;
                scn->scn_last_checkpoint = ddi_get_lbolt();
        } else if (sync_type == SYNC_CACHED) {
                VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT,
                    DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
                    &scn->scn_phys_cached, tx));
        }
}

/* ARGSUSED */
static int
dsl_scan_setup_check(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        if (dsl_scan_is_running(scn))
                return (SET_ERROR(EBUSY));

        return (0);
}

static void
dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
        pool_scan_func_t *funcp = arg;
        dmu_object_type_t ot = 0;
        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;

        ASSERT(!dsl_scan_is_running(scn));
        ASSERT(*funcp > POOL_SCAN_NONE && *funcp < POOL_SCAN_FUNCS);
        bzero(&scn->scn_phys, sizeof (scn->scn_phys));
        scn->scn_phys.scn_func = *funcp;
        scn->scn_phys.scn_state = DSS_SCANNING;
        scn->scn_phys.scn_min_txg = 0;
        scn->scn_phys.scn_max_txg = tx->tx_txg;
        scn->scn_phys.scn_ddt_class_max = DDT_CLASSES - 1; /* the entire DDT */
        scn->scn_phys.scn_start_time = gethrestime_sec();
        scn->scn_phys.scn_errors = 0;
        scn->scn_phys.scn_to_examine = spa->spa_root_vdev->vdev_stat.vs_alloc;
        scn->scn_issued_before_pass = 0;
        scn->scn_restart_txg = 0;
        scn->scn_done_txg = 0;
        scn->scn_last_checkpoint = 0;
        scn->scn_checkpointing = B_FALSE;
        spa_scan_stat_init(spa);

        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                scn->scn_phys.scn_ddt_class_max = zfs_scrub_ddt_class_max;

                /* rewrite all disk labels */
                vdev_config_dirty(spa->spa_root_vdev);

                if (vdev_resilver_needed(spa->spa_root_vdev,
                    &scn->scn_phys.scn_min_txg, &scn->scn_phys.scn_max_txg)) {
                        spa_event_notify(spa, NULL, NULL,
                            ESC_ZFS_RESILVER_START);
                } else {
                        spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_START);
                }

                spa->spa_scrub_started = B_TRUE;
                /*
                 * If this is an incremental scrub, limit the DDT scrub phase
                 * to just the auto-ditto class (for correctness); the rest
                 * of the scrub should go faster using top-down pruning.
                 */
                if (scn->scn_phys.scn_min_txg > TXG_INITIAL)
                        scn->scn_phys.scn_ddt_class_max = DDT_CLASS_DITTO;

        }

        /* back to the generic stuff */

        if (dp->dp_blkstats == NULL) {
                dp->dp_blkstats =
                    kmem_alloc(sizeof (zfs_all_blkstats_t), KM_SLEEP);
                mutex_init(&dp->dp_blkstats->zab_lock, NULL,
                    MUTEX_DEFAULT, NULL);
        }
        bzero(&dp->dp_blkstats->zab_type, sizeof (dp->dp_blkstats->zab_type));

        if (spa_version(spa) < SPA_VERSION_DSL_SCRUB)
                ot = DMU_OT_ZAP_OTHER;

        scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset,
            ot ? ot : DMU_OT_SCAN_QUEUE, DMU_OT_NONE, 0, tx);

        bcopy(&scn->scn_phys, &scn->scn_phys_cached, sizeof (scn->scn_phys));

        dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);

        spa_history_log_internal(spa, "scan setup", tx,
            "func=%u mintxg=%llu maxtxg=%llu",
            *funcp, scn->scn_phys.scn_min_txg, scn->scn_phys.scn_max_txg);
}

/*
 * Called by the ZFS_IOC_POOL_SCAN ioctl to start a scrub or resilver.
 * Can also be called to resume a paused scrub.
 */
int
dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
{
        spa_t *spa = dp->dp_spa;
        dsl_scan_t *scn = dp->dp_scan;

        /*
         * Purge all vdev caches and probe all devices.  We do this here
         * rather than in sync context because this requires a writer lock
         * on the spa_config lock, which we can't do from sync context.  The
         * spa_scrub_reopen flag indicates that vdev_open() should not
         * attempt to start another scrub.
         */
        spa_vdev_state_enter(spa, SCL_NONE);
        spa->spa_scrub_reopen = B_TRUE;
        vdev_reopen(spa->spa_root_vdev);
        spa->spa_scrub_reopen = B_FALSE;
        (void) spa_vdev_state_exit(spa, NULL, 0);

        if (func == POOL_SCAN_SCRUB && dsl_scan_is_paused_scrub(scn)) {
                /* got scrub start cmd, resume paused scrub */
                int err = dsl_scrub_set_pause_resume(scn->scn_dp,
                    POOL_SCRUB_NORMAL);
                if (err == 0) {
                        spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_RESUME);
                        return (ECANCELED);
                }
                return (SET_ERROR(err));
        }

        return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
            dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
}

/* ARGSUSED */
static void
dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
{
        static const char *old_names[] = {
                "scrub_bookmark",
                "scrub_ddt_bookmark",
                "scrub_ddt_class_max",
                "scrub_queue",
                "scrub_min_txg",
                "scrub_max_txg",
                "scrub_func",
                "scrub_errors",
                NULL
        };

        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;
        int i;

        /* Remove any remnants of an old-style scrub. */
        for (i = 0; old_names[i]; i++) {
                (void) zap_remove(dp->dp_meta_objset,
                    DMU_POOL_DIRECTORY_OBJECT, old_names[i], tx);
        }

        if (scn->scn_phys.scn_queue_obj != 0) {
                VERIFY0(dmu_object_free(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, tx));
                scn->scn_phys.scn_queue_obj = 0;
        }
        scan_ds_queue_clear(scn);

        scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;

        /*
         * If we were "restarted" from a stopped state, don't bother
         * with anything else.
         */
        if (!dsl_scan_is_running(scn)) {
                ASSERT(!scn->scn_is_sorted);
                return;
        }

        if (scn->scn_is_sorted) {
                scan_io_queues_destroy(scn);
                scn->scn_is_sorted = B_FALSE;

                if (scn->scn_taskq != NULL) {
                        taskq_destroy(scn->scn_taskq);
                        scn->scn_taskq = NULL;
                }
        }

        scn->scn_phys.scn_state = complete ? DSS_FINISHED : DSS_CANCELED;

        if (dsl_scan_restarting(scn, tx))
                spa_history_log_internal(spa, "scan aborted, restarting", tx,
                    "errors=%llu", spa_get_errlog_size(spa));
        else if (!complete)
                spa_history_log_internal(spa, "scan cancelled", tx,
                    "errors=%llu", spa_get_errlog_size(spa));
        else
                spa_history_log_internal(spa, "scan done", tx,
                    "errors=%llu", spa_get_errlog_size(spa));

        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                spa->spa_scrub_started = B_FALSE;
                spa->spa_scrub_active = B_FALSE;

                /*
                 * If the scrub/resilver completed, update all DTLs to
                 * reflect this.  Whether it succeeded or not, vacate
                 * all temporary scrub DTLs.
                 *
                 * As the scrub does not currently support traversing
                 * data that have been freed but are part of a checkpoint,
                 * we don't mark the scrub as done in the DTLs as faults
                 * may still exist in those vdevs.
                 */
                if (complete &&
                    !spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) {
                        vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
                            scn->scn_phys.scn_max_txg, B_TRUE);

                        spa_event_notify(spa, NULL, NULL,
                            scn->scn_phys.scn_min_txg ?
                            ESC_ZFS_RESILVER_FINISH : ESC_ZFS_SCRUB_FINISH);
                } else {
                        vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
                            0, B_TRUE);
                }
                spa_errlog_rotate(spa);

                /*
                 * We may have finished replacing a device.
                 * Let the async thread assess this and handle the detach.
                 */
                spa_async_request(spa, SPA_ASYNC_RESILVER_DONE);
        }

        scn->scn_phys.scn_end_time = gethrestime_sec();

        ASSERT(!dsl_scan_is_running(scn));
}

/* ARGSUSED */
static int
dsl_scan_cancel_check(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        if (!dsl_scan_is_running(scn))
                return (SET_ERROR(ENOENT));
        return (0);
}

/* ARGSUSED */
static void
dsl_scan_cancel_sync(void *arg, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;

        dsl_scan_done(scn, B_FALSE, tx);
        dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);
        spa_event_notify(scn->scn_dp->dp_spa, NULL, NULL, ESC_ZFS_SCRUB_ABORT);
}

int
dsl_scan_cancel(dsl_pool_t *dp)
{
        return (dsl_sync_task(spa_name(dp->dp_spa), dsl_scan_cancel_check,
            dsl_scan_cancel_sync, NULL, 3, ZFS_SPACE_CHECK_RESERVED));
}

static int
dsl_scrub_pause_resume_check(void *arg, dmu_tx_t *tx)
{
        pool_scrub_cmd_t *cmd = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        dsl_scan_t *scn = dp->dp_scan;

        if (*cmd == POOL_SCRUB_PAUSE) {
                /* can't pause a scrub when there is no in-progress scrub */
                if (!dsl_scan_scrubbing(dp))
                        return (SET_ERROR(ENOENT));

                /* can't pause a paused scrub */
                if (dsl_scan_is_paused_scrub(scn))
                        return (SET_ERROR(EBUSY));
        } else if (*cmd != POOL_SCRUB_NORMAL) {
                return (SET_ERROR(ENOTSUP));
        }

        return (0);
}

static void
dsl_scrub_pause_resume_sync(void *arg, dmu_tx_t *tx)
{
        pool_scrub_cmd_t *cmd = arg;
        dsl_pool_t *dp = dmu_tx_pool(tx);
        spa_t *spa = dp->dp_spa;
        dsl_scan_t *scn = dp->dp_scan;

        if (*cmd == POOL_SCRUB_PAUSE) {
                /* can't pause a scrub when there is no in-progress scrub */
                spa->spa_scan_pass_scrub_pause = gethrestime_sec();
                scn->scn_phys.scn_flags |= DSF_SCRUB_PAUSED;
                dsl_scan_sync_state(scn, tx, SYNC_CACHED);
                spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_PAUSED);
        } else {
                ASSERT3U(*cmd, ==, POOL_SCRUB_NORMAL);
                if (dsl_scan_is_paused_scrub(scn)) {
                        /*
                         * We need to keep track of how much time we spend
                         * paused per pass so that we can adjust the scrub rate
                         * shown in the output of 'zpool status'
                         */
                        spa->spa_scan_pass_scrub_spent_paused +=
                            gethrestime_sec() - spa->spa_scan_pass_scrub_pause;
                        spa->spa_scan_pass_scrub_pause = 0;
                        scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
                        dsl_scan_sync_state(scn, tx, SYNC_CACHED);
                }
        }
}

/*
 * Set scrub pause/resume state if it makes sense to do so
 */
int
dsl_scrub_set_pause_resume(const dsl_pool_t *dp, pool_scrub_cmd_t cmd)
{
        return (dsl_sync_task(spa_name(dp->dp_spa),
            dsl_scrub_pause_resume_check, dsl_scrub_pause_resume_sync, &cmd, 3,
            ZFS_SPACE_CHECK_RESERVED));
}


/* start a new scan, or restart an existing one. */
void
dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
{
        if (txg == 0) {
                dmu_tx_t *tx;
                tx = dmu_tx_create_dd(dp->dp_mos_dir);
                VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));

                txg = dmu_tx_get_txg(tx);
                dp->dp_scan->scn_restart_txg = txg;
                dmu_tx_commit(tx);
        } else {
                dp->dp_scan->scn_restart_txg = txg;
        }
        zfs_dbgmsg("restarting resilver txg=%llu", txg);
}

void
dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bp)
{
        zio_free(dp->dp_spa, txg, bp);
}

void
dsl_free_sync(zio_t *pio, dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp)
{
        ASSERT(dsl_pool_sync_context(dp));
        zio_nowait(zio_free_sync(pio, dp->dp_spa, txg, bpp, BP_GET_PSIZE(bpp),
            pio->io_flags));
}

static int
scan_ds_queue_compare(const void *a, const void *b)
{
        const scan_ds_t *sds_a = a, *sds_b = b;

        if (sds_a->sds_dsobj < sds_b->sds_dsobj)
                return (-1);
        if (sds_a->sds_dsobj == sds_b->sds_dsobj)
                return (0);
        return (1);
}

static void
scan_ds_queue_clear(dsl_scan_t *scn)
{
        void *cookie = NULL;
        scan_ds_t *sds;
        while ((sds = avl_destroy_nodes(&scn->scn_queue, &cookie)) != NULL) {
                kmem_free(sds, sizeof (*sds));
        }
}

static boolean_t
scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj, uint64_t *txg)
{
        scan_ds_t srch, *sds;

        srch.sds_dsobj = dsobj;
        sds = avl_find(&scn->scn_queue, &srch, NULL);
        if (sds != NULL && txg != NULL)
                *txg = sds->sds_txg;
        return (sds != NULL);
}

static void
scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg)
{
        scan_ds_t *sds;
        avl_index_t where;

        sds = kmem_zalloc(sizeof (*sds), KM_SLEEP);
        sds->sds_dsobj = dsobj;
        sds->sds_txg = txg;

        VERIFY3P(avl_find(&scn->scn_queue, sds, &where), ==, NULL);
        avl_insert(&scn->scn_queue, sds, where);
}

static void
scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj)
{
        scan_ds_t srch, *sds;

        srch.sds_dsobj = dsobj;

        sds = avl_find(&scn->scn_queue, &srch, NULL);
        VERIFY(sds != NULL);
        avl_remove(&scn->scn_queue, sds);
        kmem_free(sds, sizeof (*sds));
}

static void
scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;
        dmu_object_type_t ot = (spa_version(spa) >= SPA_VERSION_DSL_SCRUB) ?
            DMU_OT_SCAN_QUEUE : DMU_OT_ZAP_OTHER;

        ASSERT0(scn->scn_bytes_pending);
        ASSERT(scn->scn_phys.scn_queue_obj != 0);

        VERIFY0(dmu_object_free(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, tx));
        scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset, ot,
            DMU_OT_NONE, 0, tx);
        for (scan_ds_t *sds = avl_first(&scn->scn_queue);
            sds != NULL; sds = AVL_NEXT(&scn->scn_queue, sds)) {
                VERIFY0(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, sds->sds_dsobj,
                    sds->sds_txg, tx));
        }
}

/*
 * Computes the memory limit state that we're currently in. A sorted scan
 * needs quite a bit of memory to hold the sorting queue, so we need to
 * reasonably constrain the size so it doesn't impact overall system
 * performance. We compute two limits:
 * 1) Hard memory limit: if the amount of memory used by the sorting
 *      queues on a pool gets above this value, we stop the metadata
 *      scanning portion and start issuing the queued up and sorted
 *      I/Os to reduce memory usage.
 *      This limit is calculated as a fraction of physmem (by default 5%).
 *      We constrain the lower bound of the hard limit to an absolute
 *      minimum of zfs_scan_mem_lim_min (default: 16 MiB). We also constrain
 *      the upper bound to 5% of the total pool size - no chance we'll
 *      ever need that much memory, but just to keep the value in check.
 * 2) Soft memory limit: once we hit the hard memory limit, we start
 *      issuing I/O to reduce queue memory usage, but we don't want to
 *      completely empty out the queues, since we might be able to find I/Os
 *      that will fill in the gaps of our non-sequential IOs at some point
 *      in the future. So we stop the issuing of I/Os once the amount of
 *      memory used drops below the soft limit (at which point we stop issuing
 *      I/O and start scanning metadata again).
 *
 *      This limit is calculated by subtracting a fraction of the hard
 *      limit from the hard limit. By default this fraction is 5%, so
 *      the soft limit is 95% of the hard limit. We cap the size of the
 *      difference between the hard and soft limits at an absolute
 *      maximum of zfs_scan_mem_lim_soft_max (default: 128 MiB) - this is
 *      sufficient to not cause too frequent switching between the
 *      metadata scan and I/O issue (even at 2k recordsize, 128 MiB's
 *      worth of queues is about 1.2 GiB of on-pool data, so scanning
 *      that should take at least a decent fraction of a second).
 */
static boolean_t
dsl_scan_should_clear(dsl_scan_t *scn)
{
        vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
        uint64_t mlim_hard, mlim_soft, mused;
        uint64_t alloc = metaslab_class_get_alloc(spa_normal_class(
            scn->scn_dp->dp_spa));

        mlim_hard = MAX((physmem / zfs_scan_mem_lim_fact) * PAGESIZE,
            zfs_scan_mem_lim_min);
        mlim_hard = MIN(mlim_hard, alloc / 20);
        mlim_soft = mlim_hard - MIN(mlim_hard / zfs_scan_mem_lim_soft_fact,
            zfs_scan_mem_lim_soft_max);
        mused = 0;
        for (uint64_t i = 0; i < rvd->vdev_children; i++) {
                vdev_t *tvd = rvd->vdev_child[i];
                dsl_scan_io_queue_t *queue;

                mutex_enter(&tvd->vdev_scan_io_queue_lock);
                queue = tvd->vdev_scan_io_queue;
                if (queue != NULL) {
                        /* #extents in exts_by_size = # in exts_by_addr */
                        mused += avl_numnodes(&queue->q_exts_by_size) *
                            sizeof (range_seg_t) +
                            avl_numnodes(&queue->q_sios_by_addr) *
                            sizeof (scan_io_t);
                }
                mutex_exit(&tvd->vdev_scan_io_queue_lock);
        }

        dprintf("current scan memory usage: %llu bytes\n", (longlong_t)mused);

        if (mused == 0)
                ASSERT0(scn->scn_bytes_pending);

        /*
         * If we are above our hard limit, we need to clear out memory.
         * If we are below our soft limit, we need to accumulate sequential IOs.
         * Otherwise, we should keep doing whatever we are currently doing.
         */
        if (mused >= mlim_hard)
                return (B_TRUE);
        else if (mused < mlim_soft)
                return (B_FALSE);
        else
                return (scn->scn_clearing);
}

static boolean_t
dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
{
        /* we never skip user/group accounting objects */
        if (zb && (int64_t)zb->zb_object < 0)
                return (B_FALSE);

        if (scn->scn_suspending)
                return (B_TRUE); /* we're already suspending */

        if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark))
                return (B_FALSE); /* we're resuming */

        /* We only know how to resume from level-0 blocks. */
        if (zb && zb->zb_level != 0)
                return (B_FALSE);

        /*
         * We suspend if:
         *  - we have scanned for at least the minimum time (default 1 sec
         *    for scrub, 3 sec for resilver), and either we have sufficient
         *    dirty data that we are starting to write more quickly
         *    (default 30%), or someone is explicitly waiting for this txg
         *    to complete.
         *  or
         *  - the spa is shutting down because this pool is being exported
         *    or the machine is rebooting.
         *  or
         *  - the scan queue has reached its memory use limit
         */
        uint64_t elapsed_nanosecs = gethrtime();
        uint64_t curr_time_ns = gethrtime();
        uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
        uint64_t sync_time_ns = curr_time_ns -
            scn->scn_dp->dp_spa->spa_sync_starttime;

        int dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
        int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
            zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;

        if ((NSEC2MSEC(scan_time_ns) > mintime &&
            (dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent ||
            txg_sync_waiting(scn->scn_dp) ||
            NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
            spa_shutting_down(scn->scn_dp->dp_spa) ||
            (zfs_scan_strict_mem_lim && dsl_scan_should_clear(scn))) {
                if (zb) {
                        dprintf("suspending at bookmark %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
                        scn->scn_phys.scn_bookmark = *zb;
                } else {
                        dsl_scan_phys_t *scnp = &scn->scn_phys;

                        dprintf("suspending at at DDT bookmark "
                            "%llx/%llx/%llx/%llx\n",
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
                }
                scn->scn_suspending = B_TRUE;
                return (B_TRUE);
        }
        return (B_FALSE);
}

typedef struct zil_scan_arg {
        dsl_pool_t      *zsa_dp;
        zil_header_t    *zsa_zh;
} zil_scan_arg_t;

/* ARGSUSED */
static int
dsl_scan_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
{
        zil_scan_arg_t *zsa = arg;
        dsl_pool_t *dp = zsa->zsa_dp;
        dsl_scan_t *scn = dp->dp_scan;
        zil_header_t *zh = zsa->zsa_zh;
        zbookmark_phys_t zb;

        if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                return (0);

        /*
         * One block ("stubby") can be allocated a long time ago; we
         * want to visit that one because it has been allocated
         * (on-disk) even if it hasn't been claimed (even though for
         * scrub there's nothing to do to it).
         */
        if (claim_txg == 0 && bp->blk_birth >= spa_min_claim_txg(dp->dp_spa))
                return (0);

        SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
            ZB_ZIL_OBJECT, ZB_ZIL_LEVEL, bp->blk_cksum.zc_word[ZIL_ZC_SEQ]);

        VERIFY(0 == scan_funcs[scn->scn_phys.scn_func](dp, bp, &zb));
        return (0);
}

/* ARGSUSED */
static int
dsl_scan_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
{
        if (lrc->lrc_txtype == TX_WRITE) {
                zil_scan_arg_t *zsa = arg;
                dsl_pool_t *dp = zsa->zsa_dp;
                dsl_scan_t *scn = dp->dp_scan;
                zil_header_t *zh = zsa->zsa_zh;
                lr_write_t *lr = (lr_write_t *)lrc;
                blkptr_t *bp = &lr->lr_blkptr;
                zbookmark_phys_t zb;

                if (BP_IS_HOLE(bp) ||
                    bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                        return (0);

                /*
                 * birth can be < claim_txg if this record's txg is
                 * already txg sync'ed (but this log block contains
                 * other records that are not synced)
                 */
                if (claim_txg == 0 || bp->blk_birth < claim_txg)
                        return (0);

                SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
                    lr->lr_foid, ZB_ZIL_LEVEL,
                    lr->lr_offset / BP_GET_LSIZE(bp));

                VERIFY(0 == scan_funcs[scn->scn_phys.scn_func](dp, bp, &zb));
        }
        return (0);
}

static void
dsl_scan_zil(dsl_pool_t *dp, zil_header_t *zh)
{
        uint64_t claim_txg = zh->zh_claim_txg;
        zil_scan_arg_t zsa = { dp, zh };
        zilog_t *zilog;

        ASSERT(spa_writeable(dp->dp_spa));

        /*
         * We only want to visit blocks that have been claimed
         * but not yet replayed.
         */
        if (claim_txg == 0)
                return;

        zilog = zil_alloc(dp->dp_meta_objset, zh);

        (void) zil_parse(zilog, dsl_scan_zil_block, dsl_scan_zil_record, &zsa,
            claim_txg);

        zil_free(zilog);
}

/*
 * We compare scan_prefetch_issue_ctx_t's based on their bookmarks. The idea
 * here is to sort the AVL tree by the order each block will be needed.
 */
static int
scan_prefetch_queue_compare(const void *a, const void *b)
{
        const scan_prefetch_issue_ctx_t *spic_a = a, *spic_b = b;
        const scan_prefetch_ctx_t *spc_a = spic_a->spic_spc;
        const scan_prefetch_ctx_t *spc_b = spic_b->spic_spc;

        return (zbookmark_compare(spc_a->spc_datablkszsec,
            spc_a->spc_indblkshift, spc_b->spc_datablkszsec,
            spc_b->spc_indblkshift, &spic_a->spic_zb, &spic_b->spic_zb));
}

static void
scan_prefetch_ctx_rele(scan_prefetch_ctx_t *spc, void *tag)
{
        if (zfs_refcount_remove(&spc->spc_refcnt, tag) == 0) {
                zfs_refcount_destroy(&spc->spc_refcnt);
                kmem_free(spc, sizeof (scan_prefetch_ctx_t));
        }
}

static scan_prefetch_ctx_t *
scan_prefetch_ctx_create(dsl_scan_t *scn, dnode_phys_t *dnp, void *tag)
{
        scan_prefetch_ctx_t *spc;

        spc = kmem_alloc(sizeof (scan_prefetch_ctx_t), KM_SLEEP);
        zfs_refcount_create(&spc->spc_refcnt);
        zfs_refcount_add(&spc->spc_refcnt, tag);
        spc->spc_scn = scn;
        if (dnp != NULL) {
                spc->spc_datablkszsec = dnp->dn_datablkszsec;
                spc->spc_indblkshift = dnp->dn_indblkshift;
                spc->spc_root = B_FALSE;
        } else {
                spc->spc_datablkszsec = 0;
                spc->spc_indblkshift = 0;
                spc->spc_root = B_TRUE;
        }

        return (spc);
}

static void
scan_prefetch_ctx_add_ref(scan_prefetch_ctx_t *spc, void *tag)
{
        zfs_refcount_add(&spc->spc_refcnt, tag);
}

static boolean_t
dsl_scan_check_prefetch_resume(scan_prefetch_ctx_t *spc,
    const zbookmark_phys_t *zb)
{
        zbookmark_phys_t *last_zb = &spc->spc_scn->scn_prefetch_bookmark;
        dnode_phys_t tmp_dnp;
        dnode_phys_t *dnp = (spc->spc_root) ? NULL : &tmp_dnp;

        if (zb->zb_objset != last_zb->zb_objset)
                return (B_TRUE);
        if ((int64_t)zb->zb_object < 0)
                return (B_FALSE);

        tmp_dnp.dn_datablkszsec = spc->spc_datablkszsec;
        tmp_dnp.dn_indblkshift = spc->spc_indblkshift;

        if (zbookmark_subtree_completed(dnp, zb, last_zb))
                return (B_TRUE);

        return (B_FALSE);
}

static void
dsl_scan_prefetch(scan_prefetch_ctx_t *spc, blkptr_t *bp, zbookmark_phys_t *zb)
{
        avl_index_t idx;
        dsl_scan_t *scn = spc->spc_scn;
        spa_t *spa = scn->scn_dp->dp_spa;
        scan_prefetch_issue_ctx_t *spic;

        if (zfs_no_scrub_prefetch)
                return;

        if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg ||
            (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE &&
            BP_GET_TYPE(bp) != DMU_OT_OBJSET))
                return;

        if (dsl_scan_check_prefetch_resume(spc, zb))
                return;

        scan_prefetch_ctx_add_ref(spc, scn);
        spic = kmem_alloc(sizeof (scan_prefetch_issue_ctx_t), KM_SLEEP);
        spic->spic_spc = spc;
        spic->spic_bp = *bp;
        spic->spic_zb = *zb;

        /*
         * Add the IO to the queue of blocks to prefetch. This allows us to
         * prioritize blocks that we will need first for the main traversal
         * thread.
         */
        mutex_enter(&spa->spa_scrub_lock);
        if (avl_find(&scn->scn_prefetch_queue, spic, &idx) != NULL) {
                /* this block is already queued for prefetch */
                kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
                scan_prefetch_ctx_rele(spc, scn);
                mutex_exit(&spa->spa_scrub_lock);
                return;
        }

        avl_insert(&scn->scn_prefetch_queue, spic, idx);
        cv_broadcast(&spa->spa_scrub_io_cv);
        mutex_exit(&spa->spa_scrub_lock);
}

static void
dsl_scan_prefetch_dnode(dsl_scan_t *scn, dnode_phys_t *dnp,
    uint64_t objset, uint64_t object)
{
        int i;
        zbookmark_phys_t zb;
        scan_prefetch_ctx_t *spc;

        if (dnp->dn_nblkptr == 0 && !(dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR))
                return;

        SET_BOOKMARK(&zb, objset, object, 0, 0);

        spc = scan_prefetch_ctx_create(scn, dnp, FTAG);

        for (i = 0; i < dnp->dn_nblkptr; i++) {
                zb.zb_level = BP_GET_LEVEL(&dnp->dn_blkptr[i]);
                zb.zb_blkid = i;
                dsl_scan_prefetch(spc, &dnp->dn_blkptr[i], &zb);
        }

        if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
                zb.zb_level = 0;
                zb.zb_blkid = DMU_SPILL_BLKID;
                dsl_scan_prefetch(spc, &dnp->dn_spill, &zb);
        }

        scan_prefetch_ctx_rele(spc, FTAG);
}

void
dsl_scan_prefetch_cb(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
    arc_buf_t *buf, void *private)
{
        scan_prefetch_ctx_t *spc = private;
        dsl_scan_t *scn = spc->spc_scn;
        spa_t *spa = scn->scn_dp->dp_spa;

        /* broadcast that the IO has completed for rate limitting purposes */
        mutex_enter(&spa->spa_scrub_lock);
        ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
        spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
        cv_broadcast(&spa->spa_scrub_io_cv);
        mutex_exit(&spa->spa_scrub_lock);

        /* if there was an error or we are done prefetching, just cleanup */
        if (buf == NULL || scn->scn_suspending)
                goto out;

        if (BP_GET_LEVEL(bp) > 0) {
                int i;
                blkptr_t *cbp;
                int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
                zbookmark_phys_t czb;

                for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
                        SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1, zb->zb_blkid * epb + i);
                        dsl_scan_prefetch(spc, cbp, &czb);
                }
        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                dnode_phys_t *cdnp = buf->b_data;
                int i;
                int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;

                for (i = 0, cdnp = buf->b_data; i < epb;
                    i += cdnp->dn_extra_slots + 1,
                    cdnp += cdnp->dn_extra_slots + 1) {
                        dsl_scan_prefetch_dnode(scn, cdnp,
                            zb->zb_objset, zb->zb_blkid * epb + i);
                }
        } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
                objset_phys_t *osp = buf->b_data;

                dsl_scan_prefetch_dnode(scn, &osp->os_meta_dnode,
                    zb->zb_objset, DMU_META_DNODE_OBJECT);

                if (OBJSET_BUF_HAS_USERUSED(buf)) {
                        dsl_scan_prefetch_dnode(scn,
                            &osp->os_groupused_dnode, zb->zb_objset,
                            DMU_GROUPUSED_OBJECT);
                        dsl_scan_prefetch_dnode(scn,
                            &osp->os_userused_dnode, zb->zb_objset,
                            DMU_USERUSED_OBJECT);
                }
        }

out:
        if (buf != NULL)
                arc_buf_destroy(buf, private);
        scan_prefetch_ctx_rele(spc, scn);
}

/* ARGSUSED */
static void
dsl_scan_prefetch_thread(void *arg)
{
        dsl_scan_t *scn = arg;
        spa_t *spa = scn->scn_dp->dp_spa;
        vdev_t *rvd = spa->spa_root_vdev;
        uint64_t maxinflight = rvd->vdev_children * zfs_top_maxinflight;
        scan_prefetch_issue_ctx_t *spic;

        /* loop until we are told to stop */
        while (!scn->scn_prefetch_stop) {
                arc_flags_t flags = ARC_FLAG_NOWAIT |
                    ARC_FLAG_PRESCIENT_PREFETCH | ARC_FLAG_PREFETCH;
                int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
                
                mutex_enter(&spa->spa_scrub_lock);

                /*
                 * Wait until we have an IO to issue and are not above our
                 * maximum in flight limit.
                 */
                while (!scn->scn_prefetch_stop &&
                    (avl_numnodes(&scn->scn_prefetch_queue) == 0 ||
                    spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)) {
                        cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
                }

                /* recheck if we should stop since we waited for the cv */
                if (scn->scn_prefetch_stop) {
                        mutex_exit(&spa->spa_scrub_lock);
                        break;
                }

                /* remove the prefetch IO from the tree */
                spic = avl_first(&scn->scn_prefetch_queue);
                spa->spa_scrub_inflight += BP_GET_PSIZE(&spic->spic_bp);
                avl_remove(&scn->scn_prefetch_queue, spic);

                mutex_exit(&spa->spa_scrub_lock);

                /* issue the prefetch asynchronously */
                (void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa,
                    &spic->spic_bp, dsl_scan_prefetch_cb, spic->spic_spc,
                    ZIO_PRIORITY_SCRUB, zio_flags, &flags, &spic->spic_zb);

                kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
        }

        ASSERT(scn->scn_prefetch_stop);

        /* free any prefetches we didn't get to complete */
        mutex_enter(&spa->spa_scrub_lock);
        while ((spic = avl_first(&scn->scn_prefetch_queue)) != NULL) {
                avl_remove(&scn->scn_prefetch_queue, spic);
                scan_prefetch_ctx_rele(spic->spic_spc, scn);
                kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
        }
        ASSERT0(avl_numnodes(&scn->scn_prefetch_queue));
        mutex_exit(&spa->spa_scrub_lock);
}

static boolean_t
dsl_scan_check_resume(dsl_scan_t *scn, const dnode_phys_t *dnp,
    const zbookmark_phys_t *zb)
{
        /*
         * We never skip over user/group accounting objects (obj<0)
         */
        if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark) &&
            (int64_t)zb->zb_object >= 0) {
                /*
                 * If we already visited this bp & everything below (in
                 * a prior txg sync), don't bother doing it again.
                 */
                if (zbookmark_subtree_completed(dnp, zb,
                    &scn->scn_phys.scn_bookmark))
                        return (B_TRUE);

                /*
                 * If we found the block we're trying to resume from, or
                 * we went past it to a different object, zero it out to
                 * indicate that it's OK to start checking for suspending
                 * again.
                 */
                if (bcmp(zb, &scn->scn_phys.scn_bookmark, sizeof (*zb)) == 0 ||
                    zb->zb_object > scn->scn_phys.scn_bookmark.zb_object) {
                        dprintf("resuming at %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
                        bzero(&scn->scn_phys.scn_bookmark, sizeof (*zb));
                }
        }
        return (B_FALSE);
}

static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
    dmu_objset_type_t ostype, dmu_tx_t *tx);
static void dsl_scan_visitdnode(
    dsl_scan_t *, dsl_dataset_t *ds, dmu_objset_type_t ostype,
    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);

/*
 * Return nonzero on i/o error.
 * Return new buf to write out in *bufp.
 */
static int
dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
    dnode_phys_t *dnp, const blkptr_t *bp,
    const zbookmark_phys_t *zb, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
        int err;

        if (BP_GET_LEVEL(bp) > 0) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                int i;
                blkptr_t *cbp;
                int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
                for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
                        zbookmark_phys_t czb;

                        SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1,
                            zb->zb_blkid * epb + i);
                        dsl_scan_visitbp(cbp, &czb, dnp,
                            ds, scn, ostype, tx);
                }
                arc_buf_destroy(buf, &buf);
        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                dnode_phys_t *cdnp;
                int i;
                int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
                for (i = 0, cdnp = buf->b_data; i < epb;
                    i += cdnp->dn_extra_slots + 1,
                    cdnp += cdnp->dn_extra_slots + 1) {
                        dsl_scan_visitdnode(scn, ds, ostype,
                            cdnp, zb->zb_blkid * epb + i, tx);
                }

                arc_buf_destroy(buf, &buf);
        } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                objset_phys_t *osp;
                arc_buf_t *buf;

                err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }

                osp = buf->b_data;

                dsl_scan_visitdnode(scn, ds, osp->os_type,
                    &osp->os_meta_dnode, DMU_META_DNODE_OBJECT, tx);

                if (OBJSET_BUF_HAS_USERUSED(buf)) {
                        /*
                         * We also always visit user/group accounting
                         * objects, and never skip them, even if we are
                         * suspending.  This is necessary so that the space
                         * deltas from this txg get integrated.
                         */
                        dsl_scan_visitdnode(scn, ds, osp->os_type,
                            &osp->os_groupused_dnode,
                            DMU_GROUPUSED_OBJECT, tx);
                        dsl_scan_visitdnode(scn, ds, osp->os_type,
                            &osp->os_userused_dnode,
                            DMU_USERUSED_OBJECT, tx);
                }
                arc_buf_destroy(buf, &buf);
        }

        return (0);
}

static void
dsl_scan_visitdnode(dsl_scan_t *scn, dsl_dataset_t *ds,
    dmu_objset_type_t ostype, dnode_phys_t *dnp,
    uint64_t object, dmu_tx_t *tx)
{
        int j;

        for (j = 0; j < dnp->dn_nblkptr; j++) {
                zbookmark_phys_t czb;

                SET_BOOKMARK(&czb, ds ? ds->ds_object : 0, object,
                    dnp->dn_nlevels - 1, j);
                dsl_scan_visitbp(&dnp->dn_blkptr[j],
                    &czb, dnp, ds, scn, ostype, tx);
        }

        if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
                zbookmark_phys_t czb;
                SET_BOOKMARK(&czb, ds ? ds->ds_object : 0, object,
                    0, DMU_SPILL_BLKID);
                dsl_scan_visitbp(DN_SPILL_BLKPTR(dnp),
                    &czb, dnp, ds, scn, ostype, tx);
        }
}

/*
 * The arguments are in this order because mdb can only print the
 * first 5; we want them to be useful.
 */
static void
dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
    dmu_objset_type_t ostype, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        blkptr_t *bp_toread = NULL;

        if (dsl_scan_check_suspend(scn, zb))
                return;

        if (dsl_scan_check_resume(scn, dnp, zb))
                return;

        scn->scn_visited_this_txg++;

        dprintf_bp(bp,
            "visiting ds=%p/%llu zb=%llx/%llx/%llx/%llx bp=%p",
            ds, ds ? ds->ds_object : 0,
            zb->zb_objset, zb->zb_object, zb->zb_level, zb->zb_blkid,
            bp);

        if (BP_IS_HOLE(bp)) {
                scn->scn_holes_this_txg++;
                return;
        }

        if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg) {
                scn->scn_lt_min_this_txg++;
                return;
        }

        bp_toread = kmem_alloc(sizeof (blkptr_t), KM_SLEEP);
        *bp_toread = *bp;

        if (dsl_scan_recurse(scn, ds, ostype, dnp, bp_toread, zb, tx) != 0)
                return;

        /*
         * If dsl_scan_ddt() has already visited this block, it will have
         * already done any translations or scrubbing, so don't call the
         * callback again.
         */
        if (ddt_class_contains(dp->dp_spa,
            scn->scn_phys.scn_ddt_class_max, bp)) {
                scn->scn_ddt_contained_this_txg++;
                goto out;
        }

        /*
         * If this block is from the future (after cur_max_txg), then we
         * are doing this on behalf of a deleted snapshot, and we will
         * revisit the future block on the next pass of this dataset.
         * Don't scan it now unless we need to because something
         * under it was modified.
         */
        if (BP_PHYSICAL_BIRTH(bp) > scn->scn_phys.scn_cur_max_txg) {
                scn->scn_gt_max_this_txg++;
                goto out;
        }

        scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
out:
        kmem_free(bp_toread, sizeof (blkptr_t));
}

static void
dsl_scan_visit_rootbp(dsl_scan_t *scn, dsl_dataset_t *ds, blkptr_t *bp,
    dmu_tx_t *tx)
{
        zbookmark_phys_t zb;
        scan_prefetch_ctx_t *spc;

        SET_BOOKMARK(&zb, ds ? ds->ds_object : DMU_META_OBJSET,
            ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);

        if (ZB_IS_ZERO(&scn->scn_phys.scn_bookmark)) {
                SET_BOOKMARK(&scn->scn_prefetch_bookmark,
                    zb.zb_objset, 0, 0, 0);
        } else {
                scn->scn_prefetch_bookmark = scn->scn_phys.scn_bookmark;
        }

        scn->scn_objsets_visited_this_txg++;

        spc = scan_prefetch_ctx_create(scn, NULL, FTAG);
        dsl_scan_prefetch(spc, bp, &zb);
        scan_prefetch_ctx_rele(spc, FTAG);

        dsl_scan_visitbp(bp, &zb, NULL, ds, scn, DMU_OST_NONE, tx);

        dprintf_ds(ds, "finished scan%s", "");
}

static void
ds_destroyed_scn_phys(dsl_dataset_t *ds, dsl_scan_phys_t *scn_phys)
{
        if (scn_phys->scn_bookmark.zb_objset == ds->ds_object) {
                if (ds->ds_is_snapshot) {
                        /*
                         * Note:
                         *  - scn_cur_{min,max}_txg stays the same.
                         *  - Setting the flag is not really necessary if
                         *    scn_cur_max_txg == scn_max_txg, because there
                         *    is nothing after this snapshot that we care
                         *    about.  However, we set it anyway and then
                         *    ignore it when we retraverse it in
                         *    dsl_scan_visitds().
                         */
                        scn_phys->scn_bookmark.zb_objset =
                            dsl_dataset_phys(ds)->ds_next_snap_obj;
                        zfs_dbgmsg("destroying ds %llu; currently traversing; "
                            "reset zb_objset to %llu",
                            (u_longlong_t)ds->ds_object,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
                        scn_phys->scn_flags |= DSF_VISIT_DS_AGAIN;
                } else {
                        SET_BOOKMARK(&scn_phys->scn_bookmark,
                            ZB_DESTROYED_OBJSET, 0, 0, 0);
                        zfs_dbgmsg("destroying ds %llu; currently traversing; "
                            "reset bookmark to -1,0,0,0",
                            (u_longlong_t)ds->ds_object);
                }
        }
}

/*
 * Invoked when a dataset is destroyed. We need to make sure that:
 *
 * 1) If it is the dataset that was currently being scanned, we write
 *      a new dsl_scan_phys_t and marking the objset reference in it
 *      as destroyed.
 * 2) Remove it from the work queue, if it was present.
 *
 * If the dataset was actually a snapshot, instead of marking the dataset
 * as destroyed, we instead substitute the next snapshot in line.
 */
void
dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;

        if (!dsl_scan_is_running(scn))
                return;

        ds_destroyed_scn_phys(ds, &scn->scn_phys);
        ds_destroyed_scn_phys(ds, &scn->scn_phys_cached);

        if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
                scan_ds_queue_remove(scn, ds->ds_object);
                if (ds->ds_is_snapshot)
                        scan_ds_queue_insert(scn,
                            dsl_dataset_phys(ds)->ds_next_snap_obj, mintxg);
        }

        if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
            ds->ds_object, &mintxg) == 0) {
                ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
                if (ds->ds_is_snapshot) {
                        /*
                         * We keep the same mintxg; it could be >
                         * ds_creation_txg if the previous snapshot was
                         * deleted too.
                         */
                        VERIFY(zap_add_int_key(dp->dp_meta_objset,
                            scn->scn_phys.scn_queue_obj,
                            dsl_dataset_phys(ds)->ds_next_snap_obj,
                            mintxg, tx) == 0);
                        zfs_dbgmsg("destroying ds %llu; in queue; "
                            "replacing with %llu",
                            (u_longlong_t)ds->ds_object,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
                } else {
                        zfs_dbgmsg("destroying ds %llu; in queue; removing",
                            (u_longlong_t)ds->ds_object);
                }
        }

        /*
         * dsl_scan_sync() should be called after this, and should sync
         * out our changed state, but just to be safe, do it here.
         */
        dsl_scan_sync_state(scn, tx, SYNC_CACHED);
}

static void
ds_snapshotted_bookmark(dsl_dataset_t *ds, zbookmark_phys_t *scn_bookmark)
{
        if (scn_bookmark->zb_objset == ds->ds_object) {
                scn_bookmark->zb_objset =
                    dsl_dataset_phys(ds)->ds_prev_snap_obj;
                zfs_dbgmsg("snapshotting ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds->ds_object,
                    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
        }
}

/*
 * Called when a dataset is snapshotted. If we were currently traversing
 * this snapshot, we reset our bookmark to point at the newly created
 * snapshot. We also modify our work queue to remove the old snapshot and
 * replace with the new one.
 */
void
dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;

        if (!dsl_scan_is_running(scn))
                return;

        ASSERT(dsl_dataset_phys(ds)->ds_prev_snap_obj != 0);

        ds_snapshotted_bookmark(ds, &scn->scn_phys.scn_bookmark);
        ds_snapshotted_bookmark(ds, &scn->scn_phys_cached.scn_bookmark);

        if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
                scan_ds_queue_remove(scn, ds->ds_object);
                scan_ds_queue_insert(scn,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg);
        }

        if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
            ds->ds_object, &mintxg) == 0) {
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
                VERIFY(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg, tx) == 0);
                zfs_dbgmsg("snapshotting ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds->ds_object,
                    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
        }

        dsl_scan_sync_state(scn, tx, SYNC_CACHED);
}

static void
ds_clone_swapped_bookmark(dsl_dataset_t *ds1, dsl_dataset_t *ds2,
    zbookmark_phys_t *scn_bookmark)
{
        if (scn_bookmark->zb_objset == ds1->ds_object) {
                scn_bookmark->zb_objset = ds2->ds_object;
                zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds1->ds_object,
                    (u_longlong_t)ds2->ds_object);
        } else if (scn_bookmark->zb_objset == ds2->ds_object) {
                scn_bookmark->zb_objset = ds1->ds_object;
                zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
                    "reset zb_objset to %llu",
                    (u_longlong_t)ds2->ds_object,
                    (u_longlong_t)ds1->ds_object);
        }
}

/*
 * Called when an origin dataset and its clone are swapped.  If we were
 * currently traversing the dataset, we need to switch to traversing the
 * newly promoted clone.
 */
void
dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
{
        dsl_pool_t *dp = ds1->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg1, mintxg2;
        boolean_t ds1_queued, ds2_queued;

        if (!dsl_scan_is_running(scn))
                return;

        ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys.scn_bookmark);
        ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys_cached.scn_bookmark);

        /*
         * Handle the in-memory scan queue.
         */
        ds1_queued = scan_ds_queue_contains(scn, ds1->ds_object, &mintxg1);
        ds2_queued = scan_ds_queue_contains(scn, ds2->ds_object, &mintxg2);

        /* Sanity checking. */
        if (ds1_queued) {
                ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
        }
        if (ds2_queued) {
                ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
        }

        if (ds1_queued && ds2_queued) {
                /*
                 * If both are queued, we don't need to do anything.
                 * The swapping code below would not handle this case correctly,
                 * since we can't insert ds2 if it is already there. That's
                 * because scan_ds_queue_insert() prohibits a duplicate insert
                 * and panics.
                 */
        } else if (ds1_queued) {
                scan_ds_queue_remove(scn, ds1->ds_object);
                scan_ds_queue_insert(scn, ds2->ds_object, mintxg1);
        } else if (ds2_queued) {
                scan_ds_queue_remove(scn, ds2->ds_object);
                scan_ds_queue_insert(scn, ds1->ds_object, mintxg2);
        }

        /*
         * Handle the on-disk scan queue.
         * The on-disk state is an out-of-date version of the in-memory state,
         * so the in-memory and on-disk values for ds1_queued and ds2_queued may
         * be different. Therefore we need to apply the swap logic to the
         * on-disk state independently of the in-memory state.
         */
        ds1_queued = zap_lookup_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds1->ds_object, &mintxg1) == 0;
        ds2_queued = zap_lookup_int_key(dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg2) == 0;

        /* Sanity checking. */
        if (ds1_queued) {
                ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
        }
        if (ds2_queued) {
                ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
                ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
        }

        if (ds1_queued && ds2_queued) {
                /*
                 * If both are queued, we don't need to do anything.
                 * Alternatively, we could check for EEXIST from
                 * zap_add_int_key() and back out to the original state, but
                 * that would be more work than checking for this case upfront.
                 */
        } else if (ds1_queued) {
                VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds1->ds_object, tx));
                VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg1, tx));
                zfs_dbgmsg("clone_swap ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds1->ds_object,
                    (u_longlong_t)ds2->ds_object);
        } else if (ds2_queued) {
                VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds2->ds_object, tx));
                VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg2, tx));
                zfs_dbgmsg("clone_swap ds %llu; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds2->ds_object,
                    (u_longlong_t)ds1->ds_object);
        }

        dsl_scan_sync_state(scn, tx, SYNC_CACHED);
}

/* ARGSUSED */
static int
enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
{
        uint64_t originobj = *(uint64_t *)arg;
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;

        if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != originobj)
                return (0);

        err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
        if (err)
                return (err);

        while (dsl_dataset_phys(ds)->ds_prev_snap_obj != originobj) {
                dsl_dataset_t *prev;
                err = dsl_dataset_hold_obj(dp,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);

                dsl_dataset_rele(ds, FTAG);
                if (err)
                        return (err);
                ds = prev;
        }
        scan_ds_queue_insert(scn, ds->ds_object,
            dsl_dataset_phys(ds)->ds_prev_snap_txg);
        dsl_dataset_rele(ds, FTAG);
        return (0);
}

static void
dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
{
        dsl_pool_t *dp = scn->scn_dp;
        dsl_dataset_t *ds;

        VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));

        if (scn->scn_phys.scn_cur_min_txg >=
            scn->scn_phys.scn_max_txg) {
                /*
                 * This can happen if this snapshot was created after the
                 * scan started, and we already completed a previous snapshot
                 * that was created after the scan started.  This snapshot
                 * only references blocks with:
                 *
                 *      birth < our ds_creation_txg
                 *      cur_min_txg is no less than ds_creation_txg.
                 *      We have already visited these blocks.
                 * or
                 *      birth > scn_max_txg
                 *      The scan requested not to visit these blocks.
                 *
                 * Subsequent snapshots (and clones) can reference our
                 * blocks, or blocks with even higher birth times.
                 * Therefore we do not need to visit them either,
                 * so we do not add them to the work queue.
                 *
                 * Note that checking for cur_min_txg >= cur_max_txg
                 * is not sufficient, because in that case we may need to
                 * visit subsequent snapshots.  This happens when min_txg > 0,
                 * which raises cur_min_txg.  In this case we will visit
                 * this dataset but skip all of its blocks, because the
                 * rootbp's birth time is < cur_min_txg.  Then we will
                 * add the next snapshots/clones to the work queue.
                 */
                char *dsname = kmem_alloc(MAXNAMELEN, KM_SLEEP);
                dsl_dataset_name(ds, dsname);
                zfs_dbgmsg("scanning dataset %llu (%s) is unnecessary because "
                    "cur_min_txg (%llu) >= max_txg (%llu)",
                    (longlong_t)dsobj, dsname,
                    (longlong_t)scn->scn_phys.scn_cur_min_txg,
                    (longlong_t)scn->scn_phys.scn_max_txg);
                kmem_free(dsname, MAXNAMELEN);

                goto out;
        }

        /*
         * Only the ZIL in the head (non-snapshot) is valid. Even though
         * snapshots can have ZIL block pointers (which may be the same
         * BP as in the head), they must be ignored. In addition, $ORIGIN
         * doesn't have a objset (i.e. its ds_bp is a hole) so we don't
         * need to look for a ZIL in it either. So we traverse the ZIL here,
         * rather than in scan_recurse(), because the regular snapshot
         * block-sharing rules don't apply to it.
         */
        if (DSL_SCAN_IS_SCRUB_RESILVER(scn) && !dsl_dataset_is_snapshot(ds) &&
            (dp->dp_origin_snap == NULL ||
            ds->ds_dir != dp->dp_origin_snap->ds_dir)) {
                objset_t *os;
                if (dmu_objset_from_ds(ds, &os) != 0) {
                        goto out;
                }
                dsl_scan_zil(dp, &os->os_zil_header);
        }

        /*
         * Iterate over the bps in this ds.
         */
        dmu_buf_will_dirty(ds->ds_dbuf, tx);
        rrw_enter(&ds->ds_bp_rwlock, RW_READER, FTAG);
        dsl_scan_visit_rootbp(scn, ds, &dsl_dataset_phys(ds)->ds_bp, tx);
        rrw_exit(&ds->ds_bp_rwlock, FTAG);

        char *dsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
        dsl_dataset_name(ds, dsname);
        zfs_dbgmsg("scanned dataset %llu (%s) with min=%llu max=%llu; "
            "suspending=%u",
            (longlong_t)dsobj, dsname,
            (longlong_t)scn->scn_phys.scn_cur_min_txg,
            (longlong_t)scn->scn_phys.scn_cur_max_txg,
            (int)scn->scn_suspending);
        kmem_free(dsname, ZFS_MAX_DATASET_NAME_LEN);

        if (scn->scn_suspending)
                goto out;

        /*
         * We've finished this pass over this dataset.
         */

        /*
         * If we did not completely visit this dataset, do another pass.
         */
        if (scn->scn_phys.scn_flags & DSF_VISIT_DS_AGAIN) {
                zfs_dbgmsg("incomplete pass; visiting again");
                scn->scn_phys.scn_flags &= ~DSF_VISIT_DS_AGAIN;
                scan_ds_queue_insert(scn, ds->ds_object,
                    scn->scn_phys.scn_cur_max_txg);
                goto out;
        }

        /*
         * Add descendent datasets to work queue.
         */
        if (dsl_dataset_phys(ds)->ds_next_snap_obj != 0) {
                scan_ds_queue_insert(scn,
                    dsl_dataset_phys(ds)->ds_next_snap_obj,
                    dsl_dataset_phys(ds)->ds_creation_txg);
        }
        if (dsl_dataset_phys(ds)->ds_num_children > 1) {
                boolean_t usenext = B_FALSE;
                if (dsl_dataset_phys(ds)->ds_next_clones_obj != 0) {
                        uint64_t count;
                        /*
                         * A bug in a previous version of the code could
                         * cause upgrade_clones_cb() to not set
                         * ds_next_snap_obj when it should, leading to a
                         * missing entry.  Therefore we can only use the
                         * next_clones_obj when its count is correct.
                         */
                        int err = zap_count(dp->dp_meta_objset,
                            dsl_dataset_phys(ds)->ds_next_clones_obj, &count);
                        if (err == 0 &&
                            count == dsl_dataset_phys(ds)->ds_num_children - 1)
                                usenext = B_TRUE;
                }

                if (usenext) {
                        zap_cursor_t zc;
                        zap_attribute_t za;
                        for (zap_cursor_init(&zc, dp->dp_meta_objset,
                            dsl_dataset_phys(ds)->ds_next_clones_obj);
                            zap_cursor_retrieve(&zc, &za) == 0;
                            (void) zap_cursor_advance(&zc)) {
                                scan_ds_queue_insert(scn,
                                    zfs_strtonum(za.za_name, NULL),
                                    dsl_dataset_phys(ds)->ds_creation_txg);
                        }
                        zap_cursor_fini(&zc);
                } else {
                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
                            enqueue_clones_cb, &ds->ds_object,
                            DS_FIND_CHILDREN));
                }
        }

out:
        dsl_dataset_rele(ds, FTAG);
}

/* ARGSUSED */
static int
enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
{
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;

        err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
        if (err)
                return (err);

        while (dsl_dataset_phys(ds)->ds_prev_snap_obj != 0) {
                dsl_dataset_t *prev;
                err = dsl_dataset_hold_obj(dp,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);
                if (err) {
                        dsl_dataset_rele(ds, FTAG);
                        return (err);
                }

                /*
                 * If this is a clone, we don't need to worry about it for now.
                 */
                if (dsl_dataset_phys(prev)->ds_next_snap_obj != ds->ds_object) {
                        dsl_dataset_rele(ds, FTAG);
                        dsl_dataset_rele(prev, FTAG);
                        return (0);
                }
                dsl_dataset_rele(ds, FTAG);
                ds = prev;
        }

        scan_ds_queue_insert(scn, ds->ds_object,
            dsl_dataset_phys(ds)->ds_prev_snap_txg);
        dsl_dataset_rele(ds, FTAG);
        return (0);
}

/* ARGSUSED */
void
dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
    ddt_entry_t *dde, dmu_tx_t *tx)
{
        const ddt_key_t *ddk = &dde->dde_key;
        ddt_phys_t *ddp = dde->dde_phys;
        blkptr_t bp;
        zbookmark_phys_t zb = { 0 };
        int p;

        if (scn->scn_phys.scn_state != DSS_SCANNING)
                return;

        for (p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
                if (ddp->ddp_phys_birth == 0 ||
                    ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
                        continue;
                ddt_bp_create(checksum, ddk, ddp, &bp);

                scn->scn_visited_this_txg++;
                scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
        }
}

/*
 * Scrub/dedup interaction.
 *
 * If there are N references to a deduped block, we don't want to scrub it
 * N times -- ideally, we should scrub it exactly once.
 *
 * We leverage the fact that the dde's replication class (enum ddt_class)
 * is ordered from highest replication class (DDT_CLASS_DITTO) to lowest
 * (DDT_CLASS_UNIQUE) so that we may walk the DDT in that order.
 *
 * To prevent excess scrubbing, the scrub begins by walking the DDT
 * to find all blocks with refcnt > 1, and scrubs each of these once.
 * Since there are two replication classes which contain blocks with
 * refcnt > 1, we scrub the highest replication class (DDT_CLASS_DITTO) first.
 * Finally the top-down scrub begins, only visiting blocks with refcnt == 1.
 *
 * There would be nothing more to say if a block's refcnt couldn't change
 * during a scrub, but of course it can so we must account for changes
 * in a block's replication class.
 *
 * Here's an example of what can occur:
 *
 * If a block has refcnt > 1 during the DDT scrub phase, but has refcnt == 1
 * when visited during the top-down scrub phase, it will be scrubbed twice.
 * This negates our scrub optimization, but is otherwise harmless.
 *
 * If a block has refcnt == 1 during the DDT scrub phase, but has refcnt > 1
 * on each visit during the top-down scrub phase, it will never be scrubbed.
 * To catch this, ddt_sync_entry() notifies the scrub code whenever a block's
 * reference class transitions to a higher level (i.e DDT_CLASS_UNIQUE to
 * DDT_CLASS_DUPLICATE); if it transitions from refcnt == 1 to refcnt > 1
 * while a scrub is in progress, it scrubs the block right then.
 */
static void
dsl_scan_ddt(dsl_scan_t *scn, dmu_tx_t *tx)
{
        ddt_bookmark_t *ddb = &scn->scn_phys.scn_ddt_bookmark;
        ddt_entry_t dde = { 0 };
        int error;
        uint64_t n = 0;

        while ((error = ddt_walk(scn->scn_dp->dp_spa, ddb, &dde)) == 0) {
                ddt_t *ddt;

                if (ddb->ddb_class > scn->scn_phys.scn_ddt_class_max)
                        break;
                dprintf("visiting ddb=%llu/%llu/%llu/%llx\n",
                    (longlong_t)ddb->ddb_class,
                    (longlong_t)ddb->ddb_type,
                    (longlong_t)ddb->ddb_checksum,
                    (longlong_t)ddb->ddb_cursor);

                /* There should be no pending changes to the dedup table */
                ddt = scn->scn_dp->dp_spa->spa_ddt[ddb->ddb_checksum];
                ASSERT(avl_first(&ddt->ddt_tree) == NULL);

                dsl_scan_ddt_entry(scn, ddb->ddb_checksum, &dde, tx);
                n++;

                if (dsl_scan_check_suspend(scn, NULL))
                        break;
        }

        zfs_dbgmsg("scanned %llu ddt entries with class_max = %u; "
            "suspending=%u", (longlong_t)n,
            (int)scn->scn_phys.scn_ddt_class_max, (int)scn->scn_suspending);

        ASSERT(error == 0 || error == ENOENT);
        ASSERT(error != ENOENT ||
            ddb->ddb_class > scn->scn_phys.scn_ddt_class_max);
}

static uint64_t
dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
{
        uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
        if (ds->ds_is_snapshot)
                return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
        return (smt);
}

static void
dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
{
        scan_ds_t *sds;
        dsl_pool_t *dp = scn->scn_dp;

        if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
            scn->scn_phys.scn_ddt_class_max) {
                scn->scn_phys.scn_cur_min_txg = scn->scn_phys.scn_min_txg;
                scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
                dsl_scan_ddt(scn, tx);
                if (scn->scn_suspending)
                        return;
        }

        if (scn->scn_phys.scn_bookmark.zb_objset == DMU_META_OBJSET) {
                /* First do the MOS & ORIGIN */

                scn->scn_phys.scn_cur_min_txg = scn->scn_phys.scn_min_txg;
                scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
                dsl_scan_visit_rootbp(scn, NULL,
                    &dp->dp_meta_rootbp, tx);
                spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
                if (scn->scn_suspending)
                        return;

                if (spa_version(dp->dp_spa) < SPA_VERSION_DSL_SCRUB) {
                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
                            enqueue_cb, NULL, DS_FIND_CHILDREN));
                } else {
                        dsl_scan_visitds(scn,
                            dp->dp_origin_snap->ds_object, tx);
                }
                ASSERT(!scn->scn_suspending);
        } else if (scn->scn_phys.scn_bookmark.zb_objset !=
            ZB_DESTROYED_OBJSET) {
                uint64_t dsobj = scn->scn_phys.scn_bookmark.zb_objset;
                /*
                 * If we were suspended, continue from here. Note if the
                 * ds we were suspended on was deleted, the zb_objset may
                 * be -1, so we will skip this and find a new objset
                 * below.
                 */
                dsl_scan_visitds(scn, dsobj, tx);
                if (scn->scn_suspending)
                        return;
        }

        /*
         * In case we suspended right at the end of the ds, zero the
         * bookmark so we don't think that we're still trying to resume.
         */
        bzero(&scn->scn_phys.scn_bookmark, sizeof (zbookmark_phys_t));

        /*
         * Keep pulling things out of the dataset avl queue. Updates to the
         * persistent zap-object-as-queue happen only at checkpoints.
         */
        while ((sds = avl_first(&scn->scn_queue)) != NULL) {
                dsl_dataset_t *ds;
                uint64_t dsobj = sds->sds_dsobj;
                uint64_t txg = sds->sds_txg;

                /* dequeue and free the ds from the queue */
                scan_ds_queue_remove(scn, dsobj);
                sds = NULL;     /* must not be touched after removal */

                /* Set up min / max txg */
                VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
                if (txg != 0) {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg, txg);
                } else {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
                            dsl_dataset_phys(ds)->ds_prev_snap_txg);
                }
                scn->scn_phys.scn_cur_max_txg = dsl_scan_ds_maxtxg(ds);
                dsl_dataset_rele(ds, FTAG);

                dsl_scan_visitds(scn, dsobj, tx);
                if (scn->scn_suspending)
                        return;
        }
        /* No more objsets to fetch, we're done */
        scn->scn_phys.scn_bookmark.zb_objset = ZB_DESTROYED_OBJSET;
        ASSERT0(scn->scn_suspending);
}

static uint64_t
dsl_scan_count_leaves(vdev_t *vd)
{
        uint64_t i, leaves = 0;
        
        /* we only count leaves that belong to the main pool and are readable */
        if (vd->vdev_islog || vd->vdev_isspare ||
            vd->vdev_isl2cache || !vdev_readable(vd))
                return (0);
        
        if (vd->vdev_ops->vdev_op_leaf)
                return (1);
        
        for (i = 0; i < vd->vdev_children; i++) {
                leaves += dsl_scan_count_leaves(vd->vdev_child[i]);
        }
        
        return (leaves);
}


static void
scan_io_queues_update_zio_stats(dsl_scan_io_queue_t *q, const blkptr_t *bp)
{
        int i;
        uint64_t cur_size = 0;

        for (i = 0; i < BP_GET_NDVAS(bp); i++) {
                cur_size += DVA_GET_ASIZE(&bp->blk_dva[i]);
        }

        q->q_total_zio_size_this_txg += cur_size;
        q->q_zios_this_txg++;
}

static void
scan_io_queues_update_seg_stats(dsl_scan_io_queue_t *q, uint64_t start,
    uint64_t end)
{
        q->q_total_seg_size_this_txg += end - start;
        q->q_segs_this_txg++;
}

static boolean_t
scan_io_queue_check_suspend(dsl_scan_t *scn)
{
        /* See comment in dsl_scan_check_suspend() */
        uint64_t curr_time_ns = gethrtime();
        uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
        uint64_t sync_time_ns = curr_time_ns -
            scn->scn_dp->dp_spa->spa_sync_starttime;
        int dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
        int mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
            zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;
       
        return ((NSEC2MSEC(scan_time_ns) > mintime &&
            (dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent ||
            txg_sync_waiting(scn->scn_dp) ||
            NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
            spa_shutting_down(scn->scn_dp->dp_spa));
}

/*
 * Given a list of scan_io_t's in io_list, this issues the io's out to
 * disk. This consumes the io_list and frees the scan_io_t's. This is
 * called when emptying queues, either when we're up against the memory
 * limit or when we have finished scanning. Returns B_TRUE if we stopped
 * processing the list before we finished. Any zios that were not issued
 * will remain in the io_list.
 */
static boolean_t
scan_io_queue_issue(dsl_scan_io_queue_t *queue, list_t *io_list)
{
        dsl_scan_t *scn = queue->q_scn;
        scan_io_t *sio;
        int64_t bytes_issued = 0;
        boolean_t suspended = B_FALSE;

        while ((sio = list_head(io_list)) != NULL) {
                blkptr_t bp;

                if (scan_io_queue_check_suspend(scn)) {
                        suspended = B_TRUE;
                        break;
                }

                sio2bp(sio, &bp, queue->q_vd->vdev_id);
                bytes_issued += sio->sio_asize;
                scan_exec_io(scn->scn_dp, &bp, sio->sio_flags,
                    &sio->sio_zb, queue);
                (void) list_remove_head(io_list);
                scan_io_queues_update_zio_stats(queue, &bp);
                kmem_free(sio, sizeof (*sio));
        }

        atomic_add_64(&scn->scn_bytes_pending, -bytes_issued);

        return (suspended);
}

/*
 * Given a range_seg_t (extent) and a list, this function passes over a
 * scan queue and gathers up the appropriate ios which fit into that
 * scan seg (starting from lowest LBA). At the end, we remove the segment
 * from the q_exts_by_addr range tree.
 */
static boolean_t
scan_io_queue_gather(dsl_scan_io_queue_t *queue, range_seg_t *rs, list_t *list)
{
        scan_io_t srch_sio, *sio, *next_sio;
        avl_index_t idx;
        uint_t num_sios = 0;
        int64_t bytes_issued = 0;

        ASSERT(rs != NULL);
        ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));

        srch_sio.sio_offset = rs->rs_start;

        /*
         * The exact start of the extent might not contain any matching zios,
         * so if that's the case, examine the next one in the tree.
         */
        sio = avl_find(&queue->q_sios_by_addr, &srch_sio, &idx);
        if (sio == NULL)
                sio = avl_nearest(&queue->q_sios_by_addr, idx, AVL_AFTER);

        while (sio != NULL && sio->sio_offset < rs->rs_end && num_sios <= 32) {
                ASSERT3U(sio->sio_offset, >=, rs->rs_start);
                ASSERT3U(sio->sio_offset + sio->sio_asize, <=, rs->rs_end);

                next_sio = AVL_NEXT(&queue->q_sios_by_addr, sio);
                avl_remove(&queue->q_sios_by_addr, sio);

                bytes_issued += sio->sio_asize;
                num_sios++;
                list_insert_tail(list, sio);
                sio = next_sio;
        }

        /*
         * We limit the number of sios we process at once to 32 to avoid
         * biting off more than we can chew. If we didn't take everything
         * in the segment we update it to reflect the work we were able to
         * complete. Otherwise, we remove it from the range tree entirely.
         */
        if (sio != NULL && sio->sio_offset < rs->rs_end) {
                range_tree_adjust_fill(queue->q_exts_by_addr, rs,
                    -bytes_issued);
                range_tree_resize_segment(queue->q_exts_by_addr, rs,
                    sio->sio_offset, rs->rs_end - sio->sio_offset);

                return (B_TRUE);
        } else {
                range_tree_remove(queue->q_exts_by_addr, rs->rs_start,
                    rs->rs_end - rs->rs_start);
                return (B_FALSE);
        }
}


/*
 * This is called from the queue emptying thread and selects the next
 * extent from which we are to issue io's. The behavior of this function
 * depends on the state of the scan, the current memory consumption and
 * whether or not we are performing a scan shutdown.
 * 1) We select extents in an elevator algorithm (LBA-order) if the scan
 *      needs to perform a checkpoint
 * 2) We select the largest available extent if we are up against the
 *      memory limit.
 * 3) Otherwise we don't select any extents.
 */
static const range_seg_t *
scan_io_queue_fetch_ext(dsl_scan_io_queue_t *queue)
{
        dsl_scan_t *scn = queue->q_scn;

        ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
        ASSERT(scn->scn_is_sorted);

        /* handle tunable overrides */
        if (scn->scn_checkpointing || scn->scn_clearing) {
                if (zfs_scan_issue_strategy == 1) {
                        return (range_tree_first(queue->q_exts_by_addr));
                } else if (zfs_scan_issue_strategy == 2) {
                        return (avl_first(&queue->q_exts_by_size));
                }
        }

        /*
         * During normal clearing, we want to issue our largest segments
         * first, keeping IO as sequential as possible, and leaving the
         * smaller extents for later with the hope that they might eventually
         * grow to larger sequential segments. However, when the scan is
         * checkpointing, no new extents will be added to the sorting queue,
         * so the way we are sorted now is as good as it will ever get.
         * In this case, we instead switch to issuing extents in LBA order.
         */
        if (scn->scn_checkpointing) {
                return (range_tree_first(queue->q_exts_by_addr));
        } else if (scn->scn_clearing) {
                return (avl_first(&queue->q_exts_by_size));
        } else {
                return (NULL);
        }
}

static void
scan_io_queues_run_one(void *arg)
{
        dsl_scan_io_queue_t *queue = arg;
        kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;
        boolean_t suspended = B_FALSE;
        range_seg_t *rs = NULL;
        scan_io_t *sio = NULL;
        list_t sio_list;
        uint64_t bytes_per_leaf = zfs_scan_vdev_limit;
        uint64_t nr_leaves = dsl_scan_count_leaves(queue->q_vd);

        ASSERT(queue->q_scn->scn_is_sorted);

        list_create(&sio_list, sizeof (scan_io_t),
            offsetof(scan_io_t, sio_nodes.sio_list_node));
        mutex_enter(q_lock);

        /* calculate maximum in-flight bytes for this txg (min 1MB) */
        queue->q_maxinflight_bytes =
            MAX(nr_leaves * bytes_per_leaf, 1ULL << 20);

        /* reset per-queue scan statistics for this txg */
        queue->q_total_seg_size_this_txg = 0;
        queue->q_segs_this_txg = 0;
        queue->q_total_zio_size_this_txg = 0;
        queue->q_zios_this_txg = 0;

        /* loop until we have run out of time or sios */
        while ((rs = (range_seg_t*)scan_io_queue_fetch_ext(queue)) != NULL) {
                uint64_t seg_start = 0, seg_end = 0;
                boolean_t more_left = B_TRUE;

                ASSERT(list_is_empty(&sio_list));

                /* loop while we still have sios left to process in this rs */
                while (more_left) {
                        scan_io_t *first_sio, *last_sio;

                        /*
                         * We have selected which extent needs to be
                         * processed next. Gather up the corresponding sios.
                         */
                        more_left = scan_io_queue_gather(queue, rs, &sio_list);
                        ASSERT(!list_is_empty(&sio_list));
                        first_sio = list_head(&sio_list);
                        last_sio = list_tail(&sio_list);

                        seg_end = last_sio->sio_offset + last_sio->sio_asize;
                        if (seg_start == 0)
                                seg_start = first_sio->sio_offset;

                        /*
                         * Issuing sios can take a long time so drop the
                         * queue lock. The sio queue won't be updated by
                         * other threads since we're in syncing context so
                         * we can be sure that our trees will remain exactly
                         * as we left them.
                         */
                        mutex_exit(q_lock);
                        suspended = scan_io_queue_issue(queue, &sio_list);
                        mutex_enter(q_lock);

                        if (suspended)
                                break;
                }
                /* update statistics for debugging purposes */
                scan_io_queues_update_seg_stats(queue, seg_start, seg_end);
                
                if (suspended)
                        break;
        }
                

        /* If we were suspended in the middle of processing,
         * requeue any unfinished sios and exit.
         */
        while ((sio = list_head(&sio_list)) != NULL) {
                list_remove(&sio_list, sio);
                scan_io_queue_insert_impl(queue, sio);
        }

        mutex_exit(q_lock);
        list_destroy(&sio_list);
}

/*
 * Performs an emptying run on all scan queues in the pool. This just
 * punches out one thread per top-level vdev, each of which processes
 * only that vdev's scan queue. We can parallelize the I/O here because
 * we know that each queue's io's only affect its own top-level vdev.
 *
 * This function waits for the queue runs to complete, and must be
 * called from dsl_scan_sync (or in general, syncing context).
 */
static void
scan_io_queues_run(dsl_scan_t *scn)
{
        spa_t *spa = scn->scn_dp->dp_spa;

        ASSERT(scn->scn_is_sorted);
        ASSERT(spa_config_held(spa, SCL_CONFIG, RW_READER));

        if (scn->scn_bytes_pending == 0)
                return;

        if (scn->scn_taskq == NULL) {
                char *tq_name = kmem_zalloc(ZFS_MAX_DATASET_NAME_LEN + 16,
                    KM_SLEEP);
                int nthreads = spa->spa_root_vdev->vdev_children;

                /*
                 * We need to make this taskq *always* execute as many
                 * threads in parallel as we have top-level vdevs and no
                 * less, otherwise strange serialization of the calls to
                 * scan_io_queues_run_one can occur during spa_sync runs
                 * and that significantly impacts performance.
                 */
                (void) snprintf(tq_name, ZFS_MAX_DATASET_NAME_LEN + 16,
                    "dsl_scan_tq_%s", spa->spa_name);
                scn->scn_taskq = taskq_create(tq_name, nthreads, minclsyspri,
                    nthreads, nthreads, TASKQ_PREPOPULATE);
                kmem_free(tq_name, ZFS_MAX_DATASET_NAME_LEN + 16);
        }

        for (uint64_t i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
                vdev_t *vd = spa->spa_root_vdev->vdev_child[i];

                mutex_enter(&vd->vdev_scan_io_queue_lock);
                if (vd->vdev_scan_io_queue != NULL) {
                        VERIFY(taskq_dispatch(scn->scn_taskq,
                            scan_io_queues_run_one, vd->vdev_scan_io_queue,
                            TQ_SLEEP) != TASKQID_INVALID);
                }
                mutex_exit(&vd->vdev_scan_io_queue_lock);
        }

        /*
         * Wait for the queues to finish issuing thir IOs for this run
         * before we return. There may still be IOs in flight at this
         * point.
         */
        taskq_wait(scn->scn_taskq);
}

static boolean_t
dsl_scan_async_block_should_pause(dsl_scan_t *scn)
{
        uint64_t elapsed_nanosecs;

        if (zfs_recover)
                return (B_FALSE);

        if (scn->scn_visited_this_txg >= zfs_async_block_max_blocks)
                return (B_TRUE);

        elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
        return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
            (NSEC2MSEC(elapsed_nanosecs) > scn->scn_async_block_min_time_ms &&
            txg_sync_waiting(scn->scn_dp)) ||
            spa_shutting_down(scn->scn_dp->dp_spa));
}

static int
dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = arg;

        if (!scn->scn_is_bptree ||
            (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_OBJSET)) {
                if (dsl_scan_async_block_should_pause(scn))
                        return (SET_ERROR(ERESTART));
        }

        zio_nowait(zio_free_sync(scn->scn_zio_root, scn->scn_dp->dp_spa,
            dmu_tx_get_txg(tx), bp, BP_GET_PSIZE(bp), 0));
        dsl_dir_diduse_space(tx->tx_pool->dp_free_dir, DD_USED_HEAD,
            -bp_get_dsize_sync(scn->scn_dp->dp_spa, bp),
            -BP_GET_PSIZE(bp), -BP_GET_UCSIZE(bp), tx);
        scn->scn_visited_this_txg++;
        return (0);
}

static void
dsl_scan_update_stats(dsl_scan_t *scn)
{
        spa_t *spa = scn->scn_dp->dp_spa;
        uint64_t i;
        uint64_t seg_size_total = 0, zio_size_total = 0;
        uint64_t seg_count_total = 0, zio_count_total = 0;

        for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
                vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
                dsl_scan_io_queue_t *queue = vd->vdev_scan_io_queue;

                if (queue == NULL)
                        continue;

                seg_size_total += queue->q_total_seg_size_this_txg;
                zio_size_total += queue->q_total_zio_size_this_txg;
                seg_count_total += queue->q_segs_this_txg;
                zio_count_total += queue->q_zios_this_txg;
        }

        if (seg_count_total == 0 || zio_count_total == 0) {
                scn->scn_avg_seg_size_this_txg = 0;
                scn->scn_avg_zio_size_this_txg = 0;
                scn->scn_segs_this_txg = 0;
                scn->scn_zios_this_txg = 0;
                return;
        }

        scn->scn_avg_seg_size_this_txg = seg_size_total / seg_count_total;
        scn->scn_avg_zio_size_this_txg = zio_size_total / zio_count_total;
        scn->scn_segs_this_txg = seg_count_total;
        scn->scn_zios_this_txg = zio_count_total;
}

static int
dsl_scan_obsolete_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = arg;
        const dva_t *dva = &bp->blk_dva[0];

        if (dsl_scan_async_block_should_pause(scn))
                return (SET_ERROR(ERESTART));

        spa_vdev_indirect_mark_obsolete(scn->scn_dp->dp_spa,
            DVA_GET_VDEV(dva), DVA_GET_OFFSET(dva),
            DVA_GET_ASIZE(dva), tx);
        scn->scn_visited_this_txg++;
        return (0);
}

boolean_t
dsl_scan_active(dsl_scan_t *scn)
{
        spa_t *spa = scn->scn_dp->dp_spa;
        uint64_t used = 0, comp, uncomp;

        if (spa->spa_load_state != SPA_LOAD_NONE)
                return (B_FALSE);
        if (spa_shutting_down(spa))
                return (B_FALSE);
        if ((dsl_scan_is_running(scn) && !dsl_scan_is_paused_scrub(scn)) ||
            (scn->scn_async_destroying && !scn->scn_async_stalled))
                return (B_TRUE);

        if (spa_version(scn->scn_dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
                (void) bpobj_space(&scn->scn_dp->dp_free_bpobj,
                    &used, &comp, &uncomp);
        }
        return (used != 0);
}

static boolean_t
dsl_scan_need_resilver(spa_t *spa, const dva_t *dva, size_t psize,
    uint64_t phys_birth)
{
        vdev_t *vd;

        vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));

        if (vd->vdev_ops == &vdev_indirect_ops) {
                /*
                 * The indirect vdev can point to multiple
                 * vdevs.  For simplicity, always create
                 * the resilver zio_t. zio_vdev_io_start()
                 * will bypass the child resilver i/o's if
                 * they are on vdevs that don't have DTL's.
                 */
                return (B_TRUE);
        }

        if (DVA_GET_GANG(dva)) {
                /*
                 * Gang members may be spread across multiple
                 * vdevs, so the best estimate we have is the
                 * scrub range, which has already been checked.
                 * XXX -- it would be better to change our
                 * allocation policy to ensure that all
                 * gang members reside on the same vdev.
                 */
                return (B_TRUE);
        }

        /*
         * Check if the txg falls within the range which must be
         * resilvered.  DVAs outside this range can always be skipped.
         */
        if (!vdev_dtl_contains(vd, DTL_PARTIAL, phys_birth, 1))
                return (B_FALSE);

        /*
         * Check if the top-level vdev must resilver this offset.
         * When the offset does not intersect with a dirty leaf DTL
         * then it may be possible to skip the resilver IO.  The psize
         * is provided instead of asize to simplify the check for RAIDZ.
         */
        if (!vdev_dtl_need_resilver(vd, DVA_GET_OFFSET(dva), psize))
                return (B_FALSE);

        return (B_TRUE);
}

static int
dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
{
        int err = 0;
        dsl_scan_t *scn = dp->dp_scan;
        spa_t *spa = dp->dp_spa;

        if (spa_suspend_async_destroy(spa))
                return (0);

        if (zfs_free_bpobj_enabled &&
            spa_version(spa) >= SPA_VERSION_DEADLISTS) {
                scn->scn_is_bptree = B_FALSE;
                scn->scn_async_block_min_time_ms = zfs_free_min_time_ms;
                scn->scn_zio_root = zio_root(spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bpobj_iterate(&dp->dp_free_bpobj,
                    dsl_scan_free_block_cb, scn, tx);
                VERIFY0(zio_wait(scn->scn_zio_root));
                scn->scn_zio_root = NULL;

                if (err != 0 && err != ERESTART)
                        zfs_panic_recover("error %u from bpobj_iterate()", err);
        }

        if (err == 0 && spa_feature_is_active(spa, SPA_FEATURE_ASYNC_DESTROY)) {
                ASSERT(scn->scn_async_destroying);
                scn->scn_is_bptree = B_TRUE;
                scn->scn_zio_root = zio_root(spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bptree_iterate(dp->dp_meta_objset,
                    dp->dp_bptree_obj, B_TRUE, dsl_scan_free_block_cb, scn, tx);
                VERIFY0(zio_wait(scn->scn_zio_root));
                scn->scn_zio_root = NULL;

                if (err == EIO || err == ECKSUM) {
                        err = 0;
                } else if (err != 0 && err != ERESTART) {
                        zfs_panic_recover("error %u from "
                            "traverse_dataset_destroyed()", err);
                }

                if (bptree_is_empty(dp->dp_meta_objset, dp->dp_bptree_obj)) {
                        /* finished; deactivate async destroy feature */
                        spa_feature_decr(spa, SPA_FEATURE_ASYNC_DESTROY, tx);
                        ASSERT(!spa_feature_is_active(spa,
                            SPA_FEATURE_ASYNC_DESTROY));
                        VERIFY0(zap_remove(dp->dp_meta_objset,
                            DMU_POOL_DIRECTORY_OBJECT,
                            DMU_POOL_BPTREE_OBJ, tx));
                        VERIFY0(bptree_free(dp->dp_meta_objset,
                            dp->dp_bptree_obj, tx));
                        dp->dp_bptree_obj = 0;
                        scn->scn_async_destroying = B_FALSE;
                        scn->scn_async_stalled = B_FALSE;
                } else {
                        /*
                         * If we didn't make progress, mark the async
                         * destroy as stalled, so that we will not initiate
                         * a spa_sync() on its behalf.  Note that we only
                         * check this if we are not finished, because if the
                         * bptree had no blocks for us to visit, we can
                         * finish without "making progress".
                         */
                        scn->scn_async_stalled =
                            (scn->scn_visited_this_txg == 0);
                }
        }
        if (scn->scn_visited_this_txg) {
                zfs_dbgmsg("freed %llu blocks in %llums from "
                    "free_bpobj/bptree txg %llu; err=%d",
                    (longlong_t)scn->scn_visited_this_txg,
                    (longlong_t)
                    NSEC2MSEC(gethrtime() - scn->scn_sync_start_time),
                    (longlong_t)tx->tx_txg, err);
                scn->scn_visited_this_txg = 0;

                /*
                 * Write out changes to the DDT that may be required as a
                 * result of the blocks freed.  This ensures that the DDT
                 * is clean when a scrub/resilver runs.
                 */
                ddt_sync(spa, tx->tx_txg);
        }
        if (err != 0)
                return (err);
        if (dp->dp_free_dir != NULL && !scn->scn_async_destroying &&
            zfs_free_leak_on_eio &&
            (dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes != 0 ||
            dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes != 0 ||
            dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes != 0)) {
                /*
                 * We have finished background destroying, but there is still
                 * some space left in the dp_free_dir. Transfer this leaked
                 * space to the dp_leak_dir.
                 */
                if (dp->dp_leak_dir == NULL) {
                        rrw_enter(&dp->dp_config_rwlock, RW_WRITER, FTAG);
                        (void) dsl_dir_create_sync(dp, dp->dp_root_dir,
                            LEAK_DIR_NAME, tx);
                        VERIFY0(dsl_pool_open_special_dir(dp,
                            LEAK_DIR_NAME, &dp->dp_leak_dir));
                        rrw_exit(&dp->dp_config_rwlock, FTAG);
                }
                dsl_dir_diduse_space(dp->dp_leak_dir, DD_USED_HEAD,
                    dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes,
                    dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes,
                    dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes, tx);
                dsl_dir_diduse_space(dp->dp_free_dir, DD_USED_HEAD,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes, tx);
        }

        if (dp->dp_free_dir != NULL && !scn->scn_async_destroying) {
                /* finished; verify that space accounting went to zero */
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes);
        }

        EQUIV(bpobj_is_open(&dp->dp_obsolete_bpobj),
            0 == zap_contains(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_OBSOLETE_BPOBJ));
        if (err == 0 && bpobj_is_open(&dp->dp_obsolete_bpobj)) {
                ASSERT(spa_feature_is_active(dp->dp_spa,
                    SPA_FEATURE_OBSOLETE_COUNTS));

                scn->scn_is_bptree = B_FALSE;
                scn->scn_async_block_min_time_ms = zfs_obsolete_min_time_ms;
                err = bpobj_iterate(&dp->dp_obsolete_bpobj,
                    dsl_scan_obsolete_block_cb, scn, tx);
                if (err != 0 && err != ERESTART)
                        zfs_panic_recover("error %u from bpobj_iterate()", err);

                if (bpobj_is_empty(&dp->dp_obsolete_bpobj))
                        dsl_pool_destroy_obsolete_bpobj(dp, tx);
        }

        return (0);
}

/*
 * This is the primary entry point for scans that is called from syncing
 * context. Scans must happen entirely during syncing context so that we
 * cna guarantee that blocks we are currently scanning will not change out
 * from under us. While a scan is active, this funciton controls how quickly
 * transaction groups proceed, instead of the normal handling provided by
 * txg_sync_thread().
 */
void
dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
{
        dsl_scan_t *scn = dp->dp_scan;
        spa_t *spa = dp->dp_spa;
        int err = 0;
        state_sync_type_t sync_type = SYNC_OPTIONAL;

        /*
         * Check for scn_restart_txg before checking spa_load_state, so
         * that we can restart an old-style scan while the pool is being
         * imported (see dsl_scan_init).
         */
        if (dsl_scan_restarting(scn, tx)) {
                pool_scan_func_t func = POOL_SCAN_SCRUB;
                dsl_scan_done(scn, B_FALSE, tx);
                if (vdev_resilver_needed(spa->spa_root_vdev, NULL, NULL))
                        func = POOL_SCAN_RESILVER;
                zfs_dbgmsg("restarting scan func=%u txg=%llu",
                    func, (longlong_t)tx->tx_txg);
                dsl_scan_setup_sync(&func, tx);
        }

        /*
         * Only process scans in sync pass 1.
         */
        if (spa_sync_pass(dp->dp_spa) > 1)
                return;

        /*
         * If the spa is shutting down, then stop scanning. This will
         * ensure that the scan does not dirty any new data during the
         * shutdown phase.
         */
        if (spa_shutting_down(spa))
                return;

        /*
         * If the scan is inactive due to a stalled async destroy, try again.
         */
        if (!scn->scn_async_stalled && !dsl_scan_active(scn))
                return;

        /* reset scan statistics */
        scn->scn_visited_this_txg = 0;
        scn->scn_holes_this_txg = 0;
        scn->scn_lt_min_this_txg = 0;
        scn->scn_gt_max_this_txg = 0;
        scn->scn_ddt_contained_this_txg = 0;
        scn->scn_objsets_visited_this_txg = 0;
        scn->scn_avg_seg_size_this_txg = 0;
        scn->scn_segs_this_txg = 0;
        scn->scn_avg_zio_size_this_txg = 0;
        scn->scn_zios_this_txg = 0;
        scn->scn_suspending = B_FALSE;
        scn->scn_sync_start_time = gethrtime();
        spa->spa_scrub_active = B_TRUE;

        /*
         * First process the async destroys.  If we pause, don't do
         * any scrubbing or resilvering.  This ensures that there are no
         * async destroys while we are scanning, so the scan code doesn't
         * have to worry about traversing it.  It is also faster to free the
         * blocks than to scrub them.
         */
        err = dsl_process_async_destroys(dp, tx);
        if (err != 0)
                return;

        if (!dsl_scan_is_running(scn) || dsl_scan_is_paused_scrub(scn))
                return;

        /*
         * Wait a few txgs after importing to begin scanning so that
         * we can get the pool imported quickly.
         */
        if (spa->spa_syncing_txg < spa->spa_first_txg + SCAN_IMPORT_WAIT_TXGS)
                return;

        /*
         * It is possible to switch from unsorted to sorted at any time,
         * but afterwards the scan will remain sorted unless reloaded from
         * a checkpoint after a reboot.
         */
        if (!zfs_scan_legacy) {
                scn->scn_is_sorted = B_TRUE;
                if (scn->scn_last_checkpoint == 0)
                        scn->scn_last_checkpoint = ddi_get_lbolt();
        }

        /*
         * For sorted scans, determine what kind of work we will be doing
         * this txg based on our memory limitations and whether or not we
         * need to perform a checkpoint.
         */
        if (scn->scn_is_sorted) {
                /*
                 * If we are over our checkpoint interval, set scn_clearing
                 * so that we can begin checkpointing immediately. The
                 * checkpoint allows us to save a consisent bookmark
                 * representing how much data we have scrubbed so far.
                 * Otherwise, use the memory limit to determine if we should
                 * scan for metadata or start issue scrub IOs. We accumulate
                 * metadata until we hit our hard memory limit at which point
                 * we issue scrub IOs until we are at our soft memory limit.
                 */
                if (scn->scn_checkpointing ||
                    ddi_get_lbolt() - scn->scn_last_checkpoint >
                    SEC_TO_TICK(zfs_scan_checkpoint_intval)) {
                        if (!scn->scn_checkpointing)
                                zfs_dbgmsg("begin scan checkpoint");

                        scn->scn_checkpointing = B_TRUE;
                        scn->scn_clearing = B_TRUE;
                } else {
                        boolean_t should_clear = dsl_scan_should_clear(scn);
                        if (should_clear && !scn->scn_clearing) {
                                zfs_dbgmsg("begin scan clearing");
                                scn->scn_clearing = B_TRUE;
                        } else if (!should_clear && scn->scn_clearing) {
                                zfs_dbgmsg("finish scan clearing");
                                scn->scn_clearing = B_FALSE;
                        }
                }
        } else {
                ASSERT0(scn->scn_checkpointing);
                ASSERT0(scn->scn_clearing);
        }

        if (!scn->scn_clearing && scn->scn_done_txg == 0) {
                /* Need to scan metadata for more blocks to scrub */
                dsl_scan_phys_t *scnp = &scn->scn_phys;
                taskqid_t prefetch_tqid;
                uint64_t bytes_per_leaf = zfs_scan_vdev_limit;
                uint64_t nr_leaves = dsl_scan_count_leaves(spa->spa_root_vdev);

                /*
                 * Calculate the max number of in-flight bytes for pool-wide
                 * scanning operations (minimum 1MB). Limits for the issuing
                 * phase are done per top-level vdev and are handled separately.
                 */
                scn->scn_maxinflight_bytes =
                    MAX(nr_leaves * bytes_per_leaf, 1ULL << 20);

                if (scnp->scn_ddt_bookmark.ddb_class <=
                    scnp->scn_ddt_class_max) {
                        ASSERT(ZB_IS_ZERO(&scnp->scn_bookmark));
                        zfs_dbgmsg("doing scan sync txg %llu; "
                            "ddt bm=%llu/%llu/%llu/%llx",
                            (longlong_t)tx->tx_txg,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
                            (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
                } else {
                        zfs_dbgmsg("doing scan sync txg %llu; "
                            "bm=%llu/%llu/%llu/%llu",
                            (longlong_t)tx->tx_txg,
                            (longlong_t)scnp->scn_bookmark.zb_objset,
                            (longlong_t)scnp->scn_bookmark.zb_object,
                            (longlong_t)scnp->scn_bookmark.zb_level,
                            (longlong_t)scnp->scn_bookmark.zb_blkid);
                }

                scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
                    NULL, ZIO_FLAG_CANFAIL);

                scn->scn_prefetch_stop = B_FALSE;
                prefetch_tqid = taskq_dispatch(dp->dp_sync_taskq,
                    dsl_scan_prefetch_thread, scn, TQ_SLEEP);
                ASSERT(prefetch_tqid != TASKQID_INVALID);

                dsl_pool_config_enter(dp, FTAG);
                dsl_scan_visit(scn, tx);
                dsl_pool_config_exit(dp, FTAG);

                mutex_enter(&dp->dp_spa->spa_scrub_lock);
                scn->scn_prefetch_stop = B_TRUE;
                cv_broadcast(&spa->spa_scrub_io_cv);
                mutex_exit(&dp->dp_spa->spa_scrub_lock);

                taskq_wait_id(dp->dp_sync_taskq, prefetch_tqid);
                (void) zio_wait(scn->scn_zio_root);
                scn->scn_zio_root = NULL;

                zfs_dbgmsg("scan visited %llu blocks in %llums "
                    "(%llu os's, %llu holes, %llu < mintxg, "
                    "%llu in ddt, %llu > maxtxg)",
                    (longlong_t)scn->scn_visited_this_txg,
                    (longlong_t)NSEC2MSEC(gethrtime() -
                    scn->scn_sync_start_time),
                    (longlong_t)scn->scn_objsets_visited_this_txg,
                    (longlong_t)scn->scn_holes_this_txg,
                    (longlong_t)scn->scn_lt_min_this_txg,
                    (longlong_t)scn->scn_ddt_contained_this_txg,
                    (longlong_t)scn->scn_gt_max_this_txg);

                if (!scn->scn_suspending) {
                        ASSERT0(avl_numnodes(&scn->scn_queue));
                        scn->scn_done_txg = tx->tx_txg + 1;
                        if (scn->scn_is_sorted) {
                                scn->scn_checkpointing = B_TRUE;
                                scn->scn_clearing = B_TRUE;
                        }
                        zfs_dbgmsg("scan complete txg %llu",
                                   (longlong_t)tx->tx_txg);
                }
        } else if (scn->scn_is_sorted && scn->scn_bytes_pending != 0) {
                /* need to issue scrubbing IOs from per-vdev queues */
                scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
                    NULL, ZIO_FLAG_CANFAIL);
                scan_io_queues_run(scn);
                (void) zio_wait(scn->scn_zio_root);
                scn->scn_zio_root = NULL;

                /* calculate and dprintf the current memory usage */
                (void) dsl_scan_should_clear(scn);
                dsl_scan_update_stats(scn);

                zfs_dbgmsg("scrubbed %llu blocks (%llu segs) in %llums "
                    "(avg_block_size = %llu, avg_seg_size = %llu)",
                    (longlong_t)scn->scn_zios_this_txg,
                    (longlong_t)scn->scn_segs_this_txg,
                    (longlong_t)NSEC2MSEC(gethrtime() -
                    scn->scn_sync_start_time),
                    (longlong_t)scn->scn_avg_zio_size_this_txg,
                    (longlong_t)scn->scn_avg_seg_size_this_txg);
        } else if (scn->scn_done_txg != 0 && scn->scn_done_txg <= tx->tx_txg) {
                /* Finished with everything. Mark the scrub as complete */
                zfs_dbgmsg("scan issuing complete txg %llu",
                    (longlong_t)tx->tx_txg);
                ASSERT3U(scn->scn_done_txg, !=, 0);
                ASSERT0(spa->spa_scrub_inflight);
                ASSERT0(scn->scn_bytes_pending);
                dsl_scan_done(scn, B_TRUE, tx);
                sync_type = SYNC_MANDATORY;
        }

        dsl_scan_sync_state(scn, tx, sync_type);
}

static void
count_block(dsl_scan_t *scn, zfs_all_blkstats_t *zab, const blkptr_t *bp)
{
        int i;

        /* update the spa's stats on how many bytes we have issued */
        for (i = 0; i < BP_GET_NDVAS(bp); i++) {
                atomic_add_64(&scn->scn_dp->dp_spa->spa_scan_pass_issued,
                    DVA_GET_ASIZE(&bp->blk_dva[i]));
        }

        /*
         * If we resume after a reboot, zab will be NULL; don't record
         * incomplete stats in that case.
         */
        if (zab == NULL)
                return;

        mutex_enter(&zab->zab_lock);

        for (i = 0; i < 4; i++) {
                int l = (i < 2) ? BP_GET_LEVEL(bp) : DN_MAX_LEVELS;
                int t = (i & 1) ? BP_GET_TYPE(bp) : DMU_OT_TOTAL;
                if (t & DMU_OT_NEWTYPE)
                        t = DMU_OT_OTHER;
                zfs_blkstat_t *zb = &zab->zab_type[l][t];
                int equal;

                zb->zb_count++;
                zb->zb_asize += BP_GET_ASIZE(bp);
                zb->zb_lsize += BP_GET_LSIZE(bp);
                zb->zb_psize += BP_GET_PSIZE(bp);
                zb->zb_gangs += BP_COUNT_GANG(bp);

                switch (BP_GET_NDVAS(bp)) {
                case 2:
                        if (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[1]))
                                zb->zb_ditto_2_of_2_samevdev++;
                        break;
                case 3:
                        equal = (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[1])) +
                            (DVA_GET_VDEV(&bp->blk_dva[0]) ==
                            DVA_GET_VDEV(&bp->blk_dva[2])) +
                            (DVA_GET_VDEV(&bp->blk_dva[1]) ==
                            DVA_GET_VDEV(&bp->blk_dva[2]));
                        if (equal == 1)
                                zb->zb_ditto_2_of_3_samevdev++;
                        else if (equal == 3)
                                zb->zb_ditto_3_of_3_samevdev++;
                        break;
                }
        }

        mutex_exit(&zab->zab_lock);
}

static void
scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue, scan_io_t *sio)
{
        avl_index_t idx;
        int64_t asize = sio->sio_asize;
        dsl_scan_t *scn = queue->q_scn;

        ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));

        if (avl_find(&queue->q_sios_by_addr, sio, &idx) != NULL) {
                /* block is already scheduled for reading */
                atomic_add_64(&scn->scn_bytes_pending, -asize);
                kmem_free(sio, sizeof (*sio));
                return;
        }
        avl_insert(&queue->q_sios_by_addr, sio, idx);
        range_tree_add(queue->q_exts_by_addr, sio->sio_offset, asize);
}

/*
 * Given all the info we got from our metadata scanning process, we
 * construct a scan_io_t and insert it into the scan sorting queue. The
 * I/O must already be suitable for us to process. This is controlled
 * by dsl_scan_enqueue().
 */
static void
scan_io_queue_insert(dsl_scan_io_queue_t *queue, const blkptr_t *bp, int dva_i,
    int zio_flags, const zbookmark_phys_t *zb)
{
        dsl_scan_t *scn = queue->q_scn;
        scan_io_t *sio = kmem_zalloc(sizeof (*sio), KM_SLEEP);

        ASSERT0(BP_IS_GANG(bp));
        ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));

        bp2sio(bp, sio, dva_i);
        sio->sio_flags = zio_flags;
        sio->sio_zb = *zb;

        /*
         * Increment the bytes pending counter now so that we can't
         * get an integer underflow in case the worker processes the
         * zio before we get to incrementing this counter.
         */
        atomic_add_64(&scn->scn_bytes_pending, sio->sio_asize);

        scan_io_queue_insert_impl(queue, sio);
}

/*
 * Given a set of I/O parameters as discovered by the metadata traversal
 * process, attempts to place the I/O into the sorted queues (if allowed),
 * or immediately executes the I/O.
 */
static void
dsl_scan_enqueue(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
    const zbookmark_phys_t *zb)
{
        spa_t *spa = dp->dp_spa;

        ASSERT(!BP_IS_EMBEDDED(bp));

        /*
         * Gang blocks are hard to issue sequentially, so we just issue them
         * here immediately instead of queuing them.
         */
        if (!dp->dp_scan->scn_is_sorted || BP_IS_GANG(bp)) {
                scan_exec_io(dp, bp, zio_flags, zb, NULL);
                return;
        }
        for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
                dva_t dva;
                vdev_t *vdev;

                dva = bp->blk_dva[i];
                vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&dva));
                ASSERT(vdev != NULL);

                mutex_enter(&vdev->vdev_scan_io_queue_lock);
                if (vdev->vdev_scan_io_queue == NULL)
                        vdev->vdev_scan_io_queue = scan_io_queue_create(vdev);
                ASSERT(dp->dp_scan != NULL);
                scan_io_queue_insert(vdev->vdev_scan_io_queue, bp,
                    i, zio_flags, zb);
                mutex_exit(&vdev->vdev_scan_io_queue_lock);
        }
}

static int
dsl_scan_scrub_cb(dsl_pool_t *dp,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
        dsl_scan_t *scn = dp->dp_scan;
        spa_t *spa = dp->dp_spa;
        uint64_t phys_birth = BP_PHYSICAL_BIRTH(bp);
        size_t psize = BP_GET_PSIZE(bp);
        boolean_t needs_io;
        int zio_flags = ZIO_FLAG_SCAN_THREAD | ZIO_FLAG_RAW | ZIO_FLAG_CANFAIL;
        int d;

        if (phys_birth <= scn->scn_phys.scn_min_txg ||
            phys_birth >= scn->scn_phys.scn_max_txg) {
                count_block(scn, dp->dp_blkstats, bp);
                return (0);
        }

        /* Embedded BP's have phys_birth==0, so we reject them above. */
        ASSERT(!BP_IS_EMBEDDED(bp));

        ASSERT(DSL_SCAN_IS_SCRUB_RESILVER(scn));
        if (scn->scn_phys.scn_func == POOL_SCAN_SCRUB) {
                zio_flags |= ZIO_FLAG_SCRUB;
                needs_io = B_TRUE;
        } else {
                ASSERT3U(scn->scn_phys.scn_func, ==, POOL_SCAN_RESILVER);
                zio_flags |= ZIO_FLAG_RESILVER;
                needs_io = B_FALSE;
        }

        /* If it's an intent log block, failure is expected. */
        if (zb->zb_level == ZB_ZIL_LEVEL)
                zio_flags |= ZIO_FLAG_SPECULATIVE;

        for (d = 0; d < BP_GET_NDVAS(bp); d++) {
                const dva_t *dva = &bp->blk_dva[d];

                /*
                 * Keep track of how much data we've examined so that
                 * zpool(1M) status can make useful progress reports.
                 */
                scn->scn_phys.scn_examined += DVA_GET_ASIZE(dva);
                spa->spa_scan_pass_exam += DVA_GET_ASIZE(dva);

                /* if it's a resilver, this may not be in the target range */
                if (!needs_io)
                        needs_io = dsl_scan_need_resilver(spa, dva, psize,
                            phys_birth);
        }

        if (needs_io && !zfs_no_scrub_io) {
                dsl_scan_enqueue(dp, bp, zio_flags, zb);
        } else {
                count_block(scn, dp->dp_blkstats, bp);
        }

        /* do not relocate this block */
        return (0);
}

static void
dsl_scan_scrub_done(zio_t *zio)
{
        spa_t *spa = zio->io_spa;
        blkptr_t *bp = zio->io_bp;
        dsl_scan_io_queue_t *queue = zio->io_private;

        abd_free(zio->io_abd);

        if (queue == NULL) {
                mutex_enter(&spa->spa_scrub_lock);
                ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
                spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
                cv_broadcast(&spa->spa_scrub_io_cv);
                mutex_exit(&spa->spa_scrub_lock);
        } else {
                mutex_enter(&queue->q_vd->vdev_scan_io_queue_lock);
                ASSERT3U(queue->q_inflight_bytes, >=, BP_GET_PSIZE(bp));
                queue->q_inflight_bytes -= BP_GET_PSIZE(bp);
                cv_broadcast(&queue->q_zio_cv);
                mutex_exit(&queue->q_vd->vdev_scan_io_queue_lock);
        }

        if (zio->io_error && (zio->io_error != ECKSUM ||
            !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
                atomic_inc_64(&spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors);
        }
}

/*
 * Given a scanning zio's information, executes the zio. The zio need
 * not necessarily be only sortable, this function simply executes the
 * zio, no matter what it is. The optional queue argument allows the
 * caller to specify that they want per top level vdev IO rate limiting
 * instead of the legacy global limiting.
 */
static void
scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue)
{
        spa_t *spa = dp->dp_spa;
        dsl_scan_t *scn = dp->dp_scan;
        size_t size = BP_GET_PSIZE(bp);
        abd_t *data = abd_alloc_for_io(size, B_FALSE);
        unsigned int scan_delay = 0;

        if (queue == NULL) {
                mutex_enter(&spa->spa_scrub_lock);
                while (spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)
                        cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
                spa->spa_scrub_inflight += BP_GET_PSIZE(bp);
                mutex_exit(&spa->spa_scrub_lock);
        } else {
                kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;

                mutex_enter(q_lock);
                while (queue->q_inflight_bytes >= queue->q_maxinflight_bytes)
                        cv_wait(&queue->q_zio_cv, q_lock);
                queue->q_inflight_bytes += BP_GET_PSIZE(bp);
                mutex_exit(q_lock);
        }

        if (zio_flags & ZIO_FLAG_RESILVER)
                scan_delay = zfs_resilver_delay;
        else {
                ASSERT(zio_flags & ZIO_FLAG_SCRUB);
                scan_delay = zfs_scrub_delay;
        }

        if (scan_delay && (ddi_get_lbolt64() - spa->spa_last_io <= zfs_scan_idle))
                delay(MAX((int)scan_delay, 0));
        
        count_block(dp->dp_scan, dp->dp_blkstats, bp);
        zio_nowait(zio_read(dp->dp_scan->scn_zio_root, spa, bp, data, size,
            dsl_scan_scrub_done, queue, ZIO_PRIORITY_SCRUB, zio_flags, zb));
}

/*
 * This is the primary extent sorting algorithm. We balance two parameters:
 * 1) how many bytes of I/O are in an extent
 * 2) how well the extent is filled with I/O (as a fraction of its total size)
 * Since we allow extents to have gaps between their constituent I/Os, it's
 * possible to have a fairly large extent that contains the same amount of
 * I/O bytes than a much smaller extent, which just packs the I/O more tightly.
 * The algorithm sorts based on a score calculated from the extent's size,
 * the relative fill volume (in %) and a "fill weight" parameter that controls
 * the split between whether we prefer larger extents or more well populated
 * extents:
 *
 * SCORE = FILL_IN_BYTES + (FILL_IN_PERCENT * FILL_IN_BYTES * FILL_WEIGHT)
 *
 * Example:
 * 1) assume extsz = 64 MiB
 * 2) assume fill = 32 MiB (extent is half full)
 * 3) assume fill_weight = 3
 * 4)   SCORE = 32M + (((32M * 100) / 64M) * 3 * 32M) / 100
 *      SCORE = 32M + (50 * 3 * 32M) / 100
 *      SCORE = 32M + (4800M / 100)
 *      SCORE = 32M + 48M
 *               ^     ^
 *               |     +--- final total relative fill-based score
 *               +--------- final total fill-based score
 *      SCORE = 80M
 *
 * As can be seen, at fill_ratio=3, the algorithm is slightly biased towards
 * extents that are more completely filled (in a 3:2 ratio) vs just larger.
 * Note that as an optimization, we replace multiplication and division by
 * 100 with bitshifting by 7 (which effecitvely multiplies and divides by 128).
 */
static int
ext_size_compare(const void *x, const void *y)
{
        const range_seg_t *rsa = x, *rsb = y;
        uint64_t sa = rsa->rs_end - rsa->rs_start,
            sb = rsb->rs_end - rsb->rs_start;
        uint64_t score_a, score_b;

        score_a = rsa->rs_fill + ((((rsa->rs_fill << 7) / sa) *
            fill_weight * rsa->rs_fill) >> 7);
        score_b = rsb->rs_fill + ((((rsb->rs_fill << 7) / sb) *
            fill_weight * rsb->rs_fill) >> 7);

        if (score_a > score_b)
                return (-1);
        if (score_a == score_b) {
                if (rsa->rs_start < rsb->rs_start)
                        return (-1);
                if (rsa->rs_start == rsb->rs_start)
                        return (0);
                return (1);
        }
        return (1);
}

/*
 * Comparator for the q_sios_by_addr tree. Sorting is simply performed
 * based on LBA-order (from lowest to highest).
 */
static int
io_addr_compare(const void *x, const void *y)
{
        const scan_io_t *a = x, *b = y;

        if (a->sio_offset < b->sio_offset)
                return (-1);
        if (a->sio_offset == b->sio_offset)
                return (0);
        return (1);
}

/* IO queues are created on demand when they are needed. */
static dsl_scan_io_queue_t *
scan_io_queue_create(vdev_t *vd)
{
        dsl_scan_t *scn = vd->vdev_spa->spa_dsl_pool->dp_scan;
        dsl_scan_io_queue_t *q = kmem_zalloc(sizeof (*q), KM_SLEEP);

        q->q_scn = scn;
        q->q_vd = vd;
        cv_init(&q->q_zio_cv, NULL, CV_DEFAULT, NULL);
        q->q_exts_by_addr = range_tree_create_impl(&rt_avl_ops,
            &q->q_exts_by_size, ext_size_compare, zfs_scan_max_ext_gap);
        avl_create(&q->q_sios_by_addr, io_addr_compare,
            sizeof (scan_io_t), offsetof(scan_io_t, sio_nodes.sio_addr_node));

        return (q);
}

/*
 * Destroys a scan queue and all segments and scan_io_t's contained in it.
 * No further execution of I/O occurs, anything pending in the queue is
 * simply freed without being executed.
 */
void
dsl_scan_io_queue_destroy(dsl_scan_io_queue_t *queue)
{
        dsl_scan_t *scn = queue->q_scn;
        scan_io_t *sio;
        void *cookie = NULL;
        int64_t bytes_dequeued = 0;

        ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));

        while ((sio = avl_destroy_nodes(&queue->q_sios_by_addr, &cookie)) !=
            NULL) {
                ASSERT(range_tree_contains(queue->q_exts_by_addr,
                    sio->sio_offset, sio->sio_asize));
                bytes_dequeued += sio->sio_asize;
                kmem_free(sio, sizeof (*sio));
        }

        atomic_add_64(&scn->scn_bytes_pending, -bytes_dequeued);
        range_tree_vacate(queue->q_exts_by_addr, NULL, queue);
        range_tree_destroy(queue->q_exts_by_addr);
        avl_destroy(&queue->q_sios_by_addr);
        cv_destroy(&queue->q_zio_cv);

        kmem_free(queue, sizeof (*queue));
}

/*
 * Properly transfers a dsl_scan_queue_t from `svd' to `tvd'. This is
 * called on behalf of vdev_top_transfer when creating or destroying
 * a mirror vdev due to zpool attach/detach.
 */
void
dsl_scan_io_queue_vdev_xfer(vdev_t *svd, vdev_t *tvd)
{
        mutex_enter(&svd->vdev_scan_io_queue_lock);
        mutex_enter(&tvd->vdev_scan_io_queue_lock);

        VERIFY3P(tvd->vdev_scan_io_queue, ==, NULL);
        tvd->vdev_scan_io_queue = svd->vdev_scan_io_queue;
        svd->vdev_scan_io_queue = NULL;
        if (tvd->vdev_scan_io_queue != NULL)
                tvd->vdev_scan_io_queue->q_vd = tvd;

        mutex_exit(&tvd->vdev_scan_io_queue_lock);
        mutex_exit(&svd->vdev_scan_io_queue_lock);
}

static void
scan_io_queues_destroy(dsl_scan_t *scn)
{
        vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;

        for (uint64_t i = 0; i < rvd->vdev_children; i++) {
                vdev_t *tvd = rvd->vdev_child[i];

                mutex_enter(&tvd->vdev_scan_io_queue_lock);
                if (tvd->vdev_scan_io_queue != NULL)
                        dsl_scan_io_queue_destroy(tvd->vdev_scan_io_queue);
                tvd->vdev_scan_io_queue = NULL;
                mutex_exit(&tvd->vdev_scan_io_queue_lock);
        }
}

static void
dsl_scan_freed_dva(spa_t *spa, const blkptr_t *bp, int dva_i)
{
        dsl_pool_t *dp = spa->spa_dsl_pool;
        dsl_scan_t *scn = dp->dp_scan;
        vdev_t *vdev;
        kmutex_t *q_lock;
        dsl_scan_io_queue_t *queue;
        scan_io_t srch, *sio;
        avl_index_t idx;
        uint64_t start, size;

        vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&bp->blk_dva[dva_i]));
        ASSERT(vdev != NULL);
        q_lock = &vdev->vdev_scan_io_queue_lock;
        queue = vdev->vdev_scan_io_queue;

        mutex_enter(q_lock);
        if (queue == NULL) {
                mutex_exit(q_lock);
                return;
        }

        bp2sio(bp, &srch, dva_i);
        start = srch.sio_offset;
        size = srch.sio_asize;

        /*
         * We can find the zio in two states:
         * 1) Cold, just sitting in the queue of zio's to be issued at
         *      some point in the future. In this case, all we do is
         *      remove the zio from the q_sios_by_addr tree, decrement
         *      its data volume from the containing range_seg_t and
         *      resort the q_exts_by_size tree to reflect that the
         *      range_seg_t has lost some of its 'fill'. We don't shorten
         *      the range_seg_t - this is usually rare enough not to be
         *      worth the extra hassle of trying keep track of precise
         *      extent boundaries.
         * 2) Hot, where the zio is currently in-flight in
         *      dsl_scan_issue_ios. In this case, we can't simply
         *      reach in and stop the in-flight zio's, so we instead
         *      block the caller. Eventually, dsl_scan_issue_ios will
         *      be done with issuing the zio's it gathered and will
         *      signal us.
         */
        sio = avl_find(&queue->q_sios_by_addr, &srch, &idx);
        if (sio != NULL) {
                int64_t asize = sio->sio_asize;
                blkptr_t tmpbp;

                /* Got it while it was cold in the queue */
                ASSERT3U(start, ==, sio->sio_offset);
                ASSERT3U(size, ==, asize);
                avl_remove(&queue->q_sios_by_addr, sio);

                ASSERT(range_tree_contains(queue->q_exts_by_addr, start, size));
                range_tree_remove_fill(queue->q_exts_by_addr, start, size);

                /*
                 * We only update scn_bytes_pending in the cold path,
                 * otherwise it will already have been accounted for as
                 * part of the zio's execution.
                 */
                atomic_add_64(&scn->scn_bytes_pending, -asize);

                /* count the block as though we issued it */
                sio2bp(sio, &tmpbp, dva_i);
                count_block(scn, dp->dp_blkstats, &tmpbp);

                kmem_free(sio, sizeof (*sio));
        }
        mutex_exit(q_lock);
}

/*
 * Callback invoked when a zio_free() zio is executing. This needs to be
 * intercepted to prevent the zio from deallocating a particular portion
 * of disk space and it then getting reallocated and written to, while we
 * still have it queued up for processing.
 */
void
dsl_scan_freed(spa_t *spa, const blkptr_t *bp)
{
        dsl_pool_t *dp = spa->spa_dsl_pool;
        dsl_scan_t *scn = dp->dp_scan;

        ASSERT(!BP_IS_EMBEDDED(bp));
        ASSERT(scn != NULL);
        if (!dsl_scan_is_running(scn))
                return;

        for (int i = 0; i < BP_GET_NDVAS(bp); i++)
                dsl_scan_freed_dva(spa, bp, i);
}