MFC r216919:

[FreeBSD/releng/8.2.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / zio.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 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <sys/zfs_context.h>
#include <sys/fm/fs/zfs.h>
#include <sys/spa.h>
#include <sys/txg.h>
#include <sys/spa_impl.h>
#include <sys/vdev_impl.h>
#include <sys/zio_impl.h>
#include <sys/zio_compress.h>
#include <sys/zio_checksum.h>

SYSCTL_DECL(_vfs_zfs);
SYSCTL_NODE(_vfs_zfs, OID_AUTO, zio, CTLFLAG_RW, 0, "ZFS ZIO");
static int zio_use_uma = 0;
TUNABLE_INT("vfs.zfs.zio.use_uma", &zio_use_uma);
SYSCTL_INT(_vfs_zfs_zio, OID_AUTO, use_uma, CTLFLAG_RDTUN, &zio_use_uma, 0,
    "Use uma(9) for ZIO allocations");

/*
 * ==========================================================================
 * I/O priority table
 * ==========================================================================
 */
uint8_t zio_priority_table[ZIO_PRIORITY_TABLE_SIZE] = {
        0,      /* ZIO_PRIORITY_NOW             */
        0,      /* ZIO_PRIORITY_SYNC_READ       */
        0,      /* ZIO_PRIORITY_SYNC_WRITE      */
        0,      /* ZIO_PRIORITY_LOG_WRITE       */
        1,      /* ZIO_PRIORITY_CACHE_FILL      */
        1,      /* ZIO_PRIORITY_AGG             */
        4,      /* ZIO_PRIORITY_FREE            */
        4,      /* ZIO_PRIORITY_ASYNC_WRITE     */
        6,      /* ZIO_PRIORITY_ASYNC_READ      */
        10,     /* ZIO_PRIORITY_RESILVER        */
        20,     /* ZIO_PRIORITY_SCRUB           */
};

/*
 * ==========================================================================
 * I/O type descriptions
 * ==========================================================================
 */
char *zio_type_name[ZIO_TYPES] = {
        "zio_null", "zio_read", "zio_write", "zio_free", "zio_claim",
        "zio_ioctl"
};

#define SYNC_PASS_DEFERRED_FREE 1       /* defer frees after this pass */
#define SYNC_PASS_DONT_COMPRESS 4       /* don't compress after this pass */
#define SYNC_PASS_REWRITE       1       /* rewrite new bps after this pass */

/*
 * ==========================================================================
 * I/O kmem caches
 * ==========================================================================
 */
kmem_cache_t *zio_cache;
kmem_cache_t *zio_link_cache;
kmem_cache_t *zio_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
kmem_cache_t *zio_data_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];

#ifdef _KERNEL
extern vmem_t *zio_alloc_arena;
#endif

/*
 * An allocating zio is one that either currently has the DVA allocate
 * stage set or will have it later in its lifetime.
 */
#define IO_IS_ALLOCATING(zio) \
        ((zio)->io_orig_pipeline & (1U << ZIO_STAGE_DVA_ALLOCATE))

void
zio_init(void)
{
        size_t c;
        zio_cache = kmem_cache_create("zio_cache",
            sizeof (zio_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
        zio_link_cache = kmem_cache_create("zio_link_cache",
            sizeof (zio_link_t), 0, NULL, NULL, NULL, NULL, NULL, 0);

        /*
         * For small buffers, we want a cache for each multiple of
         * SPA_MINBLOCKSIZE.  For medium-size buffers, we want a cache
         * for each quarter-power of 2.  For large buffers, we want
         * a cache for each multiple of PAGESIZE.
         */
        for (c = 0; c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; c++) {
                size_t size = (c + 1) << SPA_MINBLOCKSHIFT;
                size_t p2 = size;
                size_t align = 0;

                while (p2 & (p2 - 1))
                        p2 &= p2 - 1;

                if (size <= 4 * SPA_MINBLOCKSIZE) {
                        align = SPA_MINBLOCKSIZE;
                } else if (P2PHASE(size, PAGESIZE) == 0) {
                        align = PAGESIZE;
                } else if (P2PHASE(size, p2 >> 2) == 0) {
                        align = p2 >> 2;
                }

                if (align != 0) {
                        char name[36];
                        (void) sprintf(name, "zio_buf_%lu", (ulong_t)size);
                        zio_buf_cache[c] = kmem_cache_create(name, size,
                            align, NULL, NULL, NULL, NULL, NULL, KMC_NODEBUG);

                        (void) sprintf(name, "zio_data_buf_%lu", (ulong_t)size);
                        zio_data_buf_cache[c] = kmem_cache_create(name, size,
                            align, NULL, NULL, NULL, NULL, NULL, KMC_NODEBUG);
                }
        }

        while (--c != 0) {
                ASSERT(zio_buf_cache[c] != NULL);
                if (zio_buf_cache[c - 1] == NULL)
                        zio_buf_cache[c - 1] = zio_buf_cache[c];

                ASSERT(zio_data_buf_cache[c] != NULL);
                if (zio_data_buf_cache[c - 1] == NULL)
                        zio_data_buf_cache[c - 1] = zio_data_buf_cache[c];
        }

        zio_inject_init();
}

void
zio_fini(void)
{
        size_t c;
        kmem_cache_t *last_cache = NULL;
        kmem_cache_t *last_data_cache = NULL;

        for (c = 0; c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; c++) {
                if (zio_buf_cache[c] != last_cache) {
                        last_cache = zio_buf_cache[c];
                        kmem_cache_destroy(zio_buf_cache[c]);
                }
                zio_buf_cache[c] = NULL;

                if (zio_data_buf_cache[c] != last_data_cache) {
                        last_data_cache = zio_data_buf_cache[c];
                        kmem_cache_destroy(zio_data_buf_cache[c]);
                }
                zio_data_buf_cache[c] = NULL;
        }

        kmem_cache_destroy(zio_link_cache);
        kmem_cache_destroy(zio_cache);

        zio_inject_fini();
}

/*
 * ==========================================================================
 * Allocate and free I/O buffers
 * ==========================================================================
 */

/*
 * Use zio_buf_alloc to allocate ZFS metadata.  This data will appear in a
 * crashdump if the kernel panics, so use it judiciously.  Obviously, it's
 * useful to inspect ZFS metadata, but if possible, we should avoid keeping
 * excess / transient data in-core during a crashdump.
 */
void *
zio_buf_alloc(size_t size)
{
        size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

        ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

        if (zio_use_uma)
                return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
        else
                return (kmem_alloc(size, KM_SLEEP));
}

/*
 * Use zio_data_buf_alloc to allocate data.  The data will not appear in a
 * crashdump if the kernel panics.  This exists so that we will limit the amount
 * of ZFS data that shows up in a kernel crashdump.  (Thus reducing the amount
 * of kernel heap dumped to disk when the kernel panics)
 */
void *
zio_data_buf_alloc(size_t size)
{
        size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

        ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

        if (zio_use_uma)
                return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
        else
                return (kmem_alloc(size, KM_SLEEP));
}

void
zio_buf_free(void *buf, size_t size)
{
        size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

        ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

        if (zio_use_uma)
                kmem_cache_free(zio_buf_cache[c], buf);
        else
                kmem_free(buf, size);
}

void
zio_data_buf_free(void *buf, size_t size)
{
        size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

        ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

        if (zio_use_uma)
                kmem_cache_free(zio_data_buf_cache[c], buf);
        else
                kmem_free(buf, size);
}

/*
 * ==========================================================================
 * Push and pop I/O transform buffers
 * ==========================================================================
 */
static void
zio_push_transform(zio_t *zio, void *data, uint64_t size, uint64_t bufsize,
        zio_transform_func_t *transform)
{
        zio_transform_t *zt = kmem_alloc(sizeof (zio_transform_t), KM_SLEEP);

        zt->zt_orig_data = zio->io_data;
        zt->zt_orig_size = zio->io_size;
        zt->zt_bufsize = bufsize;
        zt->zt_transform = transform;

        zt->zt_next = zio->io_transform_stack;
        zio->io_transform_stack = zt;

        zio->io_data = data;
        zio->io_size = size;
}

static void
zio_pop_transforms(zio_t *zio)
{
        zio_transform_t *zt;

        while ((zt = zio->io_transform_stack) != NULL) {
                if (zt->zt_transform != NULL)
                        zt->zt_transform(zio,
                            zt->zt_orig_data, zt->zt_orig_size);

                zio_buf_free(zio->io_data, zt->zt_bufsize);

                zio->io_data = zt->zt_orig_data;
                zio->io_size = zt->zt_orig_size;
                zio->io_transform_stack = zt->zt_next;

                kmem_free(zt, sizeof (zio_transform_t));
        }
}

/*
 * ==========================================================================
 * I/O transform callbacks for subblocks and decompression
 * ==========================================================================
 */
static void
zio_subblock(zio_t *zio, void *data, uint64_t size)
{
        ASSERT(zio->io_size > size);

        if (zio->io_type == ZIO_TYPE_READ)
                bcopy(zio->io_data, data, size);
}

static void
zio_decompress(zio_t *zio, void *data, uint64_t size)
{
        if (zio->io_error == 0 &&
            zio_decompress_data(BP_GET_COMPRESS(zio->io_bp),
            zio->io_data, zio->io_size, data, size) != 0)
                zio->io_error = EIO;
}

/*
 * ==========================================================================
 * I/O parent/child relationships and pipeline interlocks
 * ==========================================================================
 */
/*
 * NOTE - Callers to zio_walk_parents() and zio_walk_children must
 *        continue calling these functions until they return NULL.
 *        Otherwise, the next caller will pick up the list walk in
 *        some indeterminate state.  (Otherwise every caller would
 *        have to pass in a cookie to keep the state represented by
 *        io_walk_link, which gets annoying.)
 */
zio_t *
zio_walk_parents(zio_t *cio)
{
        zio_link_t *zl = cio->io_walk_link;
        list_t *pl = &cio->io_parent_list;

        zl = (zl == NULL) ? list_head(pl) : list_next(pl, zl);
        cio->io_walk_link = zl;

        if (zl == NULL)
                return (NULL);

        ASSERT(zl->zl_child == cio);
        return (zl->zl_parent);
}

zio_t *
zio_walk_children(zio_t *pio)
{
        zio_link_t *zl = pio->io_walk_link;
        list_t *cl = &pio->io_child_list;

        zl = (zl == NULL) ? list_head(cl) : list_next(cl, zl);
        pio->io_walk_link = zl;

        if (zl == NULL)
                return (NULL);

        ASSERT(zl->zl_parent == pio);
        return (zl->zl_child);
}

zio_t *
zio_unique_parent(zio_t *cio)
{
        zio_t *pio = zio_walk_parents(cio);

        VERIFY(zio_walk_parents(cio) == NULL);
        return (pio);
}

void
zio_add_child(zio_t *pio, zio_t *cio)
{
        zio_link_t *zl = kmem_cache_alloc(zio_link_cache, KM_SLEEP);

        /*
         * Logical I/Os can have logical, gang, or vdev children.
         * Gang I/Os can have gang or vdev children.
         * Vdev I/Os can only have vdev children.
         * The following ASSERT captures all of these constraints.
         */
        ASSERT(cio->io_child_type <= pio->io_child_type);

        zl->zl_parent = pio;
        zl->zl_child = cio;

        mutex_enter(&cio->io_lock);
        mutex_enter(&pio->io_lock);

        ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);

        for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                pio->io_children[cio->io_child_type][w] += !cio->io_state[w];

        list_insert_head(&pio->io_child_list, zl);
        list_insert_head(&cio->io_parent_list, zl);

        mutex_exit(&pio->io_lock);
        mutex_exit(&cio->io_lock);
}

static void
zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
{
        ASSERT(zl->zl_parent == pio);
        ASSERT(zl->zl_child == cio);

        mutex_enter(&cio->io_lock);
        mutex_enter(&pio->io_lock);

        list_remove(&pio->io_child_list, zl);
        list_remove(&cio->io_parent_list, zl);

        mutex_exit(&pio->io_lock);
        mutex_exit(&cio->io_lock);

        kmem_cache_free(zio_link_cache, zl);
}

static boolean_t
zio_wait_for_children(zio_t *zio, enum zio_child child, enum zio_wait_type wait)
{
        uint64_t *countp = &zio->io_children[child][wait];
        boolean_t waiting = B_FALSE;

        mutex_enter(&zio->io_lock);
        ASSERT(zio->io_stall == NULL);
        if (*countp != 0) {
                zio->io_stage--;
                zio->io_stall = countp;
                waiting = B_TRUE;
        }
        mutex_exit(&zio->io_lock);

        return (waiting);
}

static void
zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait)
{
        uint64_t *countp = &pio->io_children[zio->io_child_type][wait];
        int *errorp = &pio->io_child_error[zio->io_child_type];

        mutex_enter(&pio->io_lock);
        if (zio->io_error && !(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE))
                *errorp = zio_worst_error(*errorp, zio->io_error);
        pio->io_reexecute |= zio->io_reexecute;
        ASSERT3U(*countp, >, 0);
        if (--*countp == 0 && pio->io_stall == countp) {
                pio->io_stall = NULL;
                mutex_exit(&pio->io_lock);
                zio_execute(pio);
        } else {
                mutex_exit(&pio->io_lock);
        }
}

static void
zio_inherit_child_errors(zio_t *zio, enum zio_child c)
{
        if (zio->io_child_error[c] != 0 && zio->io_error == 0)
                zio->io_error = zio->io_child_error[c];
}

/*
 * ==========================================================================
 * Create the various types of I/O (read, write, free, etc)
 * ==========================================================================
 */
static zio_t *
zio_create(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
    void *data, uint64_t size, zio_done_func_t *done, void *private,
    zio_type_t type, int priority, int flags, vdev_t *vd, uint64_t offset,
    const zbookmark_t *zb, uint8_t stage, uint32_t pipeline)
{
        zio_t *zio;

        ASSERT3U(size, <=, SPA_MAXBLOCKSIZE);
        ASSERT(P2PHASE(size, SPA_MINBLOCKSIZE) == 0);
        ASSERT(P2PHASE(offset, SPA_MINBLOCKSIZE) == 0);

        ASSERT(!vd || spa_config_held(spa, SCL_STATE_ALL, RW_READER));
        ASSERT(!bp || !(flags & ZIO_FLAG_CONFIG_WRITER));
        ASSERT(vd || stage == ZIO_STAGE_OPEN);

        zio = kmem_cache_alloc(zio_cache, KM_SLEEP);
        bzero(zio, sizeof (zio_t));

        mutex_init(&zio->io_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&zio->io_cv, NULL, CV_DEFAULT, NULL);

        list_create(&zio->io_parent_list, sizeof (zio_link_t),
            offsetof(zio_link_t, zl_parent_node));
        list_create(&zio->io_child_list, sizeof (zio_link_t),
            offsetof(zio_link_t, zl_child_node));

        if (vd != NULL)
                zio->io_child_type = ZIO_CHILD_VDEV;
        else if (flags & ZIO_FLAG_GANG_CHILD)
                zio->io_child_type = ZIO_CHILD_GANG;
        else
                zio->io_child_type = ZIO_CHILD_LOGICAL;

        if (bp != NULL) {
                zio->io_bp = bp;
                zio->io_bp_copy = *bp;
                zio->io_bp_orig = *bp;
                if (type != ZIO_TYPE_WRITE)
                        zio->io_bp = &zio->io_bp_copy;  /* so caller can free */
                if (zio->io_child_type == ZIO_CHILD_LOGICAL)
                        zio->io_logical = zio;
                if (zio->io_child_type > ZIO_CHILD_GANG && BP_IS_GANG(bp))
                        pipeline |= ZIO_GANG_STAGES;
        }

        zio->io_spa = spa;
        zio->io_txg = txg;
        zio->io_data = data;
        zio->io_size = size;
        zio->io_done = done;
        zio->io_private = private;
        zio->io_type = type;
        zio->io_priority = priority;
        zio->io_vd = vd;
        zio->io_offset = offset;
        zio->io_orig_flags = zio->io_flags = flags;
        zio->io_orig_stage = zio->io_stage = stage;
        zio->io_orig_pipeline = zio->io_pipeline = pipeline;

        zio->io_state[ZIO_WAIT_READY] = (stage >= ZIO_STAGE_READY);
        zio->io_state[ZIO_WAIT_DONE] = (stage >= ZIO_STAGE_DONE);

        if (zb != NULL)
                zio->io_bookmark = *zb;

        if (pio != NULL) {
                if (zio->io_logical == NULL)
                        zio->io_logical = pio->io_logical;
                if (zio->io_child_type == ZIO_CHILD_GANG)
                        zio->io_gang_leader = pio->io_gang_leader;
                zio_add_child(pio, zio);
        }

        return (zio);
}

static void
zio_destroy(zio_t *zio)
{
        list_destroy(&zio->io_parent_list);
        list_destroy(&zio->io_child_list);
        mutex_destroy(&zio->io_lock);
        cv_destroy(&zio->io_cv);
        kmem_cache_free(zio_cache, zio);
}

zio_t *
zio_null(zio_t *pio, spa_t *spa, vdev_t *vd, zio_done_func_t *done,
    void *private, int flags)
{
        zio_t *zio;

        zio = zio_create(pio, spa, 0, NULL, NULL, 0, done, private,
            ZIO_TYPE_NULL, ZIO_PRIORITY_NOW, flags, vd, 0, NULL,
            ZIO_STAGE_OPEN, ZIO_INTERLOCK_PIPELINE);

        return (zio);
}

zio_t *
zio_root(spa_t *spa, zio_done_func_t *done, void *private, int flags)
{
        return (zio_null(NULL, spa, NULL, done, private, flags));
}

zio_t *
zio_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
    void *data, uint64_t size, zio_done_func_t *done, void *private,
    int priority, int flags, const zbookmark_t *zb)
{
        zio_t *zio;

        zio = zio_create(pio, spa, bp->blk_birth, (blkptr_t *)bp,
            data, size, done, private,
            ZIO_TYPE_READ, priority, flags, NULL, 0, zb,
            ZIO_STAGE_OPEN, ZIO_READ_PIPELINE);

        return (zio);
}

zio_t *
zio_write(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
    void *data, uint64_t size, zio_prop_t *zp,
    zio_done_func_t *ready, zio_done_func_t *done, void *private,
    int priority, int flags, const zbookmark_t *zb)
{
        zio_t *zio;

        ASSERT(zp->zp_checksum >= ZIO_CHECKSUM_OFF &&
            zp->zp_checksum < ZIO_CHECKSUM_FUNCTIONS &&
            zp->zp_compress >= ZIO_COMPRESS_OFF &&
            zp->zp_compress < ZIO_COMPRESS_FUNCTIONS &&
            zp->zp_type < DMU_OT_NUMTYPES &&
            zp->zp_level < 32 &&
            zp->zp_ndvas > 0 &&
            zp->zp_ndvas <= spa_max_replication(spa));
        ASSERT(ready != NULL);

        zio = zio_create(pio, spa, txg, bp, data, size, done, private,
            ZIO_TYPE_WRITE, priority, flags, NULL, 0, zb,
            ZIO_STAGE_OPEN, ZIO_WRITE_PIPELINE);

        zio->io_ready = ready;
        zio->io_prop = *zp;

        return (zio);
}

zio_t *
zio_rewrite(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp, void *data,
    uint64_t size, zio_done_func_t *done, void *private, int priority,
    int flags, zbookmark_t *zb)
{
        zio_t *zio;

        zio = zio_create(pio, spa, txg, bp, data, size, done, private,
            ZIO_TYPE_WRITE, priority, flags, NULL, 0, zb,
            ZIO_STAGE_OPEN, ZIO_REWRITE_PIPELINE);

        return (zio);
}

zio_t *
zio_free(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
    zio_done_func_t *done, void *private, int flags)
{
        zio_t *zio;

        ASSERT(!BP_IS_HOLE(bp));

        if (bp->blk_fill == BLK_FILL_ALREADY_FREED)
                return (zio_null(pio, spa, NULL, NULL, NULL, flags));

        if (txg == spa->spa_syncing_txg &&
            spa_sync_pass(spa) > SYNC_PASS_DEFERRED_FREE) {
                bplist_enqueue_deferred(&spa->spa_sync_bplist, bp);
                return (zio_null(pio, spa, NULL, NULL, NULL, flags));
        }

        zio = zio_create(pio, spa, txg, bp, NULL, BP_GET_PSIZE(bp),
            done, private, ZIO_TYPE_FREE, ZIO_PRIORITY_FREE, flags,
            NULL, 0, NULL, ZIO_STAGE_OPEN, ZIO_FREE_PIPELINE);

        return (zio);
}

zio_t *
zio_claim(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
    zio_done_func_t *done, void *private, int flags)
{
        zio_t *zio;

        /*
         * A claim is an allocation of a specific block.  Claims are needed
         * to support immediate writes in the intent log.  The issue is that
         * immediate writes contain committed data, but in a txg that was
         * *not* committed.  Upon opening the pool after an unclean shutdown,
         * the intent log claims all blocks that contain immediate write data
         * so that the SPA knows they're in use.
         *
         * All claims *must* be resolved in the first txg -- before the SPA
         * starts allocating blocks -- so that nothing is allocated twice.
         */
        ASSERT3U(spa->spa_uberblock.ub_rootbp.blk_birth, <, spa_first_txg(spa));
        ASSERT3U(spa_first_txg(spa), <=, txg);

        zio = zio_create(pio, spa, txg, bp, NULL, BP_GET_PSIZE(bp),
            done, private, ZIO_TYPE_CLAIM, ZIO_PRIORITY_NOW, flags,
            NULL, 0, NULL, ZIO_STAGE_OPEN, ZIO_CLAIM_PIPELINE);

        return (zio);
}

zio_t *
zio_ioctl(zio_t *pio, spa_t *spa, vdev_t *vd, int cmd,
    zio_done_func_t *done, void *private, int priority, int flags)
{
        zio_t *zio;
        int c;

        if (vd->vdev_children == 0) {
                zio = zio_create(pio, spa, 0, NULL, NULL, 0, done, private,
                    ZIO_TYPE_IOCTL, priority, flags, vd, 0, NULL,
                    ZIO_STAGE_OPEN, ZIO_IOCTL_PIPELINE);

                zio->io_cmd = cmd;
        } else {
                zio = zio_null(pio, spa, NULL, NULL, NULL, flags);

                for (c = 0; c < vd->vdev_children; c++)
                        zio_nowait(zio_ioctl(zio, spa, vd->vdev_child[c], cmd,
                            done, private, priority, flags));
        }

        return (zio);
}

zio_t *
zio_read_phys(zio_t *pio, vdev_t *vd, uint64_t offset, uint64_t size,
    void *data, int checksum, zio_done_func_t *done, void *private,
    int priority, int flags, boolean_t labels)
{
        zio_t *zio;

        ASSERT(vd->vdev_children == 0);
        ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
            offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
        ASSERT3U(offset + size, <=, vd->vdev_psize);

        zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, done, private,
            ZIO_TYPE_READ, priority, flags, vd, offset, NULL,
            ZIO_STAGE_OPEN, ZIO_READ_PHYS_PIPELINE);

        zio->io_prop.zp_checksum = checksum;

        return (zio);
}

zio_t *
zio_write_phys(zio_t *pio, vdev_t *vd, uint64_t offset, uint64_t size,
    void *data, int checksum, zio_done_func_t *done, void *private,
    int priority, int flags, boolean_t labels)
{
        zio_t *zio;

        ASSERT(vd->vdev_children == 0);
        ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
            offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
        ASSERT3U(offset + size, <=, vd->vdev_psize);

        zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, done, private,
            ZIO_TYPE_WRITE, priority, flags, vd, offset, NULL,
            ZIO_STAGE_OPEN, ZIO_WRITE_PHYS_PIPELINE);

        zio->io_prop.zp_checksum = checksum;

        if (zio_checksum_table[checksum].ci_zbt) {
                /*
                 * zbt checksums are necessarily destructive -- they modify
                 * the end of the write buffer to hold the verifier/checksum.
                 * Therefore, we must make a local copy in case the data is
                 * being written to multiple places in parallel.
                 */
                void *wbuf = zio_buf_alloc(size);
                bcopy(data, wbuf, size);
                zio_push_transform(zio, wbuf, size, size, NULL);
        }

        return (zio);
}

/*
 * Create a child I/O to do some work for us.
 */
zio_t *
zio_vdev_child_io(zio_t *pio, blkptr_t *bp, vdev_t *vd, uint64_t offset,
        void *data, uint64_t size, int type, int priority, int flags,
        zio_done_func_t *done, void *private)
{
        uint32_t pipeline = ZIO_VDEV_CHILD_PIPELINE;
        zio_t *zio;

        ASSERT(vd->vdev_parent ==
            (pio->io_vd ? pio->io_vd : pio->io_spa->spa_root_vdev));

        if (type == ZIO_TYPE_READ && bp != NULL) {
                /*
                 * If we have the bp, then the child should perform the
                 * checksum and the parent need not.  This pushes error
                 * detection as close to the leaves as possible and
                 * eliminates redundant checksums in the interior nodes.
                 */
                pipeline |= 1U << ZIO_STAGE_CHECKSUM_VERIFY;
                pio->io_pipeline &= ~(1U << ZIO_STAGE_CHECKSUM_VERIFY);
        }

        if (vd->vdev_children == 0)
                offset += VDEV_LABEL_START_SIZE;

        zio = zio_create(pio, pio->io_spa, pio->io_txg, bp, data, size,
            done, private, type, priority,
            (pio->io_flags & ZIO_FLAG_VDEV_INHERIT) |
            ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE | flags,
            vd, offset, &pio->io_bookmark,
            ZIO_STAGE_VDEV_IO_START - 1, pipeline);

        return (zio);
}

zio_t *
zio_vdev_delegated_io(vdev_t *vd, uint64_t offset, void *data, uint64_t size,
        int type, int priority, int flags, zio_done_func_t *done, void *private)
{
        zio_t *zio;

        ASSERT(vd->vdev_ops->vdev_op_leaf);

        zio = zio_create(NULL, vd->vdev_spa, 0, NULL,
            data, size, done, private, type, priority,
            flags | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_RETRY,
            vd, offset, NULL,
            ZIO_STAGE_VDEV_IO_START - 1, ZIO_VDEV_CHILD_PIPELINE);

        return (zio);
}

void
zio_flush(zio_t *zio, vdev_t *vd)
{
        zio_nowait(zio_ioctl(zio, zio->io_spa, vd, DKIOCFLUSHWRITECACHE,
            NULL, NULL, ZIO_PRIORITY_NOW,
            ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY));
}

/*
 * ==========================================================================
 * Prepare to read and write logical blocks
 * ==========================================================================
 */

static int
zio_read_bp_init(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;

        if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF &&
            zio->io_child_type == ZIO_CHILD_LOGICAL &&
            !(zio->io_flags & ZIO_FLAG_RAW)) {
                uint64_t csize = BP_GET_PSIZE(bp);
                void *cbuf = zio_buf_alloc(csize);

                zio_push_transform(zio, cbuf, csize, csize, zio_decompress);
        }

        if (!dmu_ot[BP_GET_TYPE(bp)].ot_metadata && BP_GET_LEVEL(bp) == 0)
                zio->io_flags |= ZIO_FLAG_DONT_CACHE;

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_write_bp_init(zio_t *zio)
{
        zio_prop_t *zp = &zio->io_prop;
        int compress = zp->zp_compress;
        blkptr_t *bp = zio->io_bp;
        void *cbuf;
        uint64_t lsize = zio->io_size;
        uint64_t csize = lsize;
        uint64_t cbufsize = 0;
        int pass = 1;

        /*
         * If our children haven't all reached the ready stage,
         * wait for them and then repeat this pipeline stage.
         */
        if (zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_READY) ||
            zio_wait_for_children(zio, ZIO_CHILD_LOGICAL, ZIO_WAIT_READY))
                return (ZIO_PIPELINE_STOP);

        if (!IO_IS_ALLOCATING(zio))
                return (ZIO_PIPELINE_CONTINUE);

        ASSERT(compress != ZIO_COMPRESS_INHERIT);

        if (bp->blk_birth == zio->io_txg) {
                /*
                 * We're rewriting an existing block, which means we're
                 * working on behalf of spa_sync().  For spa_sync() to
                 * converge, it must eventually be the case that we don't
                 * have to allocate new blocks.  But compression changes
                 * the blocksize, which forces a reallocate, and makes
                 * convergence take longer.  Therefore, after the first
                 * few passes, stop compressing to ensure convergence.
                 */
                pass = spa_sync_pass(zio->io_spa);

                if (pass > SYNC_PASS_DONT_COMPRESS)
                        compress = ZIO_COMPRESS_OFF;

                /* Make sure someone doesn't change their mind on overwrites */
                ASSERT(MIN(zp->zp_ndvas + BP_IS_GANG(bp),
                    spa_max_replication(zio->io_spa)) == BP_GET_NDVAS(bp));
        }

        if (compress != ZIO_COMPRESS_OFF) {
                if (!zio_compress_data(compress, zio->io_data, zio->io_size,
                    &cbuf, &csize, &cbufsize)) {
                        compress = ZIO_COMPRESS_OFF;
                } else if (csize != 0) {
                        zio_push_transform(zio, cbuf, csize, cbufsize, NULL);
                }
        }

        /*
         * The final pass of spa_sync() must be all rewrites, but the first
         * few passes offer a trade-off: allocating blocks defers convergence,
         * but newly allocated blocks are sequential, so they can be written
         * to disk faster.  Therefore, we allow the first few passes of
         * spa_sync() to allocate new blocks, but force rewrites after that.
         * There should only be a handful of blocks after pass 1 in any case.
         */
        if (bp->blk_birth == zio->io_txg && BP_GET_PSIZE(bp) == csize &&
            pass > SYNC_PASS_REWRITE) {
                ASSERT(csize != 0);
                uint32_t gang_stages = zio->io_pipeline & ZIO_GANG_STAGES;
                zio->io_pipeline = ZIO_REWRITE_PIPELINE | gang_stages;
                zio->io_flags |= ZIO_FLAG_IO_REWRITE;
        } else {
                BP_ZERO(bp);
                zio->io_pipeline = ZIO_WRITE_PIPELINE;
        }

        if (csize == 0) {
                zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
        } else {
                ASSERT(zp->zp_checksum != ZIO_CHECKSUM_GANG_HEADER);
                BP_SET_LSIZE(bp, lsize);
                BP_SET_PSIZE(bp, csize);
                BP_SET_COMPRESS(bp, compress);
                BP_SET_CHECKSUM(bp, zp->zp_checksum);
                BP_SET_TYPE(bp, zp->zp_type);
                BP_SET_LEVEL(bp, zp->zp_level);
                BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
        }

        return (ZIO_PIPELINE_CONTINUE);
}

/*
 * ==========================================================================
 * Execute the I/O pipeline
 * ==========================================================================
 */

static void
zio_taskq_dispatch(zio_t *zio, enum zio_taskq_type q)
{
        spa_t *spa = zio->io_spa;
        zio_type_t t = zio->io_type;
#ifdef _KERNEL
        struct ostask *task;
#endif

        ASSERT(q == ZIO_TASKQ_ISSUE || q == ZIO_TASKQ_INTERRUPT);

#ifdef _KERNEL
        if (q == ZIO_TASKQ_ISSUE)
                task = &zio->io_task_issue;
        else /* if (q == ZIO_TASKQ_INTERRUPT) */
                task = &zio->io_task_interrupt;
#endif

        /*
         * If we're a config writer or a probe, the normal issue and
         * interrupt threads may all be blocked waiting for the config lock.
         * In this case, select the otherwise-unused taskq for ZIO_TYPE_NULL.
         */
        if (zio->io_flags & (ZIO_FLAG_CONFIG_WRITER | ZIO_FLAG_PROBE))
                t = ZIO_TYPE_NULL;

        /*
         * A similar issue exists for the L2ARC write thread until L2ARC 2.0.
         */
        if (t == ZIO_TYPE_WRITE && zio->io_vd && zio->io_vd->vdev_aux)
                t = ZIO_TYPE_NULL;

        /*
         * If this is a high priority I/O, then use the high priority taskq.
         */
        if (zio->io_priority == ZIO_PRIORITY_NOW &&
            spa->spa_zio_taskq[t][q + 1] != NULL)
                q++;

        ASSERT3U(q, <, ZIO_TASKQ_TYPES);
#ifdef _KERNEL
        (void) taskq_dispatch_safe(spa->spa_zio_taskq[t][q],
            (task_func_t *)zio_execute, zio, task);
#else
        (void) taskq_dispatch(spa->spa_zio_taskq[t][q],
            (task_func_t *)zio_execute, zio, TQ_SLEEP);
#endif
}

static boolean_t
zio_taskq_member(zio_t *zio, enum zio_taskq_type q)
{
        kthread_t *executor = zio->io_executor;
        spa_t *spa = zio->io_spa;

        for (zio_type_t t = 0; t < ZIO_TYPES; t++)
                if (taskq_member(spa->spa_zio_taskq[t][q], executor))
                        return (B_TRUE);

        return (B_FALSE);
}

static int
zio_issue_async(zio_t *zio)
{
        zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);

        return (ZIO_PIPELINE_STOP);
}

void
zio_interrupt(zio_t *zio)
{
        zio_taskq_dispatch(zio, ZIO_TASKQ_INTERRUPT);
}

/*
 * Execute the I/O pipeline until one of the following occurs:
 * (1) the I/O completes; (2) the pipeline stalls waiting for
 * dependent child I/Os; (3) the I/O issues, so we're waiting
 * for an I/O completion interrupt; (4) the I/O is delegated by
 * vdev-level caching or aggregation; (5) the I/O is deferred
 * due to vdev-level queueing; (6) the I/O is handed off to
 * another thread.  In all cases, the pipeline stops whenever
 * there's no CPU work; it never burns a thread in cv_wait().
 *
 * There's no locking on io_stage because there's no legitimate way
 * for multiple threads to be attempting to process the same I/O.
 */
static zio_pipe_stage_t *zio_pipeline[ZIO_STAGES];

void
zio_execute(zio_t *zio)
{
        zio->io_executor = curthread;

        while (zio->io_stage < ZIO_STAGE_DONE) {
                uint32_t pipeline = zio->io_pipeline;
                zio_stage_t stage = zio->io_stage;
                int rv;

                ASSERT(!MUTEX_HELD(&zio->io_lock));

                while (((1U << ++stage) & pipeline) == 0)
                        continue;

                ASSERT(stage <= ZIO_STAGE_DONE);
                ASSERT(zio->io_stall == NULL);

                /*
                 * If we are in interrupt context and this pipeline stage
                 * will grab a config lock that is held across I/O,
                 * issue async to avoid deadlock.
                 */
                if (((1U << stage) & ZIO_CONFIG_LOCK_BLOCKING_STAGES) &&
                    zio->io_vd == NULL &&
                    zio_taskq_member(zio, ZIO_TASKQ_INTERRUPT)) {
                        zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);
                        return;
                }

                zio->io_stage = stage;
                rv = zio_pipeline[stage](zio);

                if (rv == ZIO_PIPELINE_STOP)
                        return;

                ASSERT(rv == ZIO_PIPELINE_CONTINUE);
        }
}

/*
 * ==========================================================================
 * Initiate I/O, either sync or async
 * ==========================================================================
 */
int
zio_wait(zio_t *zio)
{
        int error;

        ASSERT(zio->io_stage == ZIO_STAGE_OPEN);
        ASSERT(zio->io_executor == NULL);

        zio->io_waiter = curthread;

        zio_execute(zio);

        mutex_enter(&zio->io_lock);
        while (zio->io_executor != NULL)
                cv_wait(&zio->io_cv, &zio->io_lock);
        mutex_exit(&zio->io_lock);

        error = zio->io_error;
        zio_destroy(zio);

        return (error);
}

void
zio_nowait(zio_t *zio)
{
        ASSERT(zio->io_executor == NULL);

        if (zio->io_child_type == ZIO_CHILD_LOGICAL &&
            zio_unique_parent(zio) == NULL) {
                /*
                 * This is a logical async I/O with no parent to wait for it.
                 * We add it to the spa_async_root_zio "Godfather" I/O which
                 * will ensure they complete prior to unloading the pool.
                 */
                spa_t *spa = zio->io_spa;

                zio_add_child(spa->spa_async_zio_root, zio);
        }

        zio_execute(zio);
}

/*
 * ==========================================================================
 * Reexecute or suspend/resume failed I/O
 * ==========================================================================
 */

static void
zio_reexecute(zio_t *pio)
{
        zio_t *cio, *cio_next;

        ASSERT(pio->io_child_type == ZIO_CHILD_LOGICAL);
        ASSERT(pio->io_orig_stage == ZIO_STAGE_OPEN);
        ASSERT(pio->io_gang_leader == NULL);
        ASSERT(pio->io_gang_tree == NULL);

        pio->io_flags = pio->io_orig_flags;
        pio->io_stage = pio->io_orig_stage;
        pio->io_pipeline = pio->io_orig_pipeline;
        pio->io_reexecute = 0;
        pio->io_error = 0;
        for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                pio->io_state[w] = 0;
        for (int c = 0; c < ZIO_CHILD_TYPES; c++)
                pio->io_child_error[c] = 0;

        if (IO_IS_ALLOCATING(pio)) {
                /*
                 * Remember the failed bp so that the io_ready() callback
                 * can update its accounting upon reexecution.  The block
                 * was already freed in zio_done(); we indicate this with
                 * a fill count of -1 so that zio_free() knows to skip it.
                 */
                blkptr_t *bp = pio->io_bp;
                ASSERT(bp->blk_birth == 0 || bp->blk_birth == pio->io_txg);
                bp->blk_fill = BLK_FILL_ALREADY_FREED;
                pio->io_bp_orig = *bp;
                BP_ZERO(bp);
        }

        /*
         * As we reexecute pio's children, new children could be created.
         * New children go to the head of pio's io_child_list, however,
         * so we will (correctly) not reexecute them.  The key is that
         * the remainder of pio's io_child_list, from 'cio_next' onward,
         * cannot be affected by any side effects of reexecuting 'cio'.
         */
        for (cio = zio_walk_children(pio); cio != NULL; cio = cio_next) {
                cio_next = zio_walk_children(pio);
                mutex_enter(&pio->io_lock);
                for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                        pio->io_children[cio->io_child_type][w]++;
                mutex_exit(&pio->io_lock);
                zio_reexecute(cio);
        }

        /*
         * Now that all children have been reexecuted, execute the parent.
         * We don't reexecute "The Godfather" I/O here as it's the
         * responsibility of the caller to wait on him.
         */
        if (!(pio->io_flags & ZIO_FLAG_GODFATHER))
                zio_execute(pio);
}

void
zio_suspend(spa_t *spa, zio_t *zio)
{
        if (spa_get_failmode(spa) == ZIO_FAILURE_MODE_PANIC)
                fm_panic("Pool '%s' has encountered an uncorrectable I/O "
                    "failure and the failure mode property for this pool "
                    "is set to panic.", spa_name(spa));

        zfs_ereport_post(FM_EREPORT_ZFS_IO_FAILURE, spa, NULL, NULL, 0, 0);

        mutex_enter(&spa->spa_suspend_lock);

        if (spa->spa_suspend_zio_root == NULL)
                spa->spa_suspend_zio_root = zio_root(spa, NULL, NULL,
                    ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE |
                    ZIO_FLAG_GODFATHER);

        spa->spa_suspended = B_TRUE;

        if (zio != NULL) {
                ASSERT(!(zio->io_flags & ZIO_FLAG_GODFATHER));
                ASSERT(zio != spa->spa_suspend_zio_root);
                ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                ASSERT(zio_unique_parent(zio) == NULL);
                ASSERT(zio->io_stage == ZIO_STAGE_DONE);
                zio_add_child(spa->spa_suspend_zio_root, zio);
        }

        mutex_exit(&spa->spa_suspend_lock);
}

int
zio_resume(spa_t *spa)
{
        zio_t *pio;

        /*
         * Reexecute all previously suspended i/o.
         */
        mutex_enter(&spa->spa_suspend_lock);
        spa->spa_suspended = B_FALSE;
        cv_broadcast(&spa->spa_suspend_cv);
        pio = spa->spa_suspend_zio_root;
        spa->spa_suspend_zio_root = NULL;
        mutex_exit(&spa->spa_suspend_lock);

        if (pio == NULL)
                return (0);

        zio_reexecute(pio);
        return (zio_wait(pio));
}

void
zio_resume_wait(spa_t *spa)
{
        mutex_enter(&spa->spa_suspend_lock);
        while (spa_suspended(spa))
                cv_wait(&spa->spa_suspend_cv, &spa->spa_suspend_lock);
        mutex_exit(&spa->spa_suspend_lock);
}

/*
 * ==========================================================================
 * Gang blocks.
 *
 * A gang block is a collection of small blocks that looks to the DMU
 * like one large block.  When zio_dva_allocate() cannot find a block
 * of the requested size, due to either severe fragmentation or the pool
 * being nearly full, it calls zio_write_gang_block() to construct the
 * block from smaller fragments.
 *
 * A gang block consists of a gang header (zio_gbh_phys_t) and up to
 * three (SPA_GBH_NBLKPTRS) gang members.  The gang header is just like
 * an indirect block: it's an array of block pointers.  It consumes
 * only one sector and hence is allocatable regardless of fragmentation.
 * The gang header's bps point to its gang members, which hold the data.
 *
 * Gang blocks are self-checksumming, using the bp's <vdev, offset, txg>
 * as the verifier to ensure uniqueness of the SHA256 checksum.
 * Critically, the gang block bp's blk_cksum is the checksum of the data,
 * not the gang header.  This ensures that data block signatures (needed for
 * deduplication) are independent of how the block is physically stored.
 *
 * Gang blocks can be nested: a gang member may itself be a gang block.
 * Thus every gang block is a tree in which root and all interior nodes are
 * gang headers, and the leaves are normal blocks that contain user data.
 * The root of the gang tree is called the gang leader.
 *
 * To perform any operation (read, rewrite, free, claim) on a gang block,
 * zio_gang_assemble() first assembles the gang tree (minus data leaves)
 * in the io_gang_tree field of the original logical i/o by recursively
 * reading the gang leader and all gang headers below it.  This yields
 * an in-core tree containing the contents of every gang header and the
 * bps for every constituent of the gang block.
 *
 * With the gang tree now assembled, zio_gang_issue() just walks the gang tree
 * and invokes a callback on each bp.  To free a gang block, zio_gang_issue()
 * calls zio_free_gang() -- a trivial wrapper around zio_free() -- for each bp.
 * zio_claim_gang() provides a similarly trivial wrapper for zio_claim().
 * zio_read_gang() is a wrapper around zio_read() that omits reading gang
 * headers, since we already have those in io_gang_tree.  zio_rewrite_gang()
 * performs a zio_rewrite() of the data or, for gang headers, a zio_rewrite()
 * of the gang header plus zio_checksum_compute() of the data to update the
 * gang header's blk_cksum as described above.
 *
 * The two-phase assemble/issue model solves the problem of partial failure --
 * what if you'd freed part of a gang block but then couldn't read the
 * gang header for another part?  Assembling the entire gang tree first
 * ensures that all the necessary gang header I/O has succeeded before
 * starting the actual work of free, claim, or write.  Once the gang tree
 * is assembled, free and claim are in-memory operations that cannot fail.
 *
 * In the event that a gang write fails, zio_dva_unallocate() walks the
 * gang tree to immediately free (i.e. insert back into the space map)
 * everything we've allocated.  This ensures that we don't get ENOSPC
 * errors during repeated suspend/resume cycles due to a flaky device.
 *
 * Gang rewrites only happen during sync-to-convergence.  If we can't assemble
 * the gang tree, we won't modify the block, so we can safely defer the free
 * (knowing that the block is still intact).  If we *can* assemble the gang
 * tree, then even if some of the rewrites fail, zio_dva_unallocate() will free
 * each constituent bp and we can allocate a new block on the next sync pass.
 *
 * In all cases, the gang tree allows complete recovery from partial failure.
 * ==========================================================================
 */

static zio_t *
zio_read_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
        if (gn != NULL)
                return (pio);

        return (zio_read(pio, pio->io_spa, bp, data, BP_GET_PSIZE(bp),
            NULL, NULL, pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
            &pio->io_bookmark));
}

zio_t *
zio_rewrite_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
        zio_t *zio;

        if (gn != NULL) {
                zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
                    gn->gn_gbh, SPA_GANGBLOCKSIZE, NULL, NULL, pio->io_priority,
                    ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
                /*
                 * As we rewrite each gang header, the pipeline will compute
                 * a new gang block header checksum for it; but no one will
                 * compute a new data checksum, so we do that here.  The one
                 * exception is the gang leader: the pipeline already computed
                 * its data checksum because that stage precedes gang assembly.
                 * (Presently, nothing actually uses interior data checksums;
                 * this is just good hygiene.)
                 */
                if (gn != pio->io_gang_leader->io_gang_tree) {
                        zio_checksum_compute(zio, BP_GET_CHECKSUM(bp),
                            data, BP_GET_PSIZE(bp));
                }
        } else {
                zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
                    data, BP_GET_PSIZE(bp), NULL, NULL, pio->io_priority,
                    ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
        }

        return (zio);
}

/* ARGSUSED */
zio_t *
zio_free_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
        return (zio_free(pio, pio->io_spa, pio->io_txg, bp,
            NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio)));
}

/* ARGSUSED */
zio_t *
zio_claim_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
        return (zio_claim(pio, pio->io_spa, pio->io_txg, bp,
            NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio)));
}

static zio_gang_issue_func_t *zio_gang_issue_func[ZIO_TYPES] = {
        NULL,
        zio_read_gang,
        zio_rewrite_gang,
        zio_free_gang,
        zio_claim_gang,
        NULL
};

static void zio_gang_tree_assemble_done(zio_t *zio);

static zio_gang_node_t *
zio_gang_node_alloc(zio_gang_node_t **gnpp)
{
        zio_gang_node_t *gn;

        ASSERT(*gnpp == NULL);

        gn = kmem_zalloc(sizeof (*gn), KM_SLEEP);
        gn->gn_gbh = zio_buf_alloc(SPA_GANGBLOCKSIZE);
        *gnpp = gn;

        return (gn);
}

static void
zio_gang_node_free(zio_gang_node_t **gnpp)
{
        zio_gang_node_t *gn = *gnpp;

        for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
                ASSERT(gn->gn_child[g] == NULL);

        zio_buf_free(gn->gn_gbh, SPA_GANGBLOCKSIZE);
        kmem_free(gn, sizeof (*gn));
        *gnpp = NULL;
}

static void
zio_gang_tree_free(zio_gang_node_t **gnpp)
{
        zio_gang_node_t *gn = *gnpp;

        if (gn == NULL)
                return;

        for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
                zio_gang_tree_free(&gn->gn_child[g]);

        zio_gang_node_free(gnpp);
}

static void
zio_gang_tree_assemble(zio_t *gio, blkptr_t *bp, zio_gang_node_t **gnpp)
{
        zio_gang_node_t *gn = zio_gang_node_alloc(gnpp);

        ASSERT(gio->io_gang_leader == gio);
        ASSERT(BP_IS_GANG(bp));

        zio_nowait(zio_read(gio, gio->io_spa, bp, gn->gn_gbh,
            SPA_GANGBLOCKSIZE, zio_gang_tree_assemble_done, gn,
            gio->io_priority, ZIO_GANG_CHILD_FLAGS(gio), &gio->io_bookmark));
}

static void
zio_gang_tree_assemble_done(zio_t *zio)
{
        zio_t *gio = zio->io_gang_leader;
        zio_gang_node_t *gn = zio->io_private;
        blkptr_t *bp = zio->io_bp;

        ASSERT(gio == zio_unique_parent(zio));
        ASSERT(zio_walk_children(zio) == NULL);

        if (zio->io_error)
                return;

        if (BP_SHOULD_BYTESWAP(bp))
                byteswap_uint64_array(zio->io_data, zio->io_size);

        ASSERT(zio->io_data == gn->gn_gbh);
        ASSERT(zio->io_size == SPA_GANGBLOCKSIZE);
        ASSERT(gn->gn_gbh->zg_tail.zbt_magic == ZBT_MAGIC);

        for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
                if (!BP_IS_GANG(gbp))
                        continue;
                zio_gang_tree_assemble(gio, gbp, &gn->gn_child[g]);
        }
}

static void
zio_gang_tree_issue(zio_t *pio, zio_gang_node_t *gn, blkptr_t *bp, void *data)
{
        zio_t *gio = pio->io_gang_leader;
        zio_t *zio;

        ASSERT(BP_IS_GANG(bp) == !!gn);
        ASSERT(BP_GET_CHECKSUM(bp) == BP_GET_CHECKSUM(gio->io_bp));
        ASSERT(BP_GET_LSIZE(bp) == BP_GET_PSIZE(bp) || gn == gio->io_gang_tree);

        /*
         * If you're a gang header, your data is in gn->gn_gbh.
         * If you're a gang member, your data is in 'data' and gn == NULL.
         */
        zio = zio_gang_issue_func[gio->io_type](pio, bp, gn, data);

        if (gn != NULL) {
                ASSERT(gn->gn_gbh->zg_tail.zbt_magic == ZBT_MAGIC);

                for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                        blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
                        if (BP_IS_HOLE(gbp))
                                continue;
                        zio_gang_tree_issue(zio, gn->gn_child[g], gbp, data);
                        data = (char *)data + BP_GET_PSIZE(gbp);
                }
        }

        if (gn == gio->io_gang_tree)
                ASSERT3P((char *)gio->io_data + gio->io_size, ==, data);

        if (zio != pio)
                zio_nowait(zio);
}

static int
zio_gang_assemble(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;

        ASSERT(BP_IS_GANG(bp) && zio->io_gang_leader == NULL);
        ASSERT(zio->io_child_type > ZIO_CHILD_GANG);

        zio->io_gang_leader = zio;

        zio_gang_tree_assemble(zio, bp, &zio->io_gang_tree);

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_gang_issue(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;

        if (zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_DONE))
                return (ZIO_PIPELINE_STOP);

        ASSERT(BP_IS_GANG(bp) && zio->io_gang_leader == zio);
        ASSERT(zio->io_child_type > ZIO_CHILD_GANG);

        if (zio->io_child_error[ZIO_CHILD_GANG] == 0)
                zio_gang_tree_issue(zio, zio->io_gang_tree, bp, zio->io_data);
        else
                zio_gang_tree_free(&zio->io_gang_tree);

        zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

        return (ZIO_PIPELINE_CONTINUE);
}

static void
zio_write_gang_member_ready(zio_t *zio)
{
        zio_t *pio = zio_unique_parent(zio);
        zio_t *gio = zio->io_gang_leader;
        dva_t *cdva = zio->io_bp->blk_dva;
        dva_t *pdva = pio->io_bp->blk_dva;
        uint64_t asize;

        if (BP_IS_HOLE(zio->io_bp))
                return;

        ASSERT(BP_IS_HOLE(&zio->io_bp_orig));

        ASSERT(zio->io_child_type == ZIO_CHILD_GANG);
        ASSERT3U(zio->io_prop.zp_ndvas, ==, gio->io_prop.zp_ndvas);
        ASSERT3U(zio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(zio->io_bp));
        ASSERT3U(pio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(pio->io_bp));
        ASSERT3U(BP_GET_NDVAS(zio->io_bp), <=, BP_GET_NDVAS(pio->io_bp));

        mutex_enter(&pio->io_lock);
        for (int d = 0; d < BP_GET_NDVAS(zio->io_bp); d++) {
                ASSERT(DVA_GET_GANG(&pdva[d]));
                asize = DVA_GET_ASIZE(&pdva[d]);
                asize += DVA_GET_ASIZE(&cdva[d]);
                DVA_SET_ASIZE(&pdva[d], asize);
        }
        mutex_exit(&pio->io_lock);
}

static int
zio_write_gang_block(zio_t *pio)
{
        spa_t *spa = pio->io_spa;
        blkptr_t *bp = pio->io_bp;
        zio_t *gio = pio->io_gang_leader;
        zio_t *zio;
        zio_gang_node_t *gn, **gnpp;
        zio_gbh_phys_t *gbh;
        uint64_t txg = pio->io_txg;
        uint64_t resid = pio->io_size;
        uint64_t lsize;
        int ndvas = gio->io_prop.zp_ndvas;
        int gbh_ndvas = MIN(ndvas + 1, spa_max_replication(spa));
        zio_prop_t zp;
        int error;

        error = metaslab_alloc(spa, spa->spa_normal_class, SPA_GANGBLOCKSIZE,
            bp, gbh_ndvas, txg, pio == gio ? NULL : gio->io_bp,
            METASLAB_HINTBP_FAVOR | METASLAB_GANG_HEADER);
        if (error) {
                pio->io_error = error;
                return (ZIO_PIPELINE_CONTINUE);
        }

        if (pio == gio) {
                gnpp = &gio->io_gang_tree;
        } else {
                gnpp = pio->io_private;
                ASSERT(pio->io_ready == zio_write_gang_member_ready);
        }

        gn = zio_gang_node_alloc(gnpp);
        gbh = gn->gn_gbh;
        bzero(gbh, SPA_GANGBLOCKSIZE);

        /*
         * Create the gang header.
         */
        zio = zio_rewrite(pio, spa, txg, bp, gbh, SPA_GANGBLOCKSIZE, NULL, NULL,
            pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);

        /*
         * Create and nowait the gang children.
         */
        for (int g = 0; resid != 0; resid -= lsize, g++) {
                lsize = P2ROUNDUP(resid / (SPA_GBH_NBLKPTRS - g),
                    SPA_MINBLOCKSIZE);
                ASSERT(lsize >= SPA_MINBLOCKSIZE && lsize <= resid);

                zp.zp_checksum = gio->io_prop.zp_checksum;
                zp.zp_compress = ZIO_COMPRESS_OFF;
                zp.zp_type = DMU_OT_NONE;
                zp.zp_level = 0;
                zp.zp_ndvas = gio->io_prop.zp_ndvas;

                zio_nowait(zio_write(zio, spa, txg, &gbh->zg_blkptr[g],
                    (char *)pio->io_data + (pio->io_size - resid), lsize, &zp,
                    zio_write_gang_member_ready, NULL, &gn->gn_child[g],
                    pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
                    &pio->io_bookmark));
        }

        /*
         * Set pio's pipeline to just wait for zio to finish.
         */
        pio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

        zio_nowait(zio);

        return (ZIO_PIPELINE_CONTINUE);
}

/*
 * ==========================================================================
 * Allocate and free blocks
 * ==========================================================================
 */

static int
zio_dva_allocate(zio_t *zio)
{
        spa_t *spa = zio->io_spa;
        metaslab_class_t *mc = spa->spa_normal_class;
        blkptr_t *bp = zio->io_bp;
        int error;

        if (zio->io_gang_leader == NULL) {
                ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
                zio->io_gang_leader = zio;
        }

        ASSERT(BP_IS_HOLE(bp));
        ASSERT3U(BP_GET_NDVAS(bp), ==, 0);
        ASSERT3U(zio->io_prop.zp_ndvas, >, 0);
        ASSERT3U(zio->io_prop.zp_ndvas, <=, spa_max_replication(spa));
        ASSERT3U(zio->io_size, ==, BP_GET_PSIZE(bp));

        error = metaslab_alloc(spa, mc, zio->io_size, bp,
            zio->io_prop.zp_ndvas, zio->io_txg, NULL, 0);

        if (error) {
                if (error == ENOSPC && zio->io_size > SPA_MINBLOCKSIZE)
                        return (zio_write_gang_block(zio));
                zio->io_error = error;
        }

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_dva_free(zio_t *zio)
{
        metaslab_free(zio->io_spa, zio->io_bp, zio->io_txg, B_FALSE);

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_dva_claim(zio_t *zio)
{
        int error;

        error = metaslab_claim(zio->io_spa, zio->io_bp, zio->io_txg);
        if (error)
                zio->io_error = error;

        return (ZIO_PIPELINE_CONTINUE);
}

/*
 * Undo an allocation.  This is used by zio_done() when an I/O fails
 * and we want to give back the block we just allocated.
 * This handles both normal blocks and gang blocks.
 */
static void
zio_dva_unallocate(zio_t *zio, zio_gang_node_t *gn, blkptr_t *bp)
{
        spa_t *spa = zio->io_spa;
        boolean_t now = !(zio->io_flags & ZIO_FLAG_IO_REWRITE);

        ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp));

        if (zio->io_bp == bp && !now) {
                /*
                 * This is a rewrite for sync-to-convergence.
                 * We can't do a metaslab_free(NOW) because bp wasn't allocated
                 * during this sync pass, which means that metaslab_sync()
                 * already committed the allocation.
                 */
                ASSERT(DVA_EQUAL(BP_IDENTITY(bp),
                    BP_IDENTITY(&zio->io_bp_orig)));
                ASSERT(spa_sync_pass(spa) > 1);

                if (BP_IS_GANG(bp) && gn == NULL) {
                        /*
                         * This is a gang leader whose gang header(s) we
                         * couldn't read now, so defer the free until later.
                         * The block should still be intact because without
                         * the headers, we'd never even start the rewrite.
                         */
                        bplist_enqueue_deferred(&spa->spa_sync_bplist, bp);
                        return;
                }
        }

        if (!BP_IS_HOLE(bp))
                metaslab_free(spa, bp, bp->blk_birth, now);

        if (gn != NULL) {
                for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                        zio_dva_unallocate(zio, gn->gn_child[g],
                            &gn->gn_gbh->zg_blkptr[g]);
                }
        }
}

/*
 * Try to allocate an intent log block.  Return 0 on success, errno on failure.
 */
int
zio_alloc_blk(spa_t *spa, uint64_t size, blkptr_t *new_bp, blkptr_t *old_bp,
    uint64_t txg)
{
        int error;

        error = metaslab_alloc(spa, spa->spa_log_class, size,
            new_bp, 1, txg, old_bp, METASLAB_HINTBP_AVOID);

        if (error)
                error = metaslab_alloc(spa, spa->spa_normal_class, size,
                    new_bp, 1, txg, old_bp, METASLAB_HINTBP_AVOID);

        if (error == 0) {
                BP_SET_LSIZE(new_bp, size);
                BP_SET_PSIZE(new_bp, size);
                BP_SET_COMPRESS(new_bp, ZIO_COMPRESS_OFF);
                BP_SET_CHECKSUM(new_bp, ZIO_CHECKSUM_ZILOG);
                BP_SET_TYPE(new_bp, DMU_OT_INTENT_LOG);
                BP_SET_LEVEL(new_bp, 0);
                BP_SET_BYTEORDER(new_bp, ZFS_HOST_BYTEORDER);
        }

        return (error);
}

/*
 * Free an intent log block.  We know it can't be a gang block, so there's
 * nothing to do except metaslab_free() it.
 */
void
zio_free_blk(spa_t *spa, blkptr_t *bp, uint64_t txg)
{
        ASSERT(!BP_IS_GANG(bp));

        metaslab_free(spa, bp, txg, B_FALSE);
}

/*
 * ==========================================================================
 * Read and write to physical devices
 * ==========================================================================
 */
static int
zio_vdev_io_start(zio_t *zio)
{
        vdev_t *vd = zio->io_vd;
        uint64_t align;
        spa_t *spa = zio->io_spa;

        ASSERT(zio->io_error == 0);
        ASSERT(zio->io_child_error[ZIO_CHILD_VDEV] == 0);

        if (vd == NULL) {
                if (!(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
                        spa_config_enter(spa, SCL_ZIO, zio, RW_READER);

                /*
                 * The mirror_ops handle multiple DVAs in a single BP.
                 */
                return (vdev_mirror_ops.vdev_op_io_start(zio));
        }

        align = 1ULL << vd->vdev_top->vdev_ashift;

        if (P2PHASE(zio->io_size, align) != 0) {
                uint64_t asize = P2ROUNDUP(zio->io_size, align);
                char *abuf = zio_buf_alloc(asize);
                ASSERT(vd == vd->vdev_top);
                if (zio->io_type == ZIO_TYPE_WRITE) {
                        bcopy(zio->io_data, abuf, zio->io_size);
                        bzero(abuf + zio->io_size, asize - zio->io_size);
                }
                zio_push_transform(zio, abuf, asize, asize, zio_subblock);
        }

        ASSERT(P2PHASE(zio->io_offset, align) == 0);
        ASSERT(P2PHASE(zio->io_size, align) == 0);
        ASSERT(zio->io_type != ZIO_TYPE_WRITE || spa_writeable(spa));

        /*
         * If this is a repair I/O, and there's no self-healing involved --
         * that is, we're just resilvering what we expect to resilver --
         * then don't do the I/O unless zio's txg is actually in vd's DTL.
         * This prevents spurious resilvering with nested replication.
         * For example, given a mirror of mirrors, (A+B)+(C+D), if only
         * A is out of date, we'll read from C+D, then use the data to
         * resilver A+B -- but we don't actually want to resilver B, just A.
         * The top-level mirror has no way to know this, so instead we just
         * discard unnecessary repairs as we work our way down the vdev tree.
         * The same logic applies to any form of nested replication:
         * ditto + mirror, RAID-Z + replacing, etc.  This covers them all.
         */
        if ((zio->io_flags & ZIO_FLAG_IO_REPAIR) &&
            !(zio->io_flags & ZIO_FLAG_SELF_HEAL) &&
            zio->io_txg != 0 && /* not a delegated i/o */
            !vdev_dtl_contains(vd, DTL_PARTIAL, zio->io_txg, 1)) {
                ASSERT(zio->io_type == ZIO_TYPE_WRITE);
                zio_vdev_io_bypass(zio);
                return (ZIO_PIPELINE_CONTINUE);
        }

        if (vd->vdev_ops->vdev_op_leaf &&
            (zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE)) {

                if (zio->io_type == ZIO_TYPE_READ && vdev_cache_read(zio) == 0)
                        return (ZIO_PIPELINE_CONTINUE);

                if ((zio = vdev_queue_io(zio)) == NULL)
                        return (ZIO_PIPELINE_STOP);

                if (!vdev_accessible(vd, zio)) {
                        zio->io_error = ENXIO;
                        zio_interrupt(zio);
                        return (ZIO_PIPELINE_STOP);
                }
        }

        return (vd->vdev_ops->vdev_op_io_start(zio));
}

static int
zio_vdev_io_done(zio_t *zio)
{
        vdev_t *vd = zio->io_vd;
        vdev_ops_t *ops = vd ? vd->vdev_ops : &vdev_mirror_ops;
        boolean_t unexpected_error = B_FALSE;

        if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE))
                return (ZIO_PIPELINE_STOP);

        ASSERT(zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE);

        if (vd != NULL && vd->vdev_ops->vdev_op_leaf) {

                vdev_queue_io_done(zio);

                if (zio->io_type == ZIO_TYPE_WRITE)
                        vdev_cache_write(zio);

                if (zio_injection_enabled && zio->io_error == 0)
                        zio->io_error = zio_handle_device_injection(vd,
                            zio, EIO);

                if (zio_injection_enabled && zio->io_error == 0)
                        zio->io_error = zio_handle_label_injection(zio, EIO);

                if (zio->io_error) {
                        if (!vdev_accessible(vd, zio)) {
                                zio->io_error = ENXIO;
                        } else {
                                unexpected_error = B_TRUE;
                        }
                }
        }

        ops->vdev_op_io_done(zio);

        if (unexpected_error)
                VERIFY(vdev_probe(vd, zio) == NULL);

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_vdev_io_assess(zio_t *zio)
{
        vdev_t *vd = zio->io_vd;

        if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE))
                return (ZIO_PIPELINE_STOP);

        if (vd == NULL && !(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
                spa_config_exit(zio->io_spa, SCL_ZIO, zio);

        if (zio->io_vsd != NULL) {
                zio->io_vsd_free(zio);
                zio->io_vsd = NULL;
        }

        if (zio_injection_enabled && zio->io_error == 0)
                zio->io_error = zio_handle_fault_injection(zio, EIO);

        /*
         * If the I/O failed, determine whether we should attempt to retry it.
         */
        if (zio->io_error && vd == NULL &&
            !(zio->io_flags & (ZIO_FLAG_DONT_RETRY | ZIO_FLAG_IO_RETRY))) {
                ASSERT(!(zio->io_flags & ZIO_FLAG_DONT_QUEUE)); /* not a leaf */
                ASSERT(!(zio->io_flags & ZIO_FLAG_IO_BYPASS));  /* not a leaf */
                zio->io_error = 0;
                zio->io_flags |= ZIO_FLAG_IO_RETRY |
                    ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_AGGREGATE;
                zio->io_stage = ZIO_STAGE_VDEV_IO_START - 1;
                zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);
                return (ZIO_PIPELINE_STOP);
        }

        /*
         * If we got an error on a leaf device, convert it to ENXIO
         * if the device is not accessible at all.
         */
        if (zio->io_error && vd != NULL && vd->vdev_ops->vdev_op_leaf &&
            !vdev_accessible(vd, zio))
                zio->io_error = ENXIO;

        /*
         * If we can't write to an interior vdev (mirror or RAID-Z),
         * set vdev_cant_write so that we stop trying to allocate from it.
         */
        if (zio->io_error == ENXIO && zio->io_type == ZIO_TYPE_WRITE &&
            vd != NULL && !vd->vdev_ops->vdev_op_leaf)
                vd->vdev_cant_write = B_TRUE;

        if (zio->io_error)
                zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

        return (ZIO_PIPELINE_CONTINUE);
}

void
zio_vdev_io_reissue(zio_t *zio)
{
        ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_START);
        ASSERT(zio->io_error == 0);

        zio->io_stage--;
}

void
zio_vdev_io_redone(zio_t *zio)
{
        ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_DONE);

        zio->io_stage--;
}

void
zio_vdev_io_bypass(zio_t *zio)
{
        ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_START);
        ASSERT(zio->io_error == 0);

        zio->io_flags |= ZIO_FLAG_IO_BYPASS;
        zio->io_stage = ZIO_STAGE_VDEV_IO_ASSESS - 1;
}

/*
 * ==========================================================================
 * Generate and verify checksums
 * ==========================================================================
 */
static int
zio_checksum_generate(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;
        enum zio_checksum checksum;

        if (bp == NULL) {
                /*
                 * This is zio_write_phys().
                 * We're either generating a label checksum, or none at all.
                 */
                checksum = zio->io_prop.zp_checksum;

                if (checksum == ZIO_CHECKSUM_OFF)
                        return (ZIO_PIPELINE_CONTINUE);

                ASSERT(checksum == ZIO_CHECKSUM_LABEL);
        } else {
                if (BP_IS_GANG(bp) && zio->io_child_type == ZIO_CHILD_GANG) {
                        ASSERT(!IO_IS_ALLOCATING(zio));
                        checksum = ZIO_CHECKSUM_GANG_HEADER;
                } else {
                        checksum = BP_GET_CHECKSUM(bp);
                }
        }

        zio_checksum_compute(zio, checksum, zio->io_data, zio->io_size);

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_checksum_verify(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;
        int error;

        if (bp == NULL) {
                /*
                 * This is zio_read_phys().
                 * We're either verifying a label checksum, or nothing at all.
                 */
                if (zio->io_prop.zp_checksum == ZIO_CHECKSUM_OFF)
                        return (ZIO_PIPELINE_CONTINUE);

                ASSERT(zio->io_prop.zp_checksum == ZIO_CHECKSUM_LABEL);
        }

        if ((error = zio_checksum_error(zio)) != 0) {
                zio->io_error = error;
                if (!(zio->io_flags & ZIO_FLAG_SPECULATIVE)) {
                        zfs_ereport_post(FM_EREPORT_ZFS_CHECKSUM,
                            zio->io_spa, zio->io_vd, zio, 0, 0);
                }
        }

        return (ZIO_PIPELINE_CONTINUE);
}

/*
 * Called by RAID-Z to ensure we don't compute the checksum twice.
 */
void
zio_checksum_verified(zio_t *zio)
{
        zio->io_pipeline &= ~(1U << ZIO_STAGE_CHECKSUM_VERIFY);
}

/*
 * ==========================================================================
 * Error rank.  Error are ranked in the order 0, ENXIO, ECKSUM, EIO, other.
 * An error of 0 indictes success.  ENXIO indicates whole-device failure,
 * which may be transient (e.g. unplugged) or permament.  ECKSUM and EIO
 * indicate errors that are specific to one I/O, and most likely permanent.
 * Any other error is presumed to be worse because we weren't expecting it.
 * ==========================================================================
 */
int
zio_worst_error(int e1, int e2)
{
        static int zio_error_rank[] = { 0, ENXIO, ECKSUM, EIO };
        int r1, r2;

        for (r1 = 0; r1 < sizeof (zio_error_rank) / sizeof (int); r1++)
                if (e1 == zio_error_rank[r1])
                        break;

        for (r2 = 0; r2 < sizeof (zio_error_rank) / sizeof (int); r2++)
                if (e2 == zio_error_rank[r2])
                        break;

        return (r1 > r2 ? e1 : e2);
}

/*
 * ==========================================================================
 * I/O completion
 * ==========================================================================
 */
static int
zio_ready(zio_t *zio)
{
        blkptr_t *bp = zio->io_bp;
        zio_t *pio, *pio_next;

        if (zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_READY))
                return (ZIO_PIPELINE_STOP);

        if (zio->io_ready) {
                ASSERT(IO_IS_ALLOCATING(zio));
                ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp));
                ASSERT(zio->io_children[ZIO_CHILD_GANG][ZIO_WAIT_READY] == 0);

                zio->io_ready(zio);
        }

        if (bp != NULL && bp != &zio->io_bp_copy)
                zio->io_bp_copy = *bp;

        if (zio->io_error)
                zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

        mutex_enter(&zio->io_lock);
        zio->io_state[ZIO_WAIT_READY] = 1;
        pio = zio_walk_parents(zio);
        mutex_exit(&zio->io_lock);

        /*
         * As we notify zio's parents, new parents could be added.
         * New parents go to the head of zio's io_parent_list, however,
         * so we will (correctly) not notify them.  The remainder of zio's
         * io_parent_list, from 'pio_next' onward, cannot change because
         * all parents must wait for us to be done before they can be done.
         */
        for (; pio != NULL; pio = pio_next) {
                pio_next = zio_walk_parents(zio);
                zio_notify_parent(pio, zio, ZIO_WAIT_READY);
        }

        return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_done(zio_t *zio)
{
        spa_t *spa = zio->io_spa;
        zio_t *lio = zio->io_logical;
        blkptr_t *bp = zio->io_bp;
        vdev_t *vd = zio->io_vd;
        uint64_t psize = zio->io_size;
        zio_t *pio, *pio_next;

        /*
         * If our children haven't all completed,
         * wait for them and then repeat this pipeline stage.
         */
        if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE) ||
            zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_DONE) ||
            zio_wait_for_children(zio, ZIO_CHILD_LOGICAL, ZIO_WAIT_DONE))
                return (ZIO_PIPELINE_STOP);

        for (int c = 0; c < ZIO_CHILD_TYPES; c++)
                for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                        ASSERT(zio->io_children[c][w] == 0);

        if (bp != NULL) {
                ASSERT(bp->blk_pad[0] == 0);
                ASSERT(bp->blk_pad[1] == 0);
                ASSERT(bp->blk_pad[2] == 0);
                ASSERT(bcmp(bp, &zio->io_bp_copy, sizeof (blkptr_t)) == 0 ||
                    (bp == zio_unique_parent(zio)->io_bp));
                if (zio->io_type == ZIO_TYPE_WRITE && !BP_IS_HOLE(bp) &&
                    !(zio->io_flags & ZIO_FLAG_IO_REPAIR)) {
                        ASSERT(!BP_SHOULD_BYTESWAP(bp));
                        ASSERT3U(zio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(bp));
                        ASSERT(BP_COUNT_GANG(bp) == 0 ||
                            (BP_COUNT_GANG(bp) == BP_GET_NDVAS(bp)));
                }
        }

        /*
         * If there were child vdev or gang errors, they apply to us now.
         */
        zio_inherit_child_errors(zio, ZIO_CHILD_VDEV);
        zio_inherit_child_errors(zio, ZIO_CHILD_GANG);

        zio_pop_transforms(zio);        /* note: may set zio->io_error */

        vdev_stat_update(zio, psize);

        if (zio->io_error) {
                /*
                 * If this I/O is attached to a particular vdev,
                 * generate an error message describing the I/O failure
                 * at the block level.  We ignore these errors if the
                 * device is currently unavailable.
                 */
                if (zio->io_error != ECKSUM && vd != NULL && !vdev_is_dead(vd))
                        zfs_ereport_post(FM_EREPORT_ZFS_IO, spa, vd, zio, 0, 0);

                if ((zio->io_error == EIO ||
                    !(zio->io_flags & ZIO_FLAG_SPECULATIVE)) && zio == lio) {
                        /*
                         * For logical I/O requests, tell the SPA to log the
                         * error and generate a logical data ereport.
                         */
                        spa_log_error(spa, zio);
                        zfs_ereport_post(FM_EREPORT_ZFS_DATA, spa, NULL, zio,
                            0, 0);
                }
        }

        if (zio->io_error && zio == lio) {
                /*
                 * Determine whether zio should be reexecuted.  This will
                 * propagate all the way to the root via zio_notify_parent().
                 */
                ASSERT(vd == NULL && bp != NULL);

                if (IO_IS_ALLOCATING(zio))
                        if (zio->io_error != ENOSPC)
                                zio->io_reexecute |= ZIO_REEXECUTE_NOW;
                        else
                                zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;

                if ((zio->io_type == ZIO_TYPE_READ ||
                    zio->io_type == ZIO_TYPE_FREE) &&
                    zio->io_error == ENXIO &&
                    spa->spa_load_state == SPA_LOAD_NONE &&
                    spa_get_failmode(spa) != ZIO_FAILURE_MODE_CONTINUE)
                        zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;

                if (!(zio->io_flags & ZIO_FLAG_CANFAIL) && !zio->io_reexecute)
                        zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;
        }

        /*
         * If there were logical child errors, they apply to us now.
         * We defer this until now to avoid conflating logical child
         * errors with errors that happened to the zio itself when
         * updating vdev stats and reporting FMA events above.
         */
        zio_inherit_child_errors(zio, ZIO_CHILD_LOGICAL);

        if ((zio->io_error || zio->io_reexecute) && IO_IS_ALLOCATING(zio) &&
            zio->io_child_type == ZIO_CHILD_LOGICAL) {
                ASSERT(zio->io_child_type != ZIO_CHILD_GANG);
                zio_dva_unallocate(zio, zio->io_gang_tree, bp);
        }

        zio_gang_tree_free(&zio->io_gang_tree);

        /*
         * Godfather I/Os should never suspend.
         */
        if ((zio->io_flags & ZIO_FLAG_GODFATHER) &&
            (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND))
                zio->io_reexecute = 0;

        if (zio->io_reexecute) {
                /*
                 * This is a logical I/O that wants to reexecute.
                 *
                 * Reexecute is top-down.  When an i/o fails, if it's not
                 * the root, it simply notifies its parent and sticks around.
                 * The parent, seeing that it still has children in zio_done(),
                 * does the same.  This percolates all the way up to the root.
                 * The root i/o will reexecute or suspend the entire tree.
                 *
                 * This approach ensures that zio_reexecute() honors
                 * all the original i/o dependency relationships, e.g.
                 * parents not executing until children are ready.
                 */
                ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);

                zio->io_gang_leader = NULL;

                mutex_enter(&zio->io_lock);
                zio->io_state[ZIO_WAIT_DONE] = 1;
                mutex_exit(&zio->io_lock);

                /*
                 * "The Godfather" I/O monitors its children but is
                 * not a true parent to them. It will track them through
                 * the pipeline but severs its ties whenever they get into
                 * trouble (e.g. suspended). This allows "The Godfather"
                 * I/O to return status without blocking.
                 */
                for (pio = zio_walk_parents(zio); pio != NULL; pio = pio_next) {
                        zio_link_t *zl = zio->io_walk_link;
                        pio_next = zio_walk_parents(zio);

                        if ((pio->io_flags & ZIO_FLAG_GODFATHER) &&
                            (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND)) {
                                zio_remove_child(pio, zio, zl);
                                zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
                        }
                }

                if ((pio = zio_unique_parent(zio)) != NULL) {
                        /*
                         * We're not a root i/o, so there's nothing to do
                         * but notify our parent.  Don't propagate errors
                         * upward since we haven't permanently failed yet.
                         */
                        ASSERT(!(zio->io_flags & ZIO_FLAG_GODFATHER));
                        zio->io_flags |= ZIO_FLAG_DONT_PROPAGATE;
                        zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
                } else if (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND) {
                        /*
                         * We'd fail again if we reexecuted now, so suspend
                         * until conditions improve (e.g. device comes online).
                         */
                        zio_suspend(spa, zio);
                } else {
                        /*
                         * Reexecution is potentially a huge amount of work.
                         * Hand it off to the otherwise-unused claim taskq.
                         */
#ifdef _KERNEL
                        (void) taskq_dispatch_safe(
                            spa->spa_zio_taskq[ZIO_TYPE_CLAIM][ZIO_TASKQ_ISSUE],
                            (task_func_t *)zio_reexecute, zio,
                                &zio->io_task_issue);
#else
                        (void) taskq_dispatch(
                            spa->spa_zio_taskq[ZIO_TYPE_CLAIM][ZIO_TASKQ_ISSUE],
                                (task_func_t *)zio_reexecute, zio, TQ_SLEEP);
#endif
                }
                return (ZIO_PIPELINE_STOP);
        }

        ASSERT(zio_walk_children(zio) == NULL);
        ASSERT(zio->io_reexecute == 0);
        ASSERT(zio->io_error == 0 || (zio->io_flags & ZIO_FLAG_CANFAIL));

        /*
         * It is the responsibility of the done callback to ensure that this
         * particular zio is no longer discoverable for adoption, and as
         * such, cannot acquire any new parents.
         */
        if (zio->io_done)
                zio->io_done(zio);

        mutex_enter(&zio->io_lock);
        zio->io_state[ZIO_WAIT_DONE] = 1;
        mutex_exit(&zio->io_lock);

        for (pio = zio_walk_parents(zio); pio != NULL; pio = pio_next) {
                zio_link_t *zl = zio->io_walk_link;
                pio_next = zio_walk_parents(zio);
                zio_remove_child(pio, zio, zl);
                zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
        }

        if (zio->io_waiter != NULL) {
                mutex_enter(&zio->io_lock);
                zio->io_executor = NULL;
                cv_broadcast(&zio->io_cv);
                mutex_exit(&zio->io_lock);
        } else {
                zio_destroy(zio);
        }

        return (ZIO_PIPELINE_STOP);
}

/*
 * ==========================================================================
 * I/O pipeline definition
 * ==========================================================================
 */
static zio_pipe_stage_t *zio_pipeline[ZIO_STAGES] = {
        NULL,
        zio_issue_async,
        zio_read_bp_init,
        zio_write_bp_init,
        zio_checksum_generate,
        zio_gang_assemble,
        zio_gang_issue,
        zio_dva_allocate,
        zio_dva_free,
        zio_dva_claim,
        zio_ready,
        zio_vdev_io_start,
        zio_vdev_io_done,
        zio_vdev_io_assess,
        zio_checksum_verify,
        zio_done
};