MFV r348578: 9962 zil_commit should omit cache thrash

[FreeBSD/FreeBSD.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / zil.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) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
 * Copyright (c) 2014 Integros [integros.com]
 */

/* Portions Copyright 2010 Robert Milkowski */

#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/spa_impl.h>
#include <sys/dmu.h>
#include <sys/zap.h>
#include <sys/arc.h>
#include <sys/stat.h>
#include <sys/resource.h>
#include <sys/zil.h>
#include <sys/zil_impl.h>
#include <sys/dsl_dataset.h>
#include <sys/vdev_impl.h>
#include <sys/dmu_tx.h>
#include <sys/dsl_pool.h>
#include <sys/abd.h>

/*
 * The ZFS Intent Log (ZIL) saves "transaction records" (itxs) of system
 * calls that change the file system. Each itx has enough information to
 * be able to replay them after a system crash, power loss, or
 * equivalent failure mode. These are stored in memory until either:
 *
 *   1. they are committed to the pool by the DMU transaction group
 *      (txg), at which point they can be discarded; or
 *   2. they are committed to the on-disk ZIL for the dataset being
 *      modified (e.g. due to an fsync, O_DSYNC, or other synchronous
 *      requirement).
 *
 * In the event of a crash or power loss, the itxs contained by each
 * dataset's on-disk ZIL will be replayed when that dataset is first
 * instantianted (e.g. if the dataset is a normal fileystem, when it is
 * first mounted).
 *
 * As hinted at above, there is one ZIL per dataset (both the in-memory
 * representation, and the on-disk representation). The on-disk format
 * consists of 3 parts:
 *
 *      - a single, per-dataset, ZIL header; which points to a chain of
 *      - zero or more ZIL blocks; each of which contains
 *      - zero or more ZIL records
 *
 * A ZIL record holds the information necessary to replay a single
 * system call transaction. A ZIL block can hold many ZIL records, and
 * the blocks are chained together, similarly to a singly linked list.
 *
 * Each ZIL block contains a block pointer (blkptr_t) to the next ZIL
 * block in the chain, and the ZIL header points to the first block in
 * the chain.
 *
 * Note, there is not a fixed place in the pool to hold these ZIL
 * blocks; they are dynamically allocated and freed as needed from the
 * blocks available on the pool, though they can be preferentially
 * allocated from a dedicated "log" vdev.
 */

/*
 * This controls the amount of time that a ZIL block (lwb) will remain
 * "open" when it isn't "full", and it has a thread waiting for it to be
 * committed to stable storage. Please refer to the zil_commit_waiter()
 * function (and the comments within it) for more details.
 */
int zfs_commit_timeout_pct = 5;

/*
 * Disable intent logging replay.  This global ZIL switch affects all pools.
 */
int zil_replay_disable = 0;
SYSCTL_DECL(_vfs_zfs);
SYSCTL_INT(_vfs_zfs, OID_AUTO, zil_replay_disable, CTLFLAG_RWTUN,
    &zil_replay_disable, 0, "Disable intent logging replay");

/*
 * Disable the DKIOCFLUSHWRITECACHE commands that are normally sent to
 * the disk(s) by the ZIL after an LWB write has completed. Setting this
 * will cause ZIL corruption on power loss if a volatile out-of-order
 * write cache is enabled.
 */
boolean_t zil_nocacheflush = B_FALSE;
SYSCTL_INT(_vfs_zfs, OID_AUTO, zil_nocacheflush, CTLFLAG_RWTUN,
    &zil_nocacheflush, 0, "Disable ZIL cache flush");

boolean_t zfs_trim_enabled = B_TRUE;
SYSCTL_DECL(_vfs_zfs_trim);
SYSCTL_INT(_vfs_zfs_trim, OID_AUTO, enabled, CTLFLAG_RDTUN, &zfs_trim_enabled, 0,
    "Enable ZFS TRIM");

/*
 * Limit SLOG write size per commit executed with synchronous priority.
 * Any writes above that will be executed with lower (asynchronous) priority
 * to limit potential SLOG device abuse by single active ZIL writer.
 */
uint64_t zil_slog_bulk = 768 * 1024;
SYSCTL_QUAD(_vfs_zfs, OID_AUTO, zil_slog_bulk, CTLFLAG_RWTUN,
    &zil_slog_bulk, 0, "Maximal SLOG commit size with sync priority");

static kmem_cache_t *zil_lwb_cache;
static kmem_cache_t *zil_zcw_cache;

#define LWB_EMPTY(lwb) ((BP_GET_LSIZE(&lwb->lwb_blk) - \
    sizeof (zil_chain_t)) == (lwb->lwb_sz - lwb->lwb_nused))

static int
zil_bp_compare(const void *x1, const void *x2)
{
        const dva_t *dva1 = &((zil_bp_node_t *)x1)->zn_dva;
        const dva_t *dva2 = &((zil_bp_node_t *)x2)->zn_dva;

        int cmp = AVL_CMP(DVA_GET_VDEV(dva1), DVA_GET_VDEV(dva2));
        if (likely(cmp))
                return (cmp);

        return (AVL_CMP(DVA_GET_OFFSET(dva1), DVA_GET_OFFSET(dva2)));
}

static void
zil_bp_tree_init(zilog_t *zilog)
{
        avl_create(&zilog->zl_bp_tree, zil_bp_compare,
            sizeof (zil_bp_node_t), offsetof(zil_bp_node_t, zn_node));
}

static void
zil_bp_tree_fini(zilog_t *zilog)
{
        avl_tree_t *t = &zilog->zl_bp_tree;
        zil_bp_node_t *zn;
        void *cookie = NULL;

        while ((zn = avl_destroy_nodes(t, &cookie)) != NULL)
                kmem_free(zn, sizeof (zil_bp_node_t));

        avl_destroy(t);
}

int
zil_bp_tree_add(zilog_t *zilog, const blkptr_t *bp)
{
        avl_tree_t *t = &zilog->zl_bp_tree;
        const dva_t *dva;
        zil_bp_node_t *zn;
        avl_index_t where;

        if (BP_IS_EMBEDDED(bp))
                return (0);

        dva = BP_IDENTITY(bp);

        if (avl_find(t, dva, &where) != NULL)
                return (SET_ERROR(EEXIST));

        zn = kmem_alloc(sizeof (zil_bp_node_t), KM_SLEEP);
        zn->zn_dva = *dva;
        avl_insert(t, zn, where);

        return (0);
}

static zil_header_t *
zil_header_in_syncing_context(zilog_t *zilog)
{
        return ((zil_header_t *)zilog->zl_header);
}

static void
zil_init_log_chain(zilog_t *zilog, blkptr_t *bp)
{
        zio_cksum_t *zc = &bp->blk_cksum;

        zc->zc_word[ZIL_ZC_GUID_0] = spa_get_random(-1ULL);
        zc->zc_word[ZIL_ZC_GUID_1] = spa_get_random(-1ULL);
        zc->zc_word[ZIL_ZC_OBJSET] = dmu_objset_id(zilog->zl_os);
        zc->zc_word[ZIL_ZC_SEQ] = 1ULL;
}

/*
 * Read a log block and make sure it's valid.
 */
static int
zil_read_log_block(zilog_t *zilog, const blkptr_t *bp, blkptr_t *nbp, void *dst,
    char **end)
{
        enum zio_flag zio_flags = ZIO_FLAG_CANFAIL;
        arc_flags_t aflags = ARC_FLAG_WAIT;
        arc_buf_t *abuf = NULL;
        zbookmark_phys_t zb;
        int error;

        if (zilog->zl_header->zh_claim_txg == 0)
                zio_flags |= ZIO_FLAG_SPECULATIVE | ZIO_FLAG_SCRUB;

        if (!(zilog->zl_header->zh_flags & ZIL_CLAIM_LR_SEQ_VALID))
                zio_flags |= ZIO_FLAG_SPECULATIVE;

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

        error = arc_read(NULL, zilog->zl_spa, bp, arc_getbuf_func, &abuf,
            ZIO_PRIORITY_SYNC_READ, zio_flags, &aflags, &zb);

        if (error == 0) {
                zio_cksum_t cksum = bp->blk_cksum;

                /*
                 * Validate the checksummed log block.
                 *
                 * Sequence numbers should be... sequential.  The checksum
                 * verifier for the next block should be bp's checksum plus 1.
                 *
                 * Also check the log chain linkage and size used.
                 */
                cksum.zc_word[ZIL_ZC_SEQ]++;

                if (BP_GET_CHECKSUM(bp) == ZIO_CHECKSUM_ZILOG2) {
                        zil_chain_t *zilc = abuf->b_data;
                        char *lr = (char *)(zilc + 1);
                        uint64_t len = zilc->zc_nused - sizeof (zil_chain_t);

                        if (bcmp(&cksum, &zilc->zc_next_blk.blk_cksum,
                            sizeof (cksum)) || BP_IS_HOLE(&zilc->zc_next_blk)) {
                                error = SET_ERROR(ECKSUM);
                        } else {
                                ASSERT3U(len, <=, SPA_OLD_MAXBLOCKSIZE);
                                bcopy(lr, dst, len);
                                *end = (char *)dst + len;
                                *nbp = zilc->zc_next_blk;
                        }
                } else {
                        char *lr = abuf->b_data;
                        uint64_t size = BP_GET_LSIZE(bp);
                        zil_chain_t *zilc = (zil_chain_t *)(lr + size) - 1;

                        if (bcmp(&cksum, &zilc->zc_next_blk.blk_cksum,
                            sizeof (cksum)) || BP_IS_HOLE(&zilc->zc_next_blk) ||
                            (zilc->zc_nused > (size - sizeof (*zilc)))) {
                                error = SET_ERROR(ECKSUM);
                        } else {
                                ASSERT3U(zilc->zc_nused, <=,
                                    SPA_OLD_MAXBLOCKSIZE);
                                bcopy(lr, dst, zilc->zc_nused);
                                *end = (char *)dst + zilc->zc_nused;
                                *nbp = zilc->zc_next_blk;
                        }
                }

                arc_buf_destroy(abuf, &abuf);
        }

        return (error);
}

/*
 * Read a TX_WRITE log data block.
 */
static int
zil_read_log_data(zilog_t *zilog, const lr_write_t *lr, void *wbuf)
{
        enum zio_flag zio_flags = ZIO_FLAG_CANFAIL;
        const blkptr_t *bp = &lr->lr_blkptr;
        arc_flags_t aflags = ARC_FLAG_WAIT;
        arc_buf_t *abuf = NULL;
        zbookmark_phys_t zb;
        int error;

        if (BP_IS_HOLE(bp)) {
                if (wbuf != NULL)
                        bzero(wbuf, MAX(BP_GET_LSIZE(bp), lr->lr_length));
                return (0);
        }

        if (zilog->zl_header->zh_claim_txg == 0)
                zio_flags |= ZIO_FLAG_SPECULATIVE | ZIO_FLAG_SCRUB;

        SET_BOOKMARK(&zb, dmu_objset_id(zilog->zl_os), lr->lr_foid,
            ZB_ZIL_LEVEL, lr->lr_offset / BP_GET_LSIZE(bp));

        error = arc_read(NULL, zilog->zl_spa, bp, arc_getbuf_func, &abuf,
            ZIO_PRIORITY_SYNC_READ, zio_flags, &aflags, &zb);

        if (error == 0) {
                if (wbuf != NULL)
                        bcopy(abuf->b_data, wbuf, arc_buf_size(abuf));
                arc_buf_destroy(abuf, &abuf);
        }

        return (error);
}

/*
 * Parse the intent log, and call parse_func for each valid record within.
 */
int
zil_parse(zilog_t *zilog, zil_parse_blk_func_t *parse_blk_func,
    zil_parse_lr_func_t *parse_lr_func, void *arg, uint64_t txg)
{
        const zil_header_t *zh = zilog->zl_header;
        boolean_t claimed = !!zh->zh_claim_txg;
        uint64_t claim_blk_seq = claimed ? zh->zh_claim_blk_seq : UINT64_MAX;
        uint64_t claim_lr_seq = claimed ? zh->zh_claim_lr_seq : UINT64_MAX;
        uint64_t max_blk_seq = 0;
        uint64_t max_lr_seq = 0;
        uint64_t blk_count = 0;
        uint64_t lr_count = 0;
        blkptr_t blk, next_blk;
        char *lrbuf, *lrp;
        int error = 0;

        /*
         * Old logs didn't record the maximum zh_claim_lr_seq.
         */
        if (!(zh->zh_flags & ZIL_CLAIM_LR_SEQ_VALID))
                claim_lr_seq = UINT64_MAX;

        /*
         * Starting at the block pointed to by zh_log we read the log chain.
         * For each block in the chain we strongly check that block to
         * ensure its validity.  We stop when an invalid block is found.
         * For each block pointer in the chain we call parse_blk_func().
         * For each record in each valid block we call parse_lr_func().
         * If the log has been claimed, stop if we encounter a sequence
         * number greater than the highest claimed sequence number.
         */
        lrbuf = zio_buf_alloc(SPA_OLD_MAXBLOCKSIZE);
        zil_bp_tree_init(zilog);

        for (blk = zh->zh_log; !BP_IS_HOLE(&blk); blk = next_blk) {
                uint64_t blk_seq = blk.blk_cksum.zc_word[ZIL_ZC_SEQ];
                int reclen;
                char *end;

                if (blk_seq > claim_blk_seq)
                        break;
                if ((error = parse_blk_func(zilog, &blk, arg, txg)) != 0)
                        break;
                ASSERT3U(max_blk_seq, <, blk_seq);
                max_blk_seq = blk_seq;
                blk_count++;

                if (max_lr_seq == claim_lr_seq && max_blk_seq == claim_blk_seq)
                        break;

                error = zil_read_log_block(zilog, &blk, &next_blk, lrbuf, &end);
                if (error != 0)
                        break;

                for (lrp = lrbuf; lrp < end; lrp += reclen) {
                        lr_t *lr = (lr_t *)lrp;
                        reclen = lr->lrc_reclen;
                        ASSERT3U(reclen, >=, sizeof (lr_t));
                        if (lr->lrc_seq > claim_lr_seq)
                                goto done;
                        if ((error = parse_lr_func(zilog, lr, arg, txg)) != 0)
                                goto done;
                        ASSERT3U(max_lr_seq, <, lr->lrc_seq);
                        max_lr_seq = lr->lrc_seq;
                        lr_count++;
                }
        }
done:
        zilog->zl_parse_error = error;
        zilog->zl_parse_blk_seq = max_blk_seq;
        zilog->zl_parse_lr_seq = max_lr_seq;
        zilog->zl_parse_blk_count = blk_count;
        zilog->zl_parse_lr_count = lr_count;

        ASSERT(!claimed || !(zh->zh_flags & ZIL_CLAIM_LR_SEQ_VALID) ||
            (max_blk_seq == claim_blk_seq && max_lr_seq == claim_lr_seq));

        zil_bp_tree_fini(zilog);
        zio_buf_free(lrbuf, SPA_OLD_MAXBLOCKSIZE);

        return (error);
}

/* ARGSUSED */
static int
zil_clear_log_block(zilog_t *zilog, blkptr_t *bp, void *tx, uint64_t first_txg)
{
        ASSERT(!BP_IS_HOLE(bp));

        /*
         * As we call this function from the context of a rewind to a
         * checkpoint, each ZIL block whose txg is later than the txg
         * that we rewind to is invalid. Thus, we return -1 so
         * zil_parse() doesn't attempt to read it.
         */
        if (bp->blk_birth >= first_txg)
                return (-1);

        if (zil_bp_tree_add(zilog, bp) != 0)
                return (0);

        zio_free(zilog->zl_spa, first_txg, bp);
        return (0);
}

/* ARGSUSED */
static int
zil_noop_log_record(zilog_t *zilog, lr_t *lrc, void *tx, uint64_t first_txg)
{
        return (0);
}

static int
zil_claim_log_block(zilog_t *zilog, blkptr_t *bp, void *tx, uint64_t first_txg)
{
        /*
         * Claim log block if not already committed and not already claimed.
         * If tx == NULL, just verify that the block is claimable.
         */
        if (BP_IS_HOLE(bp) || bp->blk_birth < first_txg ||
            zil_bp_tree_add(zilog, bp) != 0)
                return (0);

        return (zio_wait(zio_claim(NULL, zilog->zl_spa,
            tx == NULL ? 0 : first_txg, bp, spa_claim_notify, NULL,
            ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE | ZIO_FLAG_SCRUB)));
}

static int
zil_claim_log_record(zilog_t *zilog, lr_t *lrc, void *tx, uint64_t first_txg)
{
        lr_write_t *lr = (lr_write_t *)lrc;
        int error;

        if (lrc->lrc_txtype != TX_WRITE)
                return (0);

        /*
         * If the block is not readable, don't claim it.  This can happen
         * in normal operation when a log block is written to disk before
         * some of the dmu_sync() blocks it points to.  In this case, the
         * transaction cannot have been committed to anyone (we would have
         * waited for all writes to be stable first), so it is semantically
         * correct to declare this the end of the log.
         */
        if (lr->lr_blkptr.blk_birth >= first_txg &&
            (error = zil_read_log_data(zilog, lr, NULL)) != 0)
                return (error);
        return (zil_claim_log_block(zilog, &lr->lr_blkptr, tx, first_txg));
}

/* ARGSUSED */
static int
zil_free_log_block(zilog_t *zilog, blkptr_t *bp, void *tx, uint64_t claim_txg)
{
        zio_free(zilog->zl_spa, dmu_tx_get_txg(tx), bp);

        return (0);
}

static int
zil_free_log_record(zilog_t *zilog, lr_t *lrc, void *tx, uint64_t claim_txg)
{
        lr_write_t *lr = (lr_write_t *)lrc;
        blkptr_t *bp = &lr->lr_blkptr;

        /*
         * If we previously claimed it, we need to free it.
         */
        if (claim_txg != 0 && lrc->lrc_txtype == TX_WRITE &&
            bp->blk_birth >= claim_txg && zil_bp_tree_add(zilog, bp) == 0 &&
            !BP_IS_HOLE(bp))
                zio_free(zilog->zl_spa, dmu_tx_get_txg(tx), bp);

        return (0);
}

static int
zil_lwb_vdev_compare(const void *x1, const void *x2)
{
        const uint64_t v1 = ((zil_vdev_node_t *)x1)->zv_vdev;
        const uint64_t v2 = ((zil_vdev_node_t *)x2)->zv_vdev;

        return (AVL_CMP(v1, v2));
}

static lwb_t *
zil_alloc_lwb(zilog_t *zilog, blkptr_t *bp, boolean_t slog, uint64_t txg)
{
        lwb_t *lwb;

        lwb = kmem_cache_alloc(zil_lwb_cache, KM_SLEEP);
        lwb->lwb_zilog = zilog;
        lwb->lwb_blk = *bp;
        lwb->lwb_slog = slog;
        lwb->lwb_state = LWB_STATE_CLOSED;
        lwb->lwb_buf = zio_buf_alloc(BP_GET_LSIZE(bp));
        lwb->lwb_max_txg = txg;
        lwb->lwb_write_zio = NULL;
        lwb->lwb_root_zio = NULL;
        lwb->lwb_tx = NULL;
        lwb->lwb_issued_timestamp = 0;
        if (BP_GET_CHECKSUM(bp) == ZIO_CHECKSUM_ZILOG2) {
                lwb->lwb_nused = sizeof (zil_chain_t);
                lwb->lwb_sz = BP_GET_LSIZE(bp);
        } else {
                lwb->lwb_nused = 0;
                lwb->lwb_sz = BP_GET_LSIZE(bp) - sizeof (zil_chain_t);
        }

        mutex_enter(&zilog->zl_lock);
        list_insert_tail(&zilog->zl_lwb_list, lwb);
        mutex_exit(&zilog->zl_lock);

        ASSERT(!MUTEX_HELD(&lwb->lwb_vdev_lock));
        ASSERT(avl_is_empty(&lwb->lwb_vdev_tree));
        VERIFY(list_is_empty(&lwb->lwb_waiters));

        return (lwb);
}

static void
zil_free_lwb(zilog_t *zilog, lwb_t *lwb)
{
        ASSERT(MUTEX_HELD(&zilog->zl_lock));
        ASSERT(!MUTEX_HELD(&lwb->lwb_vdev_lock));
        VERIFY(list_is_empty(&lwb->lwb_waiters));
        ASSERT(avl_is_empty(&lwb->lwb_vdev_tree));
        ASSERT3P(lwb->lwb_write_zio, ==, NULL);
        ASSERT3P(lwb->lwb_root_zio, ==, NULL);
        ASSERT3U(lwb->lwb_max_txg, <=, spa_syncing_txg(zilog->zl_spa));
        ASSERT(lwb->lwb_state == LWB_STATE_CLOSED ||
            lwb->lwb_state == LWB_STATE_FLUSH_DONE);

        /*
         * Clear the zilog's field to indicate this lwb is no longer
         * valid, and prevent use-after-free errors.
         */
        if (zilog->zl_last_lwb_opened == lwb)
                zilog->zl_last_lwb_opened = NULL;

        kmem_cache_free(zil_lwb_cache, lwb);
}

/*
 * Called when we create in-memory log transactions so that we know
 * to cleanup the itxs at the end of spa_sync().
 */
void
zilog_dirty(zilog_t *zilog, uint64_t txg)
{
        dsl_pool_t *dp = zilog->zl_dmu_pool;
        dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);

        ASSERT(spa_writeable(zilog->zl_spa));

        if (ds->ds_is_snapshot)
                panic("dirtying snapshot!");

        if (txg_list_add(&dp->dp_dirty_zilogs, zilog, txg)) {
                /* up the hold count until we can be written out */
                dmu_buf_add_ref(ds->ds_dbuf, zilog);

                zilog->zl_dirty_max_txg = MAX(txg, zilog->zl_dirty_max_txg);
        }
}

/*
 * Determine if the zil is dirty in the specified txg. Callers wanting to
 * ensure that the dirty state does not change must hold the itxg_lock for
 * the specified txg. Holding the lock will ensure that the zil cannot be
 * dirtied (zil_itx_assign) or cleaned (zil_clean) while we check its current
 * state.
 */
boolean_t
zilog_is_dirty_in_txg(zilog_t *zilog, uint64_t txg)
{
        dsl_pool_t *dp = zilog->zl_dmu_pool;

        if (txg_list_member(&dp->dp_dirty_zilogs, zilog, txg & TXG_MASK))
                return (B_TRUE);
        return (B_FALSE);
}

/*
 * Determine if the zil is dirty. The zil is considered dirty if it has
 * any pending itx records that have not been cleaned by zil_clean().
 */
boolean_t
zilog_is_dirty(zilog_t *zilog)
{
        dsl_pool_t *dp = zilog->zl_dmu_pool;

        for (int t = 0; t < TXG_SIZE; t++) {
                if (txg_list_member(&dp->dp_dirty_zilogs, zilog, t))
                        return (B_TRUE);
        }
        return (B_FALSE);
}

/*
 * Create an on-disk intent log.
 */
static lwb_t *
zil_create(zilog_t *zilog)
{
        const zil_header_t *zh = zilog->zl_header;
        lwb_t *lwb = NULL;
        uint64_t txg = 0;
        dmu_tx_t *tx = NULL;
        blkptr_t blk;
        int error = 0;
        boolean_t slog = FALSE;

        /*
         * Wait for any previous destroy to complete.
         */
        txg_wait_synced(zilog->zl_dmu_pool, zilog->zl_destroy_txg);

        ASSERT(zh->zh_claim_txg == 0);
        ASSERT(zh->zh_replay_seq == 0);

        blk = zh->zh_log;

        /*
         * Allocate an initial log block if:
         *    - there isn't one already
         *    - the existing block is the wrong endianess
         */
        if (BP_IS_HOLE(&blk) || BP_SHOULD_BYTESWAP(&blk)) {
                tx = dmu_tx_create(zilog->zl_os);
                VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
                dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
                txg = dmu_tx_get_txg(tx);

                if (!BP_IS_HOLE(&blk)) {
                        zio_free(zilog->zl_spa, txg, &blk);
                        BP_ZERO(&blk);
                }

                error = zio_alloc_zil(zilog->zl_spa,
                    zilog->zl_os->os_dsl_dataset->ds_object, txg, &blk, NULL,
                    ZIL_MIN_BLKSZ, &slog);

                if (error == 0)
                        zil_init_log_chain(zilog, &blk);
        }

        /*
         * Allocate a log write block (lwb) for the first log block.
         */
        if (error == 0)
                lwb = zil_alloc_lwb(zilog, &blk, slog, txg);

        /*
         * If we just allocated the first log block, commit our transaction
         * and wait for zil_sync() to stuff the block poiner into zh_log.
         * (zh is part of the MOS, so we cannot modify it in open context.)
         */
        if (tx != NULL) {
                dmu_tx_commit(tx);
                txg_wait_synced(zilog->zl_dmu_pool, txg);
        }

        ASSERT(bcmp(&blk, &zh->zh_log, sizeof (blk)) == 0);

        return (lwb);
}

/*
 * In one tx, free all log blocks and clear the log header. If keep_first
 * is set, then we're replaying a log with no content. We want to keep the
 * first block, however, so that the first synchronous transaction doesn't
 * require a txg_wait_synced() in zil_create(). We don't need to
 * txg_wait_synced() here either when keep_first is set, because both
 * zil_create() and zil_destroy() will wait for any in-progress destroys
 * to complete.
 */
void
zil_destroy(zilog_t *zilog, boolean_t keep_first)
{
        const zil_header_t *zh = zilog->zl_header;
        lwb_t *lwb;
        dmu_tx_t *tx;
        uint64_t txg;

        /*
         * Wait for any previous destroy to complete.
         */
        txg_wait_synced(zilog->zl_dmu_pool, zilog->zl_destroy_txg);

        zilog->zl_old_header = *zh;             /* debugging aid */

        if (BP_IS_HOLE(&zh->zh_log))
                return;

        tx = dmu_tx_create(zilog->zl_os);
        VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
        dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
        txg = dmu_tx_get_txg(tx);

        mutex_enter(&zilog->zl_lock);

        ASSERT3U(zilog->zl_destroy_txg, <, txg);
        zilog->zl_destroy_txg = txg;
        zilog->zl_keep_first = keep_first;

        if (!list_is_empty(&zilog->zl_lwb_list)) {
                ASSERT(zh->zh_claim_txg == 0);
                VERIFY(!keep_first);
                while ((lwb = list_head(&zilog->zl_lwb_list)) != NULL) {
                        list_remove(&zilog->zl_lwb_list, lwb);
                        if (lwb->lwb_buf != NULL)
                                zio_buf_free(lwb->lwb_buf, lwb->lwb_sz);
                        zio_free(zilog->zl_spa, txg, &lwb->lwb_blk);
                        zil_free_lwb(zilog, lwb);
                }
        } else if (!keep_first) {
                zil_destroy_sync(zilog, tx);
        }
        mutex_exit(&zilog->zl_lock);

        dmu_tx_commit(tx);
}

void
zil_destroy_sync(zilog_t *zilog, dmu_tx_t *tx)
{
        ASSERT(list_is_empty(&zilog->zl_lwb_list));
        (void) zil_parse(zilog, zil_free_log_block,
            zil_free_log_record, tx, zilog->zl_header->zh_claim_txg);
}

int
zil_claim(dsl_pool_t *dp, dsl_dataset_t *ds, void *txarg)
{
        dmu_tx_t *tx = txarg;
        zilog_t *zilog;
        uint64_t first_txg;
        zil_header_t *zh;
        objset_t *os;
        int error;

        error = dmu_objset_own_obj(dp, ds->ds_object,
            DMU_OST_ANY, B_FALSE, FTAG, &os);
        if (error != 0) {
                /*
                 * EBUSY indicates that the objset is inconsistent, in which
                 * case it can not have a ZIL.
                 */
                if (error != EBUSY) {
                        cmn_err(CE_WARN, "can't open objset for %llu, error %u",
                            (unsigned long long)ds->ds_object, error);
                }
                return (0);
        }

        zilog = dmu_objset_zil(os);
        zh = zil_header_in_syncing_context(zilog);
        ASSERT3U(tx->tx_txg, ==, spa_first_txg(zilog->zl_spa));
        first_txg = spa_min_claim_txg(zilog->zl_spa);

        /*
         * If the spa_log_state is not set to be cleared, check whether
         * the current uberblock is a checkpoint one and if the current
         * header has been claimed before moving on.
         *
         * If the current uberblock is a checkpointed uberblock then
         * one of the following scenarios took place:
         *
         * 1] We are currently rewinding to the checkpoint of the pool.
         * 2] We crashed in the middle of a checkpoint rewind but we
         *    did manage to write the checkpointed uberblock to the
         *    vdev labels, so when we tried to import the pool again
         *    the checkpointed uberblock was selected from the import
         *    procedure.
         *
         * In both cases we want to zero out all the ZIL blocks, except
         * the ones that have been claimed at the time of the checkpoint
         * (their zh_claim_txg != 0). The reason is that these blocks
         * may be corrupted since we may have reused their locations on
         * disk after we took the checkpoint.
         *
         * We could try to set spa_log_state to SPA_LOG_CLEAR earlier
         * when we first figure out whether the current uberblock is
         * checkpointed or not. Unfortunately, that would discard all
         * the logs, including the ones that are claimed, and we would
         * leak space.
         */
        if (spa_get_log_state(zilog->zl_spa) == SPA_LOG_CLEAR ||
            (zilog->zl_spa->spa_uberblock.ub_checkpoint_txg != 0 &&
            zh->zh_claim_txg == 0)) {
                if (!BP_IS_HOLE(&zh->zh_log)) {
                        (void) zil_parse(zilog, zil_clear_log_block,
                            zil_noop_log_record, tx, first_txg);
                }
                BP_ZERO(&zh->zh_log);
                dsl_dataset_dirty(dmu_objset_ds(os), tx);
                dmu_objset_disown(os, FTAG);
                return (0);
        }

        /*
         * If we are not rewinding and opening the pool normally, then
         * the min_claim_txg should be equal to the first txg of the pool.
         */
        ASSERT3U(first_txg, ==, spa_first_txg(zilog->zl_spa));

        /*
         * Claim all log blocks if we haven't already done so, and remember
         * the highest claimed sequence number.  This ensures that if we can
         * read only part of the log now (e.g. due to a missing device),
         * but we can read the entire log later, we will not try to replay
         * or destroy beyond the last block we successfully claimed.
         */
        ASSERT3U(zh->zh_claim_txg, <=, first_txg);
        if (zh->zh_claim_txg == 0 && !BP_IS_HOLE(&zh->zh_log)) {
                (void) zil_parse(zilog, zil_claim_log_block,
                    zil_claim_log_record, tx, first_txg);
                zh->zh_claim_txg = first_txg;
                zh->zh_claim_blk_seq = zilog->zl_parse_blk_seq;
                zh->zh_claim_lr_seq = zilog->zl_parse_lr_seq;
                if (zilog->zl_parse_lr_count || zilog->zl_parse_blk_count > 1)
                        zh->zh_flags |= ZIL_REPLAY_NEEDED;
                zh->zh_flags |= ZIL_CLAIM_LR_SEQ_VALID;
                dsl_dataset_dirty(dmu_objset_ds(os), tx);
        }

        ASSERT3U(first_txg, ==, (spa_last_synced_txg(zilog->zl_spa) + 1));
        dmu_objset_disown(os, FTAG);
        return (0);
}

/*
 * Check the log by walking the log chain.
 * Checksum errors are ok as they indicate the end of the chain.
 * Any other error (no device or read failure) returns an error.
 */
/* ARGSUSED */
int
zil_check_log_chain(dsl_pool_t *dp, dsl_dataset_t *ds, void *tx)
{
        zilog_t *zilog;
        objset_t *os;
        blkptr_t *bp;
        int error;

        ASSERT(tx == NULL);

        error = dmu_objset_from_ds(ds, &os);
        if (error != 0) {
                cmn_err(CE_WARN, "can't open objset %llu, error %d",
                    (unsigned long long)ds->ds_object, error);
                return (0);
        }

        zilog = dmu_objset_zil(os);
        bp = (blkptr_t *)&zilog->zl_header->zh_log;

        if (!BP_IS_HOLE(bp)) {
                vdev_t *vd;
                boolean_t valid = B_TRUE;

                /*
                 * Check the first block and determine if it's on a log device
                 * which may have been removed or faulted prior to loading this
                 * pool.  If so, there's no point in checking the rest of the
                 * log as its content should have already been synced to the
                 * pool.
                 */
                spa_config_enter(os->os_spa, SCL_STATE, FTAG, RW_READER);
                vd = vdev_lookup_top(os->os_spa, DVA_GET_VDEV(&bp->blk_dva[0]));
                if (vd->vdev_islog && vdev_is_dead(vd))
                        valid = vdev_log_state_valid(vd);
                spa_config_exit(os->os_spa, SCL_STATE, FTAG);

                if (!valid)
                        return (0);

                /*
                 * Check whether the current uberblock is checkpointed (e.g.
                 * we are rewinding) and whether the current header has been
                 * claimed or not. If it hasn't then skip verifying it. We
                 * do this because its ZIL blocks may be part of the pool's
                 * state before the rewind, which is no longer valid.
                 */
                zil_header_t *zh = zil_header_in_syncing_context(zilog);
                if (zilog->zl_spa->spa_uberblock.ub_checkpoint_txg != 0 &&
                    zh->zh_claim_txg == 0)
                        return (0);
        }

        /*
         * Because tx == NULL, zil_claim_log_block() will not actually claim
         * any blocks, but just determine whether it is possible to do so.
         * In addition to checking the log chain, zil_claim_log_block()
         * will invoke zio_claim() with a done func of spa_claim_notify(),
         * which will update spa_max_claim_txg.  See spa_load() for details.
         */
        error = zil_parse(zilog, zil_claim_log_block, zil_claim_log_record, tx,
            zilog->zl_header->zh_claim_txg ? -1ULL :
            spa_min_claim_txg(os->os_spa));

        return ((error == ECKSUM || error == ENOENT) ? 0 : error);
}

/*
 * When an itx is "skipped", this function is used to properly mark the
 * waiter as "done, and signal any thread(s) waiting on it. An itx can
 * be skipped (and not committed to an lwb) for a variety of reasons,
 * one of them being that the itx was committed via spa_sync(), prior to
 * it being committed to an lwb; this can happen if a thread calling
 * zil_commit() is racing with spa_sync().
 */
static void
zil_commit_waiter_skip(zil_commit_waiter_t *zcw)
{
        mutex_enter(&zcw->zcw_lock);
        ASSERT3B(zcw->zcw_done, ==, B_FALSE);
        zcw->zcw_done = B_TRUE;
        cv_broadcast(&zcw->zcw_cv);
        mutex_exit(&zcw->zcw_lock);
}

/*
 * This function is used when the given waiter is to be linked into an
 * lwb's "lwb_waiter" list; i.e. when the itx is committed to the lwb.
 * At this point, the waiter will no longer be referenced by the itx,
 * and instead, will be referenced by the lwb.
 */
static void
zil_commit_waiter_link_lwb(zil_commit_waiter_t *zcw, lwb_t *lwb)
{
        /*
         * The lwb_waiters field of the lwb is protected by the zilog's
         * zl_lock, thus it must be held when calling this function.
         */
        ASSERT(MUTEX_HELD(&lwb->lwb_zilog->zl_lock));

        mutex_enter(&zcw->zcw_lock);
        ASSERT(!list_link_active(&zcw->zcw_node));
        ASSERT3P(zcw->zcw_lwb, ==, NULL);
        ASSERT3P(lwb, !=, NULL);
        ASSERT(lwb->lwb_state == LWB_STATE_OPENED ||
            lwb->lwb_state == LWB_STATE_ISSUED ||
            lwb->lwb_state == LWB_STATE_WRITE_DONE);

        list_insert_tail(&lwb->lwb_waiters, zcw);
        zcw->zcw_lwb = lwb;
        mutex_exit(&zcw->zcw_lock);
}

/*
 * This function is used when zio_alloc_zil() fails to allocate a ZIL
 * block, and the given waiter must be linked to the "nolwb waiters"
 * list inside of zil_process_commit_list().
 */
static void
zil_commit_waiter_link_nolwb(zil_commit_waiter_t *zcw, list_t *nolwb)
{
        mutex_enter(&zcw->zcw_lock);
        ASSERT(!list_link_active(&zcw->zcw_node));
        ASSERT3P(zcw->zcw_lwb, ==, NULL);
        list_insert_tail(nolwb, zcw);
        mutex_exit(&zcw->zcw_lock);
}

void
zil_lwb_add_block(lwb_t *lwb, const blkptr_t *bp)
{
        avl_tree_t *t = &lwb->lwb_vdev_tree;
        avl_index_t where;
        zil_vdev_node_t *zv, zvsearch;
        int ndvas = BP_GET_NDVAS(bp);
        int i;

        if (zil_nocacheflush)
                return;

        mutex_enter(&lwb->lwb_vdev_lock);
        for (i = 0; i < ndvas; i++) {
                zvsearch.zv_vdev = DVA_GET_VDEV(&bp->blk_dva[i]);
                if (avl_find(t, &zvsearch, &where) == NULL) {
                        zv = kmem_alloc(sizeof (*zv), KM_SLEEP);
                        zv->zv_vdev = zvsearch.zv_vdev;
                        avl_insert(t, zv, where);
                }
        }
        mutex_exit(&lwb->lwb_vdev_lock);
}

static void
zil_lwb_flush_defer(lwb_t *lwb, lwb_t *nlwb)
{
        avl_tree_t *src = &lwb->lwb_vdev_tree;
        avl_tree_t *dst = &nlwb->lwb_vdev_tree;
        void *cookie = NULL;
        zil_vdev_node_t *zv;

        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_WRITE_DONE);
        ASSERT3S(nlwb->lwb_state, !=, LWB_STATE_WRITE_DONE);
        ASSERT3S(nlwb->lwb_state, !=, LWB_STATE_FLUSH_DONE);

        /*
         * While 'lwb' is at a point in its lifetime where lwb_vdev_tree does
         * not need the protection of lwb_vdev_lock (it will only be modified
         * while holding zilog->zl_lock) as its writes and those of its
         * children have all completed.  The younger 'nlwb' may be waiting on
         * future writes to additional vdevs.
         */
        mutex_enter(&nlwb->lwb_vdev_lock);
        /*
         * Tear down the 'lwb' vdev tree, ensuring that entries which do not
         * exist in 'nlwb' are moved to it, freeing any would-be duplicates.
         */
        while ((zv = avl_destroy_nodes(src, &cookie)) != NULL) {
                avl_index_t where;

                if (avl_find(dst, zv, &where) == NULL) {
                        avl_insert(dst, zv, where);
                } else {
                        kmem_free(zv, sizeof (*zv));
                }
        }
        mutex_exit(&nlwb->lwb_vdev_lock);
}

void
zil_lwb_add_txg(lwb_t *lwb, uint64_t txg)
{
        lwb->lwb_max_txg = MAX(lwb->lwb_max_txg, txg);
}

/*
 * This function is a called after all vdevs associated with a given lwb
 * write have completed their DKIOCFLUSHWRITECACHE command; or as soon
 * as the lwb write completes, if "zil_nocacheflush" is set. Further,
 * all "previous" lwb's will have completed before this function is
 * called; i.e. this function is called for all previous lwbs before
 * it's called for "this" lwb (enforced via zio the dependencies
 * configured in zil_lwb_set_zio_dependency()).
 *
 * The intention is for this function to be called as soon as the
 * contents of an lwb are considered "stable" on disk, and will survive
 * any sudden loss of power. At this point, any threads waiting for the
 * lwb to reach this state are signalled, and the "waiter" structures
 * are marked "done".
 */
static void
zil_lwb_flush_vdevs_done(zio_t *zio)
{
        lwb_t *lwb = zio->io_private;
        zilog_t *zilog = lwb->lwb_zilog;
        dmu_tx_t *tx = lwb->lwb_tx;
        zil_commit_waiter_t *zcw;

        spa_config_exit(zilog->zl_spa, SCL_STATE, lwb);

        zio_buf_free(lwb->lwb_buf, lwb->lwb_sz);

        mutex_enter(&zilog->zl_lock);

        /*
         * Ensure the lwb buffer pointer is cleared before releasing the
         * txg. If we have had an allocation failure and the txg is
         * waiting to sync then we want zil_sync() to remove the lwb so
         * that it's not picked up as the next new one in
         * zil_process_commit_list(). zil_sync() will only remove the
         * lwb if lwb_buf is null.
         */
        lwb->lwb_buf = NULL;
        lwb->lwb_tx = NULL;

        ASSERT3U(lwb->lwb_issued_timestamp, >, 0);
        zilog->zl_last_lwb_latency = gethrtime() - lwb->lwb_issued_timestamp;

        lwb->lwb_root_zio = NULL;

        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_WRITE_DONE);
        lwb->lwb_state = LWB_STATE_FLUSH_DONE;

        if (zilog->zl_last_lwb_opened == lwb) {
                /*
                 * Remember the highest committed log sequence number
                 * for ztest. We only update this value when all the log
                 * writes succeeded, because ztest wants to ASSERT that
                 * it got the whole log chain.
                 */
                zilog->zl_commit_lr_seq = zilog->zl_lr_seq;
        }

        while ((zcw = list_head(&lwb->lwb_waiters)) != NULL) {
                mutex_enter(&zcw->zcw_lock);

                ASSERT(list_link_active(&zcw->zcw_node));
                list_remove(&lwb->lwb_waiters, zcw);

                ASSERT3P(zcw->zcw_lwb, ==, lwb);
                zcw->zcw_lwb = NULL;

                zcw->zcw_zio_error = zio->io_error;

                ASSERT3B(zcw->zcw_done, ==, B_FALSE);
                zcw->zcw_done = B_TRUE;
                cv_broadcast(&zcw->zcw_cv);

                mutex_exit(&zcw->zcw_lock);
        }

        mutex_exit(&zilog->zl_lock);

        /*
         * Now that we've written this log block, we have a stable pointer
         * to the next block in the chain, so it's OK to let the txg in
         * which we allocated the next block sync.
         */
        dmu_tx_commit(tx);
}

/*
 * This is called when an lwb's write zio completes. The callback's
 * purpose is to issue the DKIOCFLUSHWRITECACHE commands for the vdevs
 * in the lwb's lwb_vdev_tree. The tree will contain the vdevs involved
 * in writing out this specific lwb's data, and in the case that cache
 * flushes have been deferred, vdevs involved in writing the data for
 * previous lwbs. The writes corresponding to all the vdevs in the
 * lwb_vdev_tree will have completed by the time this is called, due to
 * the zio dependencies configured in zil_lwb_set_zio_dependency(),
 * which takes deferred flushes into account. The lwb will be "done"
 * once zil_lwb_flush_vdevs_done() is called, which occurs in the zio
 * completion callback for the lwb's root zio.
 */
static void
zil_lwb_write_done(zio_t *zio)
{
        lwb_t *lwb = zio->io_private;
        spa_t *spa = zio->io_spa;
        zilog_t *zilog = lwb->lwb_zilog;
        avl_tree_t *t = &lwb->lwb_vdev_tree;
        void *cookie = NULL;
        zil_vdev_node_t *zv;
        lwb_t *nlwb;

        ASSERT3S(spa_config_held(spa, SCL_STATE, RW_READER), !=, 0);

        ASSERT(BP_GET_COMPRESS(zio->io_bp) == ZIO_COMPRESS_OFF);
        ASSERT(BP_GET_TYPE(zio->io_bp) == DMU_OT_INTENT_LOG);
        ASSERT(BP_GET_LEVEL(zio->io_bp) == 0);
        ASSERT(BP_GET_BYTEORDER(zio->io_bp) == ZFS_HOST_BYTEORDER);
        ASSERT(!BP_IS_GANG(zio->io_bp));
        ASSERT(!BP_IS_HOLE(zio->io_bp));
        ASSERT(BP_GET_FILL(zio->io_bp) == 0);

        abd_put(zio->io_abd);

        mutex_enter(&zilog->zl_lock);
        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_ISSUED);
        lwb->lwb_state = LWB_STATE_WRITE_DONE;
        lwb->lwb_write_zio = NULL;
        nlwb = list_next(&zilog->zl_lwb_list, lwb);
        mutex_exit(&zilog->zl_lock);

        if (avl_numnodes(t) == 0)
                return;

        /*
         * If there was an IO error, we're not going to call zio_flush()
         * on these vdevs, so we simply empty the tree and free the
         * nodes. We avoid calling zio_flush() since there isn't any
         * good reason for doing so, after the lwb block failed to be
         * written out.
         */
        if (zio->io_error != 0) {
                while ((zv = avl_destroy_nodes(t, &cookie)) != NULL)
                        kmem_free(zv, sizeof (*zv));
                return;
        }

        /*
         * If this lwb does not have any threads waiting for it to
         * complete, we want to defer issuing the DKIOCFLUSHWRITECACHE
         * command to the vdevs written to by "this" lwb, and instead
         * rely on the "next" lwb to handle the DKIOCFLUSHWRITECACHE
         * command for those vdevs. Thus, we merge the vdev tree of
         * "this" lwb with the vdev tree of the "next" lwb in the list,
         * and assume the "next" lwb will handle flushing the vdevs (or
         * deferring the flush(s) again).
         *
         * This is a useful performance optimization, especially for
         * workloads with lots of async write activity and few sync
         * write and/or fsync activity, as it has the potential to
         * coalesce multiple flush commands to a vdev into one.
         */
        if (list_head(&lwb->lwb_waiters) == NULL && nlwb != NULL) {
                zil_lwb_flush_defer(lwb, nlwb);
                ASSERT(avl_is_empty(&lwb->lwb_vdev_tree));
                return;
        }

        while ((zv = avl_destroy_nodes(t, &cookie)) != NULL) {
                vdev_t *vd = vdev_lookup_top(spa, zv->zv_vdev);
                if (vd != NULL)
                        zio_flush(lwb->lwb_root_zio, vd);
                kmem_free(zv, sizeof (*zv));
        }
}

static void
zil_lwb_set_zio_dependency(zilog_t *zilog, lwb_t *lwb)
{
        lwb_t *last_lwb_opened = zilog->zl_last_lwb_opened;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT(MUTEX_HELD(&zilog->zl_lock));

        /*
         * The zilog's "zl_last_lwb_opened" field is used to build the
         * lwb/zio dependency chain, which is used to preserve the
         * ordering of lwb completions that is required by the semantics
         * of the ZIL. Each new lwb zio becomes a parent of the
         * "previous" lwb zio, such that the new lwb's zio cannot
         * complete until the "previous" lwb's zio completes.
         *
         * This is required by the semantics of zil_commit(); the commit
         * waiters attached to the lwbs will be woken in the lwb zio's
         * completion callback, so this zio dependency graph ensures the
         * waiters are woken in the correct order (the same order the
         * lwbs were created).
         */
        if (last_lwb_opened != NULL &&
            last_lwb_opened->lwb_state != LWB_STATE_FLUSH_DONE) {
                ASSERT(last_lwb_opened->lwb_state == LWB_STATE_OPENED ||
                    last_lwb_opened->lwb_state == LWB_STATE_ISSUED ||
                    last_lwb_opened->lwb_state == LWB_STATE_WRITE_DONE);

                ASSERT3P(last_lwb_opened->lwb_root_zio, !=, NULL);
                zio_add_child(lwb->lwb_root_zio,
                    last_lwb_opened->lwb_root_zio);

                /*
                 * If the previous lwb's write hasn't already completed,
                 * we also want to order the completion of the lwb write
                 * zios (above, we only order the completion of the lwb
                 * root zios). This is required because of how we can
                 * defer the DKIOCFLUSHWRITECACHE commands for each lwb.
                 *
                 * When the DKIOCFLUSHWRITECACHE commands are defered,
                 * the previous lwb will rely on this lwb to flush the
                 * vdevs written to by that previous lwb. Thus, we need
                 * to ensure this lwb doesn't issue the flush until
                 * after the previous lwb's write completes. We ensure
                 * this ordering by setting the zio parent/child
                 * relationship here.
                 *
                 * Without this relationship on the lwb's write zio,
                 * it's possible for this lwb's write to complete prior
                 * to the previous lwb's write completing; and thus, the
                 * vdevs for the previous lwb would be flushed prior to
                 * that lwb's data being written to those vdevs (the
                 * vdevs are flushed in the lwb write zio's completion
                 * handler, zil_lwb_write_done()).
                 */
                if (last_lwb_opened->lwb_state != LWB_STATE_WRITE_DONE) {
                        ASSERT(last_lwb_opened->lwb_state == LWB_STATE_OPENED ||
                            last_lwb_opened->lwb_state == LWB_STATE_ISSUED);

                        ASSERT3P(last_lwb_opened->lwb_write_zio, !=, NULL);
                        zio_add_child(lwb->lwb_write_zio,
                            last_lwb_opened->lwb_write_zio);
                }
        }
}


/*
 * This function's purpose is to "open" an lwb such that it is ready to
 * accept new itxs being committed to it. To do this, the lwb's zio
 * structures are created, and linked to the lwb. This function is
 * idempotent; if the passed in lwb has already been opened, this
 * function is essentially a no-op.
 */
static void
zil_lwb_write_open(zilog_t *zilog, lwb_t *lwb)
{
        zbookmark_phys_t zb;
        zio_priority_t prio;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT3P(lwb, !=, NULL);
        EQUIV(lwb->lwb_root_zio == NULL, lwb->lwb_state == LWB_STATE_CLOSED);
        EQUIV(lwb->lwb_root_zio != NULL, lwb->lwb_state == LWB_STATE_OPENED);

        SET_BOOKMARK(&zb, lwb->lwb_blk.blk_cksum.zc_word[ZIL_ZC_OBJSET],
            ZB_ZIL_OBJECT, ZB_ZIL_LEVEL,
            lwb->lwb_blk.blk_cksum.zc_word[ZIL_ZC_SEQ]);

        if (lwb->lwb_root_zio == NULL) {
                abd_t *lwb_abd = abd_get_from_buf(lwb->lwb_buf,
                    BP_GET_LSIZE(&lwb->lwb_blk));

                if (!lwb->lwb_slog || zilog->zl_cur_used <= zil_slog_bulk)
                        prio = ZIO_PRIORITY_SYNC_WRITE;
                else
                        prio = ZIO_PRIORITY_ASYNC_WRITE;

                lwb->lwb_root_zio = zio_root(zilog->zl_spa,
                    zil_lwb_flush_vdevs_done, lwb, ZIO_FLAG_CANFAIL);
                ASSERT3P(lwb->lwb_root_zio, !=, NULL);

                lwb->lwb_write_zio = zio_rewrite(lwb->lwb_root_zio,
                    zilog->zl_spa, 0, &lwb->lwb_blk, lwb_abd,
                    BP_GET_LSIZE(&lwb->lwb_blk), zil_lwb_write_done, lwb,
                    prio, ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE, &zb);
                ASSERT3P(lwb->lwb_write_zio, !=, NULL);

                lwb->lwb_state = LWB_STATE_OPENED;

                mutex_enter(&zilog->zl_lock);
                zil_lwb_set_zio_dependency(zilog, lwb);
                zilog->zl_last_lwb_opened = lwb;
                mutex_exit(&zilog->zl_lock);
        }

        ASSERT3P(lwb->lwb_root_zio, !=, NULL);
        ASSERT3P(lwb->lwb_write_zio, !=, NULL);
        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_OPENED);
}

/*
 * Define a limited set of intent log block sizes.
 *
 * These must be a multiple of 4KB. Note only the amount used (again
 * aligned to 4KB) actually gets written. However, we can't always just
 * allocate SPA_OLD_MAXBLOCKSIZE as the slog space could be exhausted.
 */
uint64_t zil_block_buckets[] = {
    4096,               /* non TX_WRITE */
    8192+4096,          /* data base */
    32*1024 + 4096,     /* NFS writes */
    UINT64_MAX
};

/*
 * Start a log block write and advance to the next log block.
 * Calls are serialized.
 */
static lwb_t *
zil_lwb_write_issue(zilog_t *zilog, lwb_t *lwb)
{
        lwb_t *nlwb = NULL;
        zil_chain_t *zilc;
        spa_t *spa = zilog->zl_spa;
        blkptr_t *bp;
        dmu_tx_t *tx;
        uint64_t txg;
        uint64_t zil_blksz, wsz;
        int i, error;
        boolean_t slog;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT3P(lwb->lwb_root_zio, !=, NULL);
        ASSERT3P(lwb->lwb_write_zio, !=, NULL);
        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_OPENED);

        if (BP_GET_CHECKSUM(&lwb->lwb_blk) == ZIO_CHECKSUM_ZILOG2) {
                zilc = (zil_chain_t *)lwb->lwb_buf;
                bp = &zilc->zc_next_blk;
        } else {
                zilc = (zil_chain_t *)(lwb->lwb_buf + lwb->lwb_sz);
                bp = &zilc->zc_next_blk;
        }

        ASSERT(lwb->lwb_nused <= lwb->lwb_sz);

        /*
         * Allocate the next block and save its address in this block
         * before writing it in order to establish the log chain.
         * Note that if the allocation of nlwb synced before we wrote
         * the block that points at it (lwb), we'd leak it if we crashed.
         * Therefore, we don't do dmu_tx_commit() until zil_lwb_write_done().
         * We dirty the dataset to ensure that zil_sync() will be called
         * to clean up in the event of allocation failure or I/O failure.
         */

        tx = dmu_tx_create(zilog->zl_os);

        /*
         * Since we are not going to create any new dirty data, and we
         * can even help with clearing the existing dirty data, we
         * should not be subject to the dirty data based delays. We
         * use TXG_NOTHROTTLE to bypass the delay mechanism.
         */
        VERIFY0(dmu_tx_assign(tx, TXG_WAIT | TXG_NOTHROTTLE));

        dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
        txg = dmu_tx_get_txg(tx);

        lwb->lwb_tx = tx;

        /*
         * Log blocks are pre-allocated. Here we select the size of the next
         * block, based on size used in the last block.
         * - first find the smallest bucket that will fit the block from a
         *   limited set of block sizes. This is because it's faster to write
         *   blocks allocated from the same metaslab as they are adjacent or
         *   close.
         * - next find the maximum from the new suggested size and an array of
         *   previous sizes. This lessens a picket fence effect of wrongly
         *   guesssing the size if we have a stream of say 2k, 64k, 2k, 64k
         *   requests.
         *
         * Note we only write what is used, but we can't just allocate
         * the maximum block size because we can exhaust the available
         * pool log space.
         */
        zil_blksz = zilog->zl_cur_used + sizeof (zil_chain_t);
        for (i = 0; zil_blksz > zil_block_buckets[i]; i++)
                continue;
        zil_blksz = zil_block_buckets[i];
        if (zil_blksz == UINT64_MAX)
                zil_blksz = SPA_OLD_MAXBLOCKSIZE;
        zilog->zl_prev_blks[zilog->zl_prev_rotor] = zil_blksz;
        for (i = 0; i < ZIL_PREV_BLKS; i++)
                zil_blksz = MAX(zil_blksz, zilog->zl_prev_blks[i]);
        zilog->zl_prev_rotor = (zilog->zl_prev_rotor + 1) & (ZIL_PREV_BLKS - 1);

        BP_ZERO(bp);

        /* pass the old blkptr in order to spread log blocks across devs */
        error = zio_alloc_zil(spa, zilog->zl_os->os_dsl_dataset->ds_object,
            txg, bp, &lwb->lwb_blk, zil_blksz, &slog);
        if (error == 0) {
                ASSERT3U(bp->blk_birth, ==, txg);
                bp->blk_cksum = lwb->lwb_blk.blk_cksum;
                bp->blk_cksum.zc_word[ZIL_ZC_SEQ]++;

                /*
                 * Allocate a new log write block (lwb).
                 */
                nlwb = zil_alloc_lwb(zilog, bp, slog, txg);
        }

        if (BP_GET_CHECKSUM(&lwb->lwb_blk) == ZIO_CHECKSUM_ZILOG2) {
                /* For Slim ZIL only write what is used. */
                wsz = P2ROUNDUP_TYPED(lwb->lwb_nused, ZIL_MIN_BLKSZ, uint64_t);
                ASSERT3U(wsz, <=, lwb->lwb_sz);
                zio_shrink(lwb->lwb_write_zio, wsz);

        } else {
                wsz = lwb->lwb_sz;
        }

        zilc->zc_pad = 0;
        zilc->zc_nused = lwb->lwb_nused;
        zilc->zc_eck.zec_cksum = lwb->lwb_blk.blk_cksum;

        /*
         * clear unused data for security
         */
        bzero(lwb->lwb_buf + lwb->lwb_nused, wsz - lwb->lwb_nused);

        spa_config_enter(zilog->zl_spa, SCL_STATE, lwb, RW_READER);

        zil_lwb_add_block(lwb, &lwb->lwb_blk);
        lwb->lwb_issued_timestamp = gethrtime();
        lwb->lwb_state = LWB_STATE_ISSUED;

        zio_nowait(lwb->lwb_root_zio);
        zio_nowait(lwb->lwb_write_zio);

        /*
         * If there was an allocation failure then nlwb will be null which
         * forces a txg_wait_synced().
         */
        return (nlwb);
}

static lwb_t *
zil_lwb_commit(zilog_t *zilog, itx_t *itx, lwb_t *lwb)
{
        lr_t *lrcb, *lrc;
        lr_write_t *lrwb, *lrw;
        char *lr_buf;
        uint64_t dlen, dnow, lwb_sp, reclen, txg;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT3P(lwb, !=, NULL);
        ASSERT3P(lwb->lwb_buf, !=, NULL);

        zil_lwb_write_open(zilog, lwb);

        lrc = &itx->itx_lr;
        lrw = (lr_write_t *)lrc;

        /*
         * A commit itx doesn't represent any on-disk state; instead
         * it's simply used as a place holder on the commit list, and
         * provides a mechanism for attaching a "commit waiter" onto the
         * correct lwb (such that the waiter can be signalled upon
         * completion of that lwb). Thus, we don't process this itx's
         * log record if it's a commit itx (these itx's don't have log
         * records), and instead link the itx's waiter onto the lwb's
         * list of waiters.
         *
         * For more details, see the comment above zil_commit().
         */
        if (lrc->lrc_txtype == TX_COMMIT) {
                mutex_enter(&zilog->zl_lock);
                zil_commit_waiter_link_lwb(itx->itx_private, lwb);
                itx->itx_private = NULL;
                mutex_exit(&zilog->zl_lock);
                return (lwb);
        }

        if (lrc->lrc_txtype == TX_WRITE && itx->itx_wr_state == WR_NEED_COPY) {
                dlen = P2ROUNDUP_TYPED(
                    lrw->lr_length, sizeof (uint64_t), uint64_t);
        } else {
                dlen = 0;
        }
        reclen = lrc->lrc_reclen;
        zilog->zl_cur_used += (reclen + dlen);
        txg = lrc->lrc_txg;

        ASSERT3U(zilog->zl_cur_used, <, UINT64_MAX - (reclen + dlen));

cont:
        /*
         * If this record won't fit in the current log block, start a new one.
         * For WR_NEED_COPY optimize layout for minimal number of chunks.
         */
        lwb_sp = lwb->lwb_sz - lwb->lwb_nused;
        if (reclen > lwb_sp || (reclen + dlen > lwb_sp &&
            lwb_sp < ZIL_MAX_WASTE_SPACE && (dlen % ZIL_MAX_LOG_DATA == 0 ||
            lwb_sp < reclen + dlen % ZIL_MAX_LOG_DATA))) {
                lwb = zil_lwb_write_issue(zilog, lwb);
                if (lwb == NULL)
                        return (NULL);
                zil_lwb_write_open(zilog, lwb);
                ASSERT(LWB_EMPTY(lwb));
                lwb_sp = lwb->lwb_sz - lwb->lwb_nused;
                ASSERT3U(reclen + MIN(dlen, sizeof (uint64_t)), <=, lwb_sp);
        }

        dnow = MIN(dlen, lwb_sp - reclen);
        lr_buf = lwb->lwb_buf + lwb->lwb_nused;
        bcopy(lrc, lr_buf, reclen);
        lrcb = (lr_t *)lr_buf;          /* Like lrc, but inside lwb. */
        lrwb = (lr_write_t *)lrcb;      /* Like lrw, but inside lwb. */

        /*
         * If it's a write, fetch the data or get its blkptr as appropriate.
         */
        if (lrc->lrc_txtype == TX_WRITE) {
                if (txg > spa_freeze_txg(zilog->zl_spa))
                        txg_wait_synced(zilog->zl_dmu_pool, txg);
                if (itx->itx_wr_state != WR_COPIED) {
                        char *dbuf;
                        int error;

                        if (itx->itx_wr_state == WR_NEED_COPY) {
                                dbuf = lr_buf + reclen;
                                lrcb->lrc_reclen += dnow;
                                if (lrwb->lr_length > dnow)
                                        lrwb->lr_length = dnow;
                                lrw->lr_offset += dnow;
                                lrw->lr_length -= dnow;
                        } else {
                                ASSERT(itx->itx_wr_state == WR_INDIRECT);
                                dbuf = NULL;
                        }

                        /*
                         * We pass in the "lwb_write_zio" rather than
                         * "lwb_root_zio" so that the "lwb_write_zio"
                         * becomes the parent of any zio's created by
                         * the "zl_get_data" callback. The vdevs are
                         * flushed after the "lwb_write_zio" completes,
                         * so we want to make sure that completion
                         * callback waits for these additional zio's,
                         * such that the vdevs used by those zio's will
                         * be included in the lwb's vdev tree, and those
                         * vdevs will be properly flushed. If we passed
                         * in "lwb_root_zio" here, then these additional
                         * vdevs may not be flushed; e.g. if these zio's
                         * completed after "lwb_write_zio" completed.
                         */
                        error = zilog->zl_get_data(itx->itx_private,
                            lrwb, dbuf, lwb, lwb->lwb_write_zio);

                        if (error == EIO) {
                                txg_wait_synced(zilog->zl_dmu_pool, txg);
                                return (lwb);
                        }
                        if (error != 0) {
                                ASSERT(error == ENOENT || error == EEXIST ||
                                    error == EALREADY);
                                return (lwb);
                        }
                }
        }

        /*
         * We're actually making an entry, so update lrc_seq to be the
         * log record sequence number.  Note that this is generally not
         * equal to the itx sequence number because not all transactions
         * are synchronous, and sometimes spa_sync() gets there first.
         */
        lrcb->lrc_seq = ++zilog->zl_lr_seq;
        lwb->lwb_nused += reclen + dnow;

        zil_lwb_add_txg(lwb, txg);

        ASSERT3U(lwb->lwb_nused, <=, lwb->lwb_sz);
        ASSERT0(P2PHASE(lwb->lwb_nused, sizeof (uint64_t)));

        dlen -= dnow;
        if (dlen > 0) {
                zilog->zl_cur_used += reclen;
                goto cont;
        }

        return (lwb);
}

itx_t *
zil_itx_create(uint64_t txtype, size_t lrsize)
{
        itx_t *itx;

        lrsize = P2ROUNDUP_TYPED(lrsize, sizeof (uint64_t), size_t);

        itx = kmem_alloc(offsetof(itx_t, itx_lr) + lrsize, KM_SLEEP);
        itx->itx_lr.lrc_txtype = txtype;
        itx->itx_lr.lrc_reclen = lrsize;
        itx->itx_lr.lrc_seq = 0;        /* defensive */
        itx->itx_sync = B_TRUE;         /* default is synchronous */

        return (itx);
}

void
zil_itx_destroy(itx_t *itx)
{
        kmem_free(itx, offsetof(itx_t, itx_lr) + itx->itx_lr.lrc_reclen);
}

/*
 * Free up the sync and async itxs. The itxs_t has already been detached
 * so no locks are needed.
 */
static void
zil_itxg_clean(itxs_t *itxs)
{
        itx_t *itx;
        list_t *list;
        avl_tree_t *t;
        void *cookie;
        itx_async_node_t *ian;

        list = &itxs->i_sync_list;
        while ((itx = list_head(list)) != NULL) {
                /*
                 * In the general case, commit itxs will not be found
                 * here, as they'll be committed to an lwb via
                 * zil_lwb_commit(), and free'd in that function. Having
                 * said that, it is still possible for commit itxs to be
                 * found here, due to the following race:
                 *
                 *      - a thread calls zil_commit() which assigns the
                 *        commit itx to a per-txg i_sync_list
                 *      - zil_itxg_clean() is called (e.g. via spa_sync())
                 *        while the waiter is still on the i_sync_list
                 *
                 * There's nothing to prevent syncing the txg while the
                 * waiter is on the i_sync_list. This normally doesn't
                 * happen because spa_sync() is slower than zil_commit(),
                 * but if zil_commit() calls txg_wait_synced() (e.g.
                 * because zil_create() or zil_commit_writer_stall() is
                 * called) we will hit this case.
                 */
                if (itx->itx_lr.lrc_txtype == TX_COMMIT)
                        zil_commit_waiter_skip(itx->itx_private);

                list_remove(list, itx);
                zil_itx_destroy(itx);
        }

        cookie = NULL;
        t = &itxs->i_async_tree;
        while ((ian = avl_destroy_nodes(t, &cookie)) != NULL) {
                list = &ian->ia_list;
                while ((itx = list_head(list)) != NULL) {
                        list_remove(list, itx);
                        /* commit itxs should never be on the async lists. */
                        ASSERT3U(itx->itx_lr.lrc_txtype, !=, TX_COMMIT);
                        zil_itx_destroy(itx);
                }
                list_destroy(list);
                kmem_free(ian, sizeof (itx_async_node_t));
        }
        avl_destroy(t);

        kmem_free(itxs, sizeof (itxs_t));
}

static int
zil_aitx_compare(const void *x1, const void *x2)
{
        const uint64_t o1 = ((itx_async_node_t *)x1)->ia_foid;
        const uint64_t o2 = ((itx_async_node_t *)x2)->ia_foid;

        return (AVL_CMP(o1, o2));
}

/*
 * Remove all async itx with the given oid.
 */
static void
zil_remove_async(zilog_t *zilog, uint64_t oid)
{
        uint64_t otxg, txg;
        itx_async_node_t *ian;
        avl_tree_t *t;
        avl_index_t where;
        list_t clean_list;
        itx_t *itx;

        ASSERT(oid != 0);
        list_create(&clean_list, sizeof (itx_t), offsetof(itx_t, itx_node));

        if (spa_freeze_txg(zilog->zl_spa) != UINT64_MAX) /* ziltest support */
                otxg = ZILTEST_TXG;
        else
                otxg = spa_last_synced_txg(zilog->zl_spa) + 1;

        for (txg = otxg; txg < (otxg + TXG_CONCURRENT_STATES); txg++) {
                itxg_t *itxg = &zilog->zl_itxg[txg & TXG_MASK];

                mutex_enter(&itxg->itxg_lock);
                if (itxg->itxg_txg != txg) {
                        mutex_exit(&itxg->itxg_lock);
                        continue;
                }

                /*
                 * Locate the object node and append its list.
                 */
                t = &itxg->itxg_itxs->i_async_tree;
                ian = avl_find(t, &oid, &where);
                if (ian != NULL)
                        list_move_tail(&clean_list, &ian->ia_list);
                mutex_exit(&itxg->itxg_lock);
        }
        while ((itx = list_head(&clean_list)) != NULL) {
                list_remove(&clean_list, itx);
                /* commit itxs should never be on the async lists. */
                ASSERT3U(itx->itx_lr.lrc_txtype, !=, TX_COMMIT);
                zil_itx_destroy(itx);
        }
        list_destroy(&clean_list);
}

void
zil_itx_assign(zilog_t *zilog, itx_t *itx, dmu_tx_t *tx)
{
        uint64_t txg;
        itxg_t *itxg;
        itxs_t *itxs, *clean = NULL;

        /*
         * Object ids can be re-instantiated in the next txg so
         * remove any async transactions to avoid future leaks.
         * This can happen if a fsync occurs on the re-instantiated
         * object for a WR_INDIRECT or WR_NEED_COPY write, which gets
         * the new file data and flushes a write record for the old object.
         */
        if ((itx->itx_lr.lrc_txtype & ~TX_CI) == TX_REMOVE)
                zil_remove_async(zilog, itx->itx_oid);

        /*
         * Ensure the data of a renamed file is committed before the rename.
         */
        if ((itx->itx_lr.lrc_txtype & ~TX_CI) == TX_RENAME)
                zil_async_to_sync(zilog, itx->itx_oid);

        if (spa_freeze_txg(zilog->zl_spa) != UINT64_MAX)
                txg = ZILTEST_TXG;
        else
                txg = dmu_tx_get_txg(tx);

        itxg = &zilog->zl_itxg[txg & TXG_MASK];
        mutex_enter(&itxg->itxg_lock);
        itxs = itxg->itxg_itxs;
        if (itxg->itxg_txg != txg) {
                if (itxs != NULL) {
                        /*
                         * The zil_clean callback hasn't got around to cleaning
                         * this itxg. Save the itxs for release below.
                         * This should be rare.
                         */
                        zfs_dbgmsg("zil_itx_assign: missed itx cleanup for "
                            "txg %llu", itxg->itxg_txg);
                        clean = itxg->itxg_itxs;
                }
                itxg->itxg_txg = txg;
                itxs = itxg->itxg_itxs = kmem_zalloc(sizeof (itxs_t), KM_SLEEP);

                list_create(&itxs->i_sync_list, sizeof (itx_t),
                    offsetof(itx_t, itx_node));
                avl_create(&itxs->i_async_tree, zil_aitx_compare,
                    sizeof (itx_async_node_t),
                    offsetof(itx_async_node_t, ia_node));
        }
        if (itx->itx_sync) {
                list_insert_tail(&itxs->i_sync_list, itx);
        } else {
                avl_tree_t *t = &itxs->i_async_tree;
                uint64_t foid =
                    LR_FOID_GET_OBJ(((lr_ooo_t *)&itx->itx_lr)->lr_foid);
                itx_async_node_t *ian;
                avl_index_t where;

                ian = avl_find(t, &foid, &where);
                if (ian == NULL) {
                        ian = kmem_alloc(sizeof (itx_async_node_t), KM_SLEEP);
                        list_create(&ian->ia_list, sizeof (itx_t),
                            offsetof(itx_t, itx_node));
                        ian->ia_foid = foid;
                        avl_insert(t, ian, where);
                }
                list_insert_tail(&ian->ia_list, itx);
        }

        itx->itx_lr.lrc_txg = dmu_tx_get_txg(tx);

        /*
         * We don't want to dirty the ZIL using ZILTEST_TXG, because
         * zil_clean() will never be called using ZILTEST_TXG. Thus, we
         * need to be careful to always dirty the ZIL using the "real"
         * TXG (not itxg_txg) even when the SPA is frozen.
         */
        zilog_dirty(zilog, dmu_tx_get_txg(tx));
        mutex_exit(&itxg->itxg_lock);

        /* Release the old itxs now we've dropped the lock */
        if (clean != NULL)
                zil_itxg_clean(clean);
}

/*
 * If there are any in-memory intent log transactions which have now been
 * synced then start up a taskq to free them. We should only do this after we
 * have written out the uberblocks (i.e. txg has been comitted) so that
 * don't inadvertently clean out in-memory log records that would be required
 * by zil_commit().
 */
void
zil_clean(zilog_t *zilog, uint64_t synced_txg)
{
        itxg_t *itxg = &zilog->zl_itxg[synced_txg & TXG_MASK];
        itxs_t *clean_me;

        ASSERT3U(synced_txg, <, ZILTEST_TXG);

        mutex_enter(&itxg->itxg_lock);
        if (itxg->itxg_itxs == NULL || itxg->itxg_txg == ZILTEST_TXG) {
                mutex_exit(&itxg->itxg_lock);
                return;
        }
        ASSERT3U(itxg->itxg_txg, <=, synced_txg);
        ASSERT3U(itxg->itxg_txg, !=, 0);
        clean_me = itxg->itxg_itxs;
        itxg->itxg_itxs = NULL;
        itxg->itxg_txg = 0;
        mutex_exit(&itxg->itxg_lock);
        /*
         * Preferably start a task queue to free up the old itxs but
         * if taskq_dispatch can't allocate resources to do that then
         * free it in-line. This should be rare. Note, using TQ_SLEEP
         * created a bad performance problem.
         */
        ASSERT3P(zilog->zl_dmu_pool, !=, NULL);
        ASSERT3P(zilog->zl_dmu_pool->dp_zil_clean_taskq, !=, NULL);
        if (taskq_dispatch(zilog->zl_dmu_pool->dp_zil_clean_taskq,
            (void (*)(void *))zil_itxg_clean, clean_me, TQ_NOSLEEP) == 0)
                zil_itxg_clean(clean_me);
}

/*
 * This function will traverse the queue of itxs that need to be
 * committed, and move them onto the ZIL's zl_itx_commit_list.
 */
static void
zil_get_commit_list(zilog_t *zilog)
{
        uint64_t otxg, txg;
        list_t *commit_list = &zilog->zl_itx_commit_list;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));

        if (spa_freeze_txg(zilog->zl_spa) != UINT64_MAX) /* ziltest support */
                otxg = ZILTEST_TXG;
        else
                otxg = spa_last_synced_txg(zilog->zl_spa) + 1;

        /*
         * This is inherently racy, since there is nothing to prevent
         * the last synced txg from changing. That's okay since we'll
         * only commit things in the future.
         */
        for (txg = otxg; txg < (otxg + TXG_CONCURRENT_STATES); txg++) {
                itxg_t *itxg = &zilog->zl_itxg[txg & TXG_MASK];

                mutex_enter(&itxg->itxg_lock);
                if (itxg->itxg_txg != txg) {
                        mutex_exit(&itxg->itxg_lock);
                        continue;
                }

                /*
                 * If we're adding itx records to the zl_itx_commit_list,
                 * then the zil better be dirty in this "txg". We can assert
                 * that here since we're holding the itxg_lock which will
                 * prevent spa_sync from cleaning it. Once we add the itxs
                 * to the zl_itx_commit_list we must commit it to disk even
                 * if it's unnecessary (i.e. the txg was synced).
                 */
                ASSERT(zilog_is_dirty_in_txg(zilog, txg) ||
                    spa_freeze_txg(zilog->zl_spa) != UINT64_MAX);
                list_move_tail(commit_list, &itxg->itxg_itxs->i_sync_list);

                mutex_exit(&itxg->itxg_lock);
        }
}

/*
 * Move the async itxs for a specified object to commit into sync lists.
 */
void
zil_async_to_sync(zilog_t *zilog, uint64_t foid)
{
        uint64_t otxg, txg;
        itx_async_node_t *ian;
        avl_tree_t *t;
        avl_index_t where;

        if (spa_freeze_txg(zilog->zl_spa) != UINT64_MAX) /* ziltest support */
                otxg = ZILTEST_TXG;
        else
                otxg = spa_last_synced_txg(zilog->zl_spa) + 1;

        /*
         * This is inherently racy, since there is nothing to prevent
         * the last synced txg from changing.
         */
        for (txg = otxg; txg < (otxg + TXG_CONCURRENT_STATES); txg++) {
                itxg_t *itxg = &zilog->zl_itxg[txg & TXG_MASK];

                mutex_enter(&itxg->itxg_lock);
                if (itxg->itxg_txg != txg) {
                        mutex_exit(&itxg->itxg_lock);
                        continue;
                }

                /*
                 * If a foid is specified then find that node and append its
                 * list. Otherwise walk the tree appending all the lists
                 * to the sync list. We add to the end rather than the
                 * beginning to ensure the create has happened.
                 */
                t = &itxg->itxg_itxs->i_async_tree;
                if (foid != 0) {
                        ian = avl_find(t, &foid, &where);
                        if (ian != NULL) {
                                list_move_tail(&itxg->itxg_itxs->i_sync_list,
                                    &ian->ia_list);
                        }
                } else {
                        void *cookie = NULL;

                        while ((ian = avl_destroy_nodes(t, &cookie)) != NULL) {
                                list_move_tail(&itxg->itxg_itxs->i_sync_list,
                                    &ian->ia_list);
                                list_destroy(&ian->ia_list);
                                kmem_free(ian, sizeof (itx_async_node_t));
                        }
                }
                mutex_exit(&itxg->itxg_lock);
        }
}

/*
 * This function will prune commit itxs that are at the head of the
 * commit list (it won't prune past the first non-commit itx), and
 * either: a) attach them to the last lwb that's still pending
 * completion, or b) skip them altogether.
 *
 * This is used as a performance optimization to prevent commit itxs
 * from generating new lwbs when it's unnecessary to do so.
 */
static void
zil_prune_commit_list(zilog_t *zilog)
{
        itx_t *itx;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));

        while (itx = list_head(&zilog->zl_itx_commit_list)) {
                lr_t *lrc = &itx->itx_lr;
                if (lrc->lrc_txtype != TX_COMMIT)
                        break;

                mutex_enter(&zilog->zl_lock);

                lwb_t *last_lwb = zilog->zl_last_lwb_opened;
                if (last_lwb == NULL ||
                    last_lwb->lwb_state == LWB_STATE_FLUSH_DONE) {
                        /*
                         * All of the itxs this waiter was waiting on
                         * must have already completed (or there were
                         * never any itx's for it to wait on), so it's
                         * safe to skip this waiter and mark it done.
                         */
                        zil_commit_waiter_skip(itx->itx_private);
                } else {
                        zil_commit_waiter_link_lwb(itx->itx_private, last_lwb);
                        itx->itx_private = NULL;
                }

                mutex_exit(&zilog->zl_lock);

                list_remove(&zilog->zl_itx_commit_list, itx);
                zil_itx_destroy(itx);
        }

        IMPLY(itx != NULL, itx->itx_lr.lrc_txtype != TX_COMMIT);
}

static void
zil_commit_writer_stall(zilog_t *zilog)
{
        /*
         * When zio_alloc_zil() fails to allocate the next lwb block on
         * disk, we must call txg_wait_synced() to ensure all of the
         * lwbs in the zilog's zl_lwb_list are synced and then freed (in
         * zil_sync()), such that any subsequent ZIL writer (i.e. a call
         * to zil_process_commit_list()) will have to call zil_create(),
         * and start a new ZIL chain.
         *
         * Since zil_alloc_zil() failed, the lwb that was previously
         * issued does not have a pointer to the "next" lwb on disk.
         * Thus, if another ZIL writer thread was to allocate the "next"
         * on-disk lwb, that block could be leaked in the event of a
         * crash (because the previous lwb on-disk would not point to
         * it).
         *
         * We must hold the zilog's zl_issuer_lock while we do this, to
         * ensure no new threads enter zil_process_commit_list() until
         * all lwb's in the zl_lwb_list have been synced and freed
         * (which is achieved via the txg_wait_synced() call).
         */
        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
        txg_wait_synced(zilog->zl_dmu_pool, 0);
        ASSERT3P(list_tail(&zilog->zl_lwb_list), ==, NULL);
}

/*
 * This function will traverse the commit list, creating new lwbs as
 * needed, and committing the itxs from the commit list to these newly
 * created lwbs. Additionally, as a new lwb is created, the previous
 * lwb will be issued to the zio layer to be written to disk.
 */
static void
zil_process_commit_list(zilog_t *zilog)
{
        spa_t *spa = zilog->zl_spa;
        list_t nolwb_waiters;
        lwb_t *lwb;
        itx_t *itx;

        ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));

        /*
         * Return if there's nothing to commit before we dirty the fs by
         * calling zil_create().
         */
        if (list_head(&zilog->zl_itx_commit_list) == NULL)
                return;

        list_create(&nolwb_waiters, sizeof (zil_commit_waiter_t),
            offsetof(zil_commit_waiter_t, zcw_node));

        lwb = list_tail(&zilog->zl_lwb_list);
        if (lwb == NULL) {
                lwb = zil_create(zilog);
        } else {
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_ISSUED);
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_WRITE_DONE);
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_FLUSH_DONE);
        }

        while (itx = list_head(&zilog->zl_itx_commit_list)) {
                lr_t *lrc = &itx->itx_lr;
                uint64_t txg = lrc->lrc_txg;

                ASSERT3U(txg, !=, 0);

                if (lrc->lrc_txtype == TX_COMMIT) {
                        DTRACE_PROBE2(zil__process__commit__itx,
                            zilog_t *, zilog, itx_t *, itx);
                } else {
                        DTRACE_PROBE2(zil__process__normal__itx,
                            zilog_t *, zilog, itx_t *, itx);
                }

                boolean_t synced = txg <= spa_last_synced_txg(spa);
                boolean_t frozen = txg > spa_freeze_txg(spa);

                /*
                 * If the txg of this itx has already been synced out, then
                 * we don't need to commit this itx to an lwb. This is
                 * because the data of this itx will have already been
                 * written to the main pool. This is inherently racy, and
                 * it's still ok to commit an itx whose txg has already
                 * been synced; this will result in a write that's
                 * unnecessary, but will do no harm.
                 *
                 * With that said, we always want to commit TX_COMMIT itxs
                 * to an lwb, regardless of whether or not that itx's txg
                 * has been synced out. We do this to ensure any OPENED lwb
                 * will always have at least one zil_commit_waiter_t linked
                 * to the lwb.
                 *
                 * As a counter-example, if we skipped TX_COMMIT itx's
                 * whose txg had already been synced, the following
                 * situation could occur if we happened to be racing with
                 * spa_sync:
                 *
                 * 1. we commit a non-TX_COMMIT itx to an lwb, where the
                 *    itx's txg is 10 and the last synced txg is 9.
                 * 2. spa_sync finishes syncing out txg 10.
                 * 3. we move to the next itx in the list, it's a TX_COMMIT
                 *    whose txg is 10, so we skip it rather than committing
                 *    it to the lwb used in (1).
                 *
                 * If the itx that is skipped in (3) is the last TX_COMMIT
                 * itx in the commit list, than it's possible for the lwb
                 * used in (1) to remain in the OPENED state indefinitely.
                 *
                 * To prevent the above scenario from occuring, ensuring
                 * that once an lwb is OPENED it will transition to ISSUED
                 * and eventually DONE, we always commit TX_COMMIT itx's to
                 * an lwb here, even if that itx's txg has already been
                 * synced.
                 *
                 * Finally, if the pool is frozen, we _always_ commit the
                 * itx.  The point of freezing the pool is to prevent data
                 * from being written to the main pool via spa_sync, and
                 * instead rely solely on the ZIL to persistently store the
                 * data; i.e.  when the pool is frozen, the last synced txg
                 * value can't be trusted.
                 */
                if (frozen || !synced || lrc->lrc_txtype == TX_COMMIT) {
                        if (lwb != NULL) {
                                lwb = zil_lwb_commit(zilog, itx, lwb);
                        } else if (lrc->lrc_txtype == TX_COMMIT) {
                                ASSERT3P(lwb, ==, NULL);
                                zil_commit_waiter_link_nolwb(
                                    itx->itx_private, &nolwb_waiters);
                        }
                }

                list_remove(&zilog->zl_itx_commit_list, itx);
                zil_itx_destroy(itx);
        }

        if (lwb == NULL) {
                /*
                 * This indicates zio_alloc_zil() failed to allocate the
                 * "next" lwb on-disk. When this happens, we must stall
                 * the ZIL write pipeline; see the comment within
                 * zil_commit_writer_stall() for more details.
                 */
                zil_commit_writer_stall(zilog);

                /*
                 * Additionally, we have to signal and mark the "nolwb"
                 * waiters as "done" here, since without an lwb, we
                 * can't do this via zil_lwb_flush_vdevs_done() like
                 * normal.
                 */
                zil_commit_waiter_t *zcw;
                while (zcw = list_head(&nolwb_waiters)) {
                        zil_commit_waiter_skip(zcw);
                        list_remove(&nolwb_waiters, zcw);
                }
        } else {
                ASSERT(list_is_empty(&nolwb_waiters));
                ASSERT3P(lwb, !=, NULL);
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_ISSUED);
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_WRITE_DONE);
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_FLUSH_DONE);

                /*
                 * At this point, the ZIL block pointed at by the "lwb"
                 * variable is in one of the following states: "closed"
                 * or "open".
                 *
                 * If its "closed", then no itxs have been committed to
                 * it, so there's no point in issuing its zio (i.e.
                 * it's "empty").
                 *
                 * If its "open" state, then it contains one or more
                 * itxs that eventually need to be committed to stable
                 * storage. In this case we intentionally do not issue
                 * the lwb's zio to disk yet, and instead rely on one of
                 * the following two mechanisms for issuing the zio:
                 *
                 * 1. Ideally, there will be more ZIL activity occuring
                 * on the system, such that this function will be
                 * immediately called again (not necessarily by the same
                 * thread) and this lwb's zio will be issued via
                 * zil_lwb_commit(). This way, the lwb is guaranteed to
                 * be "full" when it is issued to disk, and we'll make
                 * use of the lwb's size the best we can.
                 *
                 * 2. If there isn't sufficient ZIL activity occuring on
                 * the system, such that this lwb's zio isn't issued via
                 * zil_lwb_commit(), zil_commit_waiter() will issue the
                 * lwb's zio. If this occurs, the lwb is not guaranteed
                 * to be "full" by the time its zio is issued, and means
                 * the size of the lwb was "too large" given the amount
                 * of ZIL activity occuring on the system at that time.
                 *
                 * We do this for a couple of reasons:
                 *
                 * 1. To try and reduce the number of IOPs needed to
                 * write the same number of itxs. If an lwb has space
                 * available in it's buffer for more itxs, and more itxs
                 * will be committed relatively soon (relative to the
                 * latency of performing a write), then it's beneficial
                 * to wait for these "next" itxs. This way, more itxs
                 * can be committed to stable storage with fewer writes.
                 *
                 * 2. To try and use the largest lwb block size that the
                 * incoming rate of itxs can support. Again, this is to
                 * try and pack as many itxs into as few lwbs as
                 * possible, without significantly impacting the latency
                 * of each individual itx.
                 */
        }
}

/*
 * This function is responsible for ensuring the passed in commit waiter
 * (and associated commit itx) is committed to an lwb. If the waiter is
 * not already committed to an lwb, all itxs in the zilog's queue of
 * itxs will be processed. The assumption is the passed in waiter's
 * commit itx will found in the queue just like the other non-commit
 * itxs, such that when the entire queue is processed, the waiter will
 * have been commited to an lwb.
 *
 * The lwb associated with the passed in waiter is not guaranteed to
 * have been issued by the time this function completes. If the lwb is
 * not issued, we rely on future calls to zil_commit_writer() to issue
 * the lwb, or the timeout mechanism found in zil_commit_waiter().
 */
static void
zil_commit_writer(zilog_t *zilog, zil_commit_waiter_t *zcw)
{
        ASSERT(!MUTEX_HELD(&zilog->zl_lock));
        ASSERT(spa_writeable(zilog->zl_spa));

        mutex_enter(&zilog->zl_issuer_lock);

        if (zcw->zcw_lwb != NULL || zcw->zcw_done) {
                /*
                 * It's possible that, while we were waiting to acquire
                 * the "zl_issuer_lock", another thread committed this
                 * waiter to an lwb. If that occurs, we bail out early,
                 * without processing any of the zilog's queue of itxs.
                 *
                 * On certain workloads and system configurations, the
                 * "zl_issuer_lock" can become highly contended. In an
                 * attempt to reduce this contention, we immediately drop
                 * the lock if the waiter has already been processed.
                 *
                 * We've measured this optimization to reduce CPU spent
                 * contending on this lock by up to 5%, using a system
                 * with 32 CPUs, low latency storage (~50 usec writes),
                 * and 1024 threads performing sync writes.
                 */
                goto out;
        }

        zil_get_commit_list(zilog);
        zil_prune_commit_list(zilog);
        zil_process_commit_list(zilog);

out:
        mutex_exit(&zilog->zl_issuer_lock);
}

static void
zil_commit_waiter_timeout(zilog_t *zilog, zil_commit_waiter_t *zcw)
{
        ASSERT(!MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT(MUTEX_HELD(&zcw->zcw_lock));
        ASSERT3B(zcw->zcw_done, ==, B_FALSE);

        lwb_t *lwb = zcw->zcw_lwb;
        ASSERT3P(lwb, !=, NULL);
        ASSERT3S(lwb->lwb_state, !=, LWB_STATE_CLOSED);

        /*
         * If the lwb has already been issued by another thread, we can
         * immediately return since there's no work to be done (the
         * point of this function is to issue the lwb). Additionally, we
         * do this prior to acquiring the zl_issuer_lock, to avoid
         * acquiring it when it's not necessary to do so.
         */
        if (lwb->lwb_state == LWB_STATE_ISSUED ||
            lwb->lwb_state == LWB_STATE_WRITE_DONE ||
            lwb->lwb_state == LWB_STATE_FLUSH_DONE)
                return;

        /*
         * In order to call zil_lwb_write_issue() we must hold the
         * zilog's "zl_issuer_lock". We can't simply acquire that lock,
         * since we're already holding the commit waiter's "zcw_lock",
         * and those two locks are aquired in the opposite order
         * elsewhere.
         */
        mutex_exit(&zcw->zcw_lock);
        mutex_enter(&zilog->zl_issuer_lock);
        mutex_enter(&zcw->zcw_lock);

        /*
         * Since we just dropped and re-acquired the commit waiter's
         * lock, we have to re-check to see if the waiter was marked
         * "done" during that process. If the waiter was marked "done",
         * the "lwb" pointer is no longer valid (it can be free'd after
         * the waiter is marked "done"), so without this check we could
         * wind up with a use-after-free error below.
         */
        if (zcw->zcw_done)
                goto out;

        ASSERT3P(lwb, ==, zcw->zcw_lwb);

        /*
         * We've already checked this above, but since we hadn't acquired
         * the zilog's zl_issuer_lock, we have to perform this check a
         * second time while holding the lock.
         *
         * We don't need to hold the zl_lock since the lwb cannot transition
         * from OPENED to ISSUED while we hold the zl_issuer_lock. The lwb
         * _can_ transition from ISSUED to DONE, but it's OK to race with
         * that transition since we treat the lwb the same, whether it's in
         * the ISSUED or DONE states.
         *
         * The important thing, is we treat the lwb differently depending on
         * if it's ISSUED or OPENED, and block any other threads that might
         * attempt to issue this lwb. For that reason we hold the
         * zl_issuer_lock when checking the lwb_state; we must not call
         * zil_lwb_write_issue() if the lwb had already been issued.
         *
         * See the comment above the lwb_state_t structure definition for
         * more details on the lwb states, and locking requirements.
         */
        if (lwb->lwb_state == LWB_STATE_ISSUED ||
            lwb->lwb_state == LWB_STATE_WRITE_DONE ||
            lwb->lwb_state == LWB_STATE_FLUSH_DONE)
                goto out;

        ASSERT3S(lwb->lwb_state, ==, LWB_STATE_OPENED);

        /*
         * As described in the comments above zil_commit_waiter() and
         * zil_process_commit_list(), we need to issue this lwb's zio
         * since we've reached the commit waiter's timeout and it still
         * hasn't been issued.
         */
        lwb_t *nlwb = zil_lwb_write_issue(zilog, lwb);

        IMPLY(nlwb != NULL, lwb->lwb_state != LWB_STATE_OPENED);

        /*
         * Since the lwb's zio hadn't been issued by the time this thread
         * reached its timeout, we reset the zilog's "zl_cur_used" field
         * to influence the zil block size selection algorithm.
         *
         * By having to issue the lwb's zio here, it means the size of the
         * lwb was too large, given the incoming throughput of itxs.  By
         * setting "zl_cur_used" to zero, we communicate this fact to the
         * block size selection algorithm, so it can take this informaiton
         * into account, and potentially select a smaller size for the
         * next lwb block that is allocated.
         */
        zilog->zl_cur_used = 0;

        if (nlwb == NULL) {
                /*
                 * When zil_lwb_write_issue() returns NULL, this
                 * indicates zio_alloc_zil() failed to allocate the
                 * "next" lwb on-disk. When this occurs, the ZIL write
                 * pipeline must be stalled; see the comment within the
                 * zil_commit_writer_stall() function for more details.
                 *
                 * We must drop the commit waiter's lock prior to
                 * calling zil_commit_writer_stall() or else we can wind
                 * up with the following deadlock:
                 *
                 * - This thread is waiting for the txg to sync while
                 *   holding the waiter's lock; txg_wait_synced() is
                 *   used within txg_commit_writer_stall().
                 *
                 * - The txg can't sync because it is waiting for this
                 *   lwb's zio callback to call dmu_tx_commit().
                 *
                 * - The lwb's zio callback can't call dmu_tx_commit()
                 *   because it's blocked trying to acquire the waiter's
                 *   lock, which occurs prior to calling dmu_tx_commit()
                 */
                mutex_exit(&zcw->zcw_lock);
                zil_commit_writer_stall(zilog);
                mutex_enter(&zcw->zcw_lock);
        }

out:
        mutex_exit(&zilog->zl_issuer_lock);
        ASSERT(MUTEX_HELD(&zcw->zcw_lock));
}

/*
 * This function is responsible for performing the following two tasks:
 *
 * 1. its primary responsibility is to block until the given "commit
 *    waiter" is considered "done".
 *
 * 2. its secondary responsibility is to issue the zio for the lwb that
 *    the given "commit waiter" is waiting on, if this function has
 *    waited "long enough" and the lwb is still in the "open" state.
 *
 * Given a sufficient amount of itxs being generated and written using
 * the ZIL, the lwb's zio will be issued via the zil_lwb_commit()
 * function. If this does not occur, this secondary responsibility will
 * ensure the lwb is issued even if there is not other synchronous
 * activity on the system.
 *
 * For more details, see zil_process_commit_list(); more specifically,
 * the comment at the bottom of that function.
 */
static void
zil_commit_waiter(zilog_t *zilog, zil_commit_waiter_t *zcw)
{
        ASSERT(!MUTEX_HELD(&zilog->zl_lock));
        ASSERT(!MUTEX_HELD(&zilog->zl_issuer_lock));
        ASSERT(spa_writeable(zilog->zl_spa));

        mutex_enter(&zcw->zcw_lock);

        /*
         * The timeout is scaled based on the lwb latency to avoid
         * significantly impacting the latency of each individual itx.
         * For more details, see the comment at the bottom of the
         * zil_process_commit_list() function.
         */
        int pct = MAX(zfs_commit_timeout_pct, 1);
#if defined(illumos) || !defined(_KERNEL)
        hrtime_t sleep = (zilog->zl_last_lwb_latency * pct) / 100;
        hrtime_t wakeup = gethrtime() + sleep;
#else
        sbintime_t sleep = nstosbt((zilog->zl_last_lwb_latency * pct) / 100);
        sbintime_t wakeup = getsbinuptime() + sleep;
#endif
        boolean_t timedout = B_FALSE;

        while (!zcw->zcw_done) {
                ASSERT(MUTEX_HELD(&zcw->zcw_lock));

                lwb_t *lwb = zcw->zcw_lwb;

                /*
                 * Usually, the waiter will have a non-NULL lwb field here,
                 * but it's possible for it to be NULL as a result of
                 * zil_commit() racing with spa_sync().
                 *
                 * When zil_clean() is called, it's possible for the itxg
                 * list (which may be cleaned via a taskq) to contain
                 * commit itxs. When this occurs, the commit waiters linked
                 * off of these commit itxs will not be committed to an
                 * lwb.  Additionally, these commit waiters will not be
                 * marked done until zil_commit_waiter_skip() is called via
                 * zil_itxg_clean().
                 *
                 * Thus, it's possible for this commit waiter (i.e. the
                 * "zcw" variable) to be found in this "in between" state;
                 * where it's "zcw_lwb" field is NULL, and it hasn't yet
                 * been skipped, so it's "zcw_done" field is still B_FALSE.
                 */
                IMPLY(lwb != NULL, lwb->lwb_state != LWB_STATE_CLOSED);

                if (lwb != NULL && lwb->lwb_state == LWB_STATE_OPENED) {
                        ASSERT3B(timedout, ==, B_FALSE);

                        /*
                         * If the lwb hasn't been issued yet, then we
                         * need to wait with a timeout, in case this
                         * function needs to issue the lwb after the
                         * timeout is reached; responsibility (2) from
                         * the comment above this function.
                         */
#if defined(illumos) || !defined(_KERNEL)
                        clock_t timeleft = cv_timedwait_hires(&zcw->zcw_cv,
                            &zcw->zcw_lock, wakeup, USEC2NSEC(1),
                            CALLOUT_FLAG_ABSOLUTE);

                        if (timeleft >= 0 || zcw->zcw_done)
                                continue;
#else
                        int wait_err = cv_timedwait_sbt(&zcw->zcw_cv,
                            &zcw->zcw_lock, wakeup, SBT_1NS, C_ABSOLUTE);
                        if (wait_err != EWOULDBLOCK || zcw->zcw_done)
                                continue;
#endif

                        timedout = B_TRUE;
                        zil_commit_waiter_timeout(zilog, zcw);

                        if (!zcw->zcw_done) {
                                /*
                                 * If the commit waiter has already been
                                 * marked "done", it's possible for the
                                 * waiter's lwb structure to have already
                                 * been freed.  Thus, we can only reliably
                                 * make these assertions if the waiter
                                 * isn't done.
                                 */
                                ASSERT3P(lwb, ==, zcw->zcw_lwb);
                                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_OPENED);
                        }
                } else {
                        /*
                         * If the lwb isn't open, then it must have already
                         * been issued. In that case, there's no need to
                         * use a timeout when waiting for the lwb to
                         * complete.
                         *
                         * Additionally, if the lwb is NULL, the waiter
                         * will soon be signalled and marked done via
                         * zil_clean() and zil_itxg_clean(), so no timeout
                         * is required.
                         */

                        IMPLY(lwb != NULL,
                            lwb->lwb_state == LWB_STATE_ISSUED ||
                            lwb->lwb_state == LWB_STATE_WRITE_DONE ||
                            lwb->lwb_state == LWB_STATE_FLUSH_DONE);
                        cv_wait(&zcw->zcw_cv, &zcw->zcw_lock);
                }
        }

        mutex_exit(&zcw->zcw_lock);
}

static zil_commit_waiter_t *
zil_alloc_commit_waiter()
{
        zil_commit_waiter_t *zcw = kmem_cache_alloc(zil_zcw_cache, KM_SLEEP);

        cv_init(&zcw->zcw_cv, NULL, CV_DEFAULT, NULL);
        mutex_init(&zcw->zcw_lock, NULL, MUTEX_DEFAULT, NULL);
        list_link_init(&zcw->zcw_node);
        zcw->zcw_lwb = NULL;
        zcw->zcw_done = B_FALSE;
        zcw->zcw_zio_error = 0;

        return (zcw);
}

static void
zil_free_commit_waiter(zil_commit_waiter_t *zcw)
{
        ASSERT(!list_link_active(&zcw->zcw_node));
        ASSERT3P(zcw->zcw_lwb, ==, NULL);
        ASSERT3B(zcw->zcw_done, ==, B_TRUE);
        mutex_destroy(&zcw->zcw_lock);
        cv_destroy(&zcw->zcw_cv);
        kmem_cache_free(zil_zcw_cache, zcw);
}

/*
 * This function is used to create a TX_COMMIT itx and assign it. This
 * way, it will be linked into the ZIL's list of synchronous itxs, and
 * then later committed to an lwb (or skipped) when
 * zil_process_commit_list() is called.
 */
static void
zil_commit_itx_assign(zilog_t *zilog, zil_commit_waiter_t *zcw)
{
        dmu_tx_t *tx = dmu_tx_create(zilog->zl_os);
        VERIFY0(dmu_tx_assign(tx, TXG_WAIT));

        itx_t *itx = zil_itx_create(TX_COMMIT, sizeof (lr_t));
        itx->itx_sync = B_TRUE;
        itx->itx_private = zcw;

        zil_itx_assign(zilog, itx, tx);

        dmu_tx_commit(tx);
}

/*
 * Commit ZFS Intent Log transactions (itxs) to stable storage.
 *
 * When writing ZIL transactions to the on-disk representation of the
 * ZIL, the itxs are committed to a Log Write Block (lwb). Multiple
 * itxs can be committed to a single lwb. Once a lwb is written and
 * committed to stable storage (i.e. the lwb is written, and vdevs have
 * been flushed), each itx that was committed to that lwb is also
 * considered to be committed to stable storage.
 *
 * When an itx is committed to an lwb, the log record (lr_t) contained
 * by the itx is copied into the lwb's zio buffer, and once this buffer
 * is written to disk, it becomes an on-disk ZIL block.
 *
 * As itxs are generated, they're inserted into the ZIL's queue of
 * uncommitted itxs. The semantics of zil_commit() are such that it will
 * block until all itxs that were in the queue when it was called, are
 * committed to stable storage.
 *
 * If "foid" is zero, this means all "synchronous" and "asynchronous"
 * itxs, for all objects in the dataset, will be committed to stable
 * storage prior to zil_commit() returning. If "foid" is non-zero, all
 * "synchronous" itxs for all objects, but only "asynchronous" itxs
 * that correspond to the foid passed in, will be committed to stable
 * storage prior to zil_commit() returning.
 *
 * Generally speaking, when zil_commit() is called, the consumer doesn't
 * actually care about _all_ of the uncommitted itxs. Instead, they're
 * simply trying to waiting for a specific itx to be committed to disk,
 * but the interface(s) for interacting with the ZIL don't allow such
 * fine-grained communication. A better interface would allow a consumer
 * to create and assign an itx, and then pass a reference to this itx to
 * zil_commit(); such that zil_commit() would return as soon as that
 * specific itx was committed to disk (instead of waiting for _all_
 * itxs to be committed).
 *
 * When a thread calls zil_commit() a special "commit itx" will be
 * generated, along with a corresponding "waiter" for this commit itx.
 * zil_commit() will wait on this waiter's CV, such that when the waiter
 * is marked done, and signalled, zil_commit() will return.
 *
 * This commit itx is inserted into the queue of uncommitted itxs. This
 * provides an easy mechanism for determining which itxs were in the
 * queue prior to zil_commit() having been called, and which itxs were
 * added after zil_commit() was called.
 *
 * The commit it is special; it doesn't have any on-disk representation.
 * When a commit itx is "committed" to an lwb, the waiter associated
 * with it is linked onto the lwb's list of waiters. Then, when that lwb
 * completes, each waiter on the lwb's list is marked done and signalled
 * -- allowing the thread waiting on the waiter to return from zil_commit().
 *
 * It's important to point out a few critical factors that allow us
 * to make use of the commit itxs, commit waiters, per-lwb lists of
 * commit waiters, and zio completion callbacks like we're doing:
 *
 *   1. The list of waiters for each lwb is traversed, and each commit
 *      waiter is marked "done" and signalled, in the zio completion
 *      callback of the lwb's zio[*].
 *
 *      * Actually, the waiters are signalled in the zio completion
 *        callback of the root zio for the DKIOCFLUSHWRITECACHE commands
 *        that are sent to the vdevs upon completion of the lwb zio.
 *
 *   2. When the itxs are inserted into the ZIL's queue of uncommitted
 *      itxs, the order in which they are inserted is preserved[*]; as
 *      itxs are added to the queue, they are added to the tail of
 *      in-memory linked lists.
 *
 *      When committing the itxs to lwbs (to be written to disk), they
 *      are committed in the same order in which the itxs were added to
 *      the uncommitted queue's linked list(s); i.e. the linked list of
 *      itxs to commit is traversed from head to tail, and each itx is
 *      committed to an lwb in that order.
 *
 *      * To clarify:
 *
 *        - the order of "sync" itxs is preserved w.r.t. other
 *          "sync" itxs, regardless of the corresponding objects.
 *        - the order of "async" itxs is preserved w.r.t. other
 *          "async" itxs corresponding to the same object.
 *        - the order of "async" itxs is *not* preserved w.r.t. other
 *          "async" itxs corresponding to different objects.
 *        - the order of "sync" itxs w.r.t. "async" itxs (or vice
 *          versa) is *not* preserved, even for itxs that correspond
 *          to the same object.
 *
 *      For more details, see: zil_itx_assign(), zil_async_to_sync(),
 *      zil_get_commit_list(), and zil_process_commit_list().
 *
 *   3. The lwbs represent a linked list of blocks on disk. Thus, any
 *      lwb cannot be considered committed to stable storage, until its
 *      "previous" lwb is also committed to stable storage. This fact,
 *      coupled with the fact described above, means that itxs are
 *      committed in (roughly) the order in which they were generated.
 *      This is essential because itxs are dependent on prior itxs.
 *      Thus, we *must not* deem an itx as being committed to stable
 *      storage, until *all* prior itxs have also been committed to
 *      stable storage.
 *
 *      To enforce this ordering of lwb zio's, while still leveraging as
 *      much of the underlying storage performance as possible, we rely
 *      on two fundamental concepts:
 *
 *          1. The creation and issuance of lwb zio's is protected by
 *             the zilog's "zl_issuer_lock", which ensures only a single
 *             thread is creating and/or issuing lwb's at a time
 *          2. The "previous" lwb is a child of the "current" lwb
 *             (leveraging the zio parent-child depenency graph)
 *
 *      By relying on this parent-child zio relationship, we can have
 *      many lwb zio's concurrently issued to the underlying storage,
 *      but the order in which they complete will be the same order in
 *      which they were created.
 */
void
zil_commit(zilog_t *zilog, uint64_t foid)
{
        /*
         * We should never attempt to call zil_commit on a snapshot for
         * a couple of reasons:
         *
         * 1. A snapshot may never be modified, thus it cannot have any
         *    in-flight itxs that would have modified the dataset.
         *
         * 2. By design, when zil_commit() is called, a commit itx will
         *    be assigned to this zilog; as a result, the zilog will be
         *    dirtied. We must not dirty the zilog of a snapshot; there's
         *    checks in the code that enforce this invariant, and will
         *    cause a panic if it's not upheld.
         */
        ASSERT3B(dmu_objset_is_snapshot(zilog->zl_os), ==, B_FALSE);

        if (zilog->zl_sync == ZFS_SYNC_DISABLED)
                return;

        if (!spa_writeable(zilog->zl_spa)) {
                /*
                 * If the SPA is not writable, there should never be any
                 * pending itxs waiting to be committed to disk. If that
                 * weren't true, we'd skip writing those itxs out, and
                 * would break the sematics of zil_commit(); thus, we're
                 * verifying that truth before we return to the caller.
                 */
                ASSERT(list_is_empty(&zilog->zl_lwb_list));
                ASSERT3P(zilog->zl_last_lwb_opened, ==, NULL);
                for (int i = 0; i < TXG_SIZE; i++)
                        ASSERT3P(zilog->zl_itxg[i].itxg_itxs, ==, NULL);
                return;
        }

        /*
         * If the ZIL is suspended, we don't want to dirty it by calling
         * zil_commit_itx_assign() below, nor can we write out
         * lwbs like would be done in zil_commit_write(). Thus, we
         * simply rely on txg_wait_synced() to maintain the necessary
         * semantics, and avoid calling those functions altogether.
         */
        if (zilog->zl_suspend > 0) {
                txg_wait_synced(zilog->zl_dmu_pool, 0);
                return;
        }

        zil_commit_impl(zilog, foid);
}

void
zil_commit_impl(zilog_t *zilog, uint64_t foid)
{
        /*
         * Move the "async" itxs for the specified foid to the "sync"
         * queues, such that they will be later committed (or skipped)
         * to an lwb when zil_process_commit_list() is called.
         *
         * Since these "async" itxs must be committed prior to this
         * call to zil_commit returning, we must perform this operation
         * before we call zil_commit_itx_assign().
         */
        zil_async_to_sync(zilog, foid);

        /*
         * We allocate a new "waiter" structure which will initially be
         * linked to the commit itx using the itx's "itx_private" field.
         * Since the commit itx doesn't represent any on-disk state,
         * when it's committed to an lwb, rather than copying the its
         * lr_t into the lwb's buffer, the commit itx's "waiter" will be
         * added to the lwb's list of waiters. Then, when the lwb is
         * committed to stable storage, each waiter in the lwb's list of
         * waiters will be marked "done", and signalled.
         *
         * We must create the waiter and assign the commit itx prior to
         * calling zil_commit_writer(), or else our specific commit itx
         * is not guaranteed to be committed to an lwb prior to calling
         * zil_commit_waiter().
         */
        zil_commit_waiter_t *zcw = zil_alloc_commit_waiter();
        zil_commit_itx_assign(zilog, zcw);

        zil_commit_writer(zilog, zcw);
        zil_commit_waiter(zilog, zcw);

        if (zcw->zcw_zio_error != 0) {
                /*
                 * If there was an error writing out the ZIL blocks that
                 * this thread is waiting on, then we fallback to
                 * relying on spa_sync() to write out the data this
                 * thread is waiting on. Obviously this has performance
                 * implications, but the expectation is for this to be
                 * an exceptional case, and shouldn't occur often.
                 */
                DTRACE_PROBE2(zil__commit__io__error,
                    zilog_t *, zilog, zil_commit_waiter_t *, zcw);
                txg_wait_synced(zilog->zl_dmu_pool, 0);
        }

        zil_free_commit_waiter(zcw);
}

/*
 * Called in syncing context to free committed log blocks and update log header.
 */
void
zil_sync(zilog_t *zilog, dmu_tx_t *tx)
{
        zil_header_t *zh = zil_header_in_syncing_context(zilog);
        uint64_t txg = dmu_tx_get_txg(tx);
        spa_t *spa = zilog->zl_spa;
        uint64_t *replayed_seq = &zilog->zl_replayed_seq[txg & TXG_MASK];
        lwb_t *lwb;

        /*
         * We don't zero out zl_destroy_txg, so make sure we don't try
         * to destroy it twice.
         */
        if (spa_sync_pass(spa) != 1)
                return;

        mutex_enter(&zilog->zl_lock);

        ASSERT(zilog->zl_stop_sync == 0);

        if (*replayed_seq != 0) {
                ASSERT(zh->zh_replay_seq < *replayed_seq);
                zh->zh_replay_seq = *replayed_seq;
                *replayed_seq = 0;
        }

        if (zilog->zl_destroy_txg == txg) {
                blkptr_t blk = zh->zh_log;

                ASSERT(list_head(&zilog->zl_lwb_list) == NULL);

                bzero(zh, sizeof (zil_header_t));
                bzero(zilog->zl_replayed_seq, sizeof (zilog->zl_replayed_seq));

                if (zilog->zl_keep_first) {
                        /*
                         * If this block was part of log chain that couldn't
                         * be claimed because a device was missing during
                         * zil_claim(), but that device later returns,
                         * then this block could erroneously appear valid.
                         * To guard against this, assign a new GUID to the new
                         * log chain so it doesn't matter what blk points to.
                         */
                        zil_init_log_chain(zilog, &blk);
                        zh->zh_log = blk;
                }
        }

        while ((lwb = list_head(&zilog->zl_lwb_list)) != NULL) {
                zh->zh_log = lwb->lwb_blk;
                if (lwb->lwb_buf != NULL || lwb->lwb_max_txg > txg)
                        break;
                list_remove(&zilog->zl_lwb_list, lwb);
                zio_free(spa, txg, &lwb->lwb_blk);
                zil_free_lwb(zilog, lwb);

                /*
                 * If we don't have anything left in the lwb list then
                 * we've had an allocation failure and we need to zero
                 * out the zil_header blkptr so that we don't end
                 * up freeing the same block twice.
                 */
                if (list_head(&zilog->zl_lwb_list) == NULL)
                        BP_ZERO(&zh->zh_log);
        }
        mutex_exit(&zilog->zl_lock);
}

/* ARGSUSED */
static int
zil_lwb_cons(void *vbuf, void *unused, int kmflag)
{
        lwb_t *lwb = vbuf;
        list_create(&lwb->lwb_waiters, sizeof (zil_commit_waiter_t),
            offsetof(zil_commit_waiter_t, zcw_node));
        avl_create(&lwb->lwb_vdev_tree, zil_lwb_vdev_compare,
            sizeof (zil_vdev_node_t), offsetof(zil_vdev_node_t, zv_node));
        mutex_init(&lwb->lwb_vdev_lock, NULL, MUTEX_DEFAULT, NULL);
        return (0);
}

/* ARGSUSED */
static void
zil_lwb_dest(void *vbuf, void *unused)
{
        lwb_t *lwb = vbuf;
        mutex_destroy(&lwb->lwb_vdev_lock);
        avl_destroy(&lwb->lwb_vdev_tree);
        list_destroy(&lwb->lwb_waiters);
}

void
zil_init(void)
{
        zil_lwb_cache = kmem_cache_create("zil_lwb_cache",
            sizeof (lwb_t), 0, zil_lwb_cons, zil_lwb_dest, NULL, NULL, NULL, 0);

        zil_zcw_cache = kmem_cache_create("zil_zcw_cache",
            sizeof (zil_commit_waiter_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
}

void
zil_fini(void)
{
        kmem_cache_destroy(zil_zcw_cache);
        kmem_cache_destroy(zil_lwb_cache);
}

void
zil_set_sync(zilog_t *zilog, uint64_t sync)
{
        zilog->zl_sync = sync;
}

void
zil_set_logbias(zilog_t *zilog, uint64_t logbias)
{
        zilog->zl_logbias = logbias;
}

zilog_t *
zil_alloc(objset_t *os, zil_header_t *zh_phys)
{
        zilog_t *zilog;

        zilog = kmem_zalloc(sizeof (zilog_t), KM_SLEEP);

        zilog->zl_header = zh_phys;
        zilog->zl_os = os;
        zilog->zl_spa = dmu_objset_spa(os);
        zilog->zl_dmu_pool = dmu_objset_pool(os);
        zilog->zl_destroy_txg = TXG_INITIAL - 1;
        zilog->zl_logbias = dmu_objset_logbias(os);
        zilog->zl_sync = dmu_objset_syncprop(os);
        zilog->zl_dirty_max_txg = 0;
        zilog->zl_last_lwb_opened = NULL;
        zilog->zl_last_lwb_latency = 0;

        mutex_init(&zilog->zl_lock, NULL, MUTEX_DEFAULT, NULL);
        mutex_init(&zilog->zl_issuer_lock, NULL, MUTEX_DEFAULT, NULL);

        for (int i = 0; i < TXG_SIZE; i++) {
                mutex_init(&zilog->zl_itxg[i].itxg_lock, NULL,
                    MUTEX_DEFAULT, NULL);
        }

        list_create(&zilog->zl_lwb_list, sizeof (lwb_t),
            offsetof(lwb_t, lwb_node));

        list_create(&zilog->zl_itx_commit_list, sizeof (itx_t),
            offsetof(itx_t, itx_node));

        cv_init(&zilog->zl_cv_suspend, NULL, CV_DEFAULT, NULL);

        return (zilog);
}

void
zil_free(zilog_t *zilog)
{
        zilog->zl_stop_sync = 1;

        ASSERT0(zilog->zl_suspend);
        ASSERT0(zilog->zl_suspending);

        ASSERT(list_is_empty(&zilog->zl_lwb_list));
        list_destroy(&zilog->zl_lwb_list);

        ASSERT(list_is_empty(&zilog->zl_itx_commit_list));
        list_destroy(&zilog->zl_itx_commit_list);

        for (int i = 0; i < TXG_SIZE; i++) {
                /*
                 * It's possible for an itx to be generated that doesn't dirty
                 * a txg (e.g. ztest TX_TRUNCATE). So there's no zil_clean()
                 * callback to remove the entry. We remove those here.
                 *
                 * Also free up the ziltest itxs.
                 */
                if (zilog->zl_itxg[i].itxg_itxs)
                        zil_itxg_clean(zilog->zl_itxg[i].itxg_itxs);
                mutex_destroy(&zilog->zl_itxg[i].itxg_lock);
        }

        mutex_destroy(&zilog->zl_issuer_lock);
        mutex_destroy(&zilog->zl_lock);

        cv_destroy(&zilog->zl_cv_suspend);

        kmem_free(zilog, sizeof (zilog_t));
}

/*
 * Open an intent log.
 */
zilog_t *
zil_open(objset_t *os, zil_get_data_t *get_data)
{
        zilog_t *zilog = dmu_objset_zil(os);

        ASSERT3P(zilog->zl_get_data, ==, NULL);
        ASSERT3P(zilog->zl_last_lwb_opened, ==, NULL);
        ASSERT(list_is_empty(&zilog->zl_lwb_list));

        zilog->zl_get_data = get_data;

        return (zilog);
}

/*
 * Close an intent log.
 */
void
zil_close(zilog_t *zilog)
{
        lwb_t *lwb;
        uint64_t txg;

        if (!dmu_objset_is_snapshot(zilog->zl_os)) {
                zil_commit(zilog, 0);
        } else {
                ASSERT3P(list_tail(&zilog->zl_lwb_list), ==, NULL);
                ASSERT0(zilog->zl_dirty_max_txg);
                ASSERT3B(zilog_is_dirty(zilog), ==, B_FALSE);
        }

        mutex_enter(&zilog->zl_lock);
        lwb = list_tail(&zilog->zl_lwb_list);
        if (lwb == NULL)
                txg = zilog->zl_dirty_max_txg;
        else
                txg = MAX(zilog->zl_dirty_max_txg, lwb->lwb_max_txg);
        mutex_exit(&zilog->zl_lock);

        /*
         * We need to use txg_wait_synced() to wait long enough for the
         * ZIL to be clean, and to wait for all pending lwbs to be
         * written out.
         */
        if (txg)
                txg_wait_synced(zilog->zl_dmu_pool, txg);

        if (txg < spa_freeze_txg(zilog->zl_spa))
                ASSERT(!zilog_is_dirty(zilog));

        zilog->zl_get_data = NULL;

        /*
         * We should have only one lwb left on the list; remove it now.
         */
        mutex_enter(&zilog->zl_lock);
        lwb = list_head(&zilog->zl_lwb_list);
        if (lwb != NULL) {
                ASSERT3P(lwb, ==, list_tail(&zilog->zl_lwb_list));
                ASSERT3S(lwb->lwb_state, !=, LWB_STATE_ISSUED);
                list_remove(&zilog->zl_lwb_list, lwb);
                zio_buf_free(lwb->lwb_buf, lwb->lwb_sz);
                zil_free_lwb(zilog, lwb);
        }
        mutex_exit(&zilog->zl_lock);
}

static char *suspend_tag = "zil suspending";

/*
 * Suspend an intent log.  While in suspended mode, we still honor
 * synchronous semantics, but we rely on txg_wait_synced() to do it.
 * On old version pools, we suspend the log briefly when taking a
 * snapshot so that it will have an empty intent log.
 *
 * Long holds are not really intended to be used the way we do here --
 * held for such a short time.  A concurrent caller of dsl_dataset_long_held()
 * could fail.  Therefore we take pains to only put a long hold if it is
 * actually necessary.  Fortunately, it will only be necessary if the
 * objset is currently mounted (or the ZVOL equivalent).  In that case it
 * will already have a long hold, so we are not really making things any worse.
 *
 * Ideally, we would locate the existing long-holder (i.e. the zfsvfs_t or
 * zvol_state_t), and use their mechanism to prevent their hold from being
 * dropped (e.g. VFS_HOLD()).  However, that would be even more pain for
 * very little gain.
 *
 * if cookiep == NULL, this does both the suspend & resume.
 * Otherwise, it returns with the dataset "long held", and the cookie
 * should be passed into zil_resume().
 */
int
zil_suspend(const char *osname, void **cookiep)
{
        objset_t *os;
        zilog_t *zilog;
        const zil_header_t *zh;
        int error;

        error = dmu_objset_hold(osname, suspend_tag, &os);
        if (error != 0)
                return (error);
        zilog = dmu_objset_zil(os);

        mutex_enter(&zilog->zl_lock);
        zh = zilog->zl_header;

        if (zh->zh_flags & ZIL_REPLAY_NEEDED) {         /* unplayed log */
                mutex_exit(&zilog->zl_lock);
                dmu_objset_rele(os, suspend_tag);
                return (SET_ERROR(EBUSY));
        }

        /*
         * Don't put a long hold in the cases where we can avoid it.  This
         * is when there is no cookie so we are doing a suspend & resume
         * (i.e. called from zil_vdev_offline()), and there's nothing to do
         * for the suspend because it's already suspended, or there's no ZIL.
         */
        if (cookiep == NULL && !zilog->zl_suspending &&
            (zilog->zl_suspend > 0 || BP_IS_HOLE(&zh->zh_log))) {
                mutex_exit(&zilog->zl_lock);
                dmu_objset_rele(os, suspend_tag);
                return (0);
        }

        dsl_dataset_long_hold(dmu_objset_ds(os), suspend_tag);
        dsl_pool_rele(dmu_objset_pool(os), suspend_tag);

        zilog->zl_suspend++;

        if (zilog->zl_suspend > 1) {
                /*
                 * Someone else is already suspending it.
                 * Just wait for them to finish.
                 */

                while (zilog->zl_suspending)
                        cv_wait(&zilog->zl_cv_suspend, &zilog->zl_lock);
                mutex_exit(&zilog->zl_lock);

                if (cookiep == NULL)
                        zil_resume(os);
                else
                        *cookiep = os;
                return (0);
        }

        /*
         * If there is no pointer to an on-disk block, this ZIL must not
         * be active (e.g. filesystem not mounted), so there's nothing
         * to clean up.
         */
        if (BP_IS_HOLE(&zh->zh_log)) {
                ASSERT(cookiep != NULL); /* fast path already handled */

                *cookiep = os;
                mutex_exit(&zilog->zl_lock);
                return (0);
        }

        zilog->zl_suspending = B_TRUE;
        mutex_exit(&zilog->zl_lock);

        /*
         * We need to use zil_commit_impl to ensure we wait for all
         * LWB_STATE_OPENED and LWB_STATE_ISSUED lwb's to be committed
         * to disk before proceeding. If we used zil_commit instead, it
         * would just call txg_wait_synced(), because zl_suspend is set.
         * txg_wait_synced() doesn't wait for these lwb's to be
         * LWB_STATE_FLUSH_DONE before returning.
         */
        zil_commit_impl(zilog, 0);

        /*
         * Now that we've ensured all lwb's are LWB_STATE_FLUSH_DONE, we
         * use txg_wait_synced() to ensure the data from the zilog has
         * migrated to the main pool before calling zil_destroy().
         */
        txg_wait_synced(zilog->zl_dmu_pool, 0);

        zil_destroy(zilog, B_FALSE);

        mutex_enter(&zilog->zl_lock);
        zilog->zl_suspending = B_FALSE;
        cv_broadcast(&zilog->zl_cv_suspend);
        mutex_exit(&zilog->zl_lock);

        if (cookiep == NULL)
                zil_resume(os);
        else
                *cookiep = os;
        return (0);
}

void
zil_resume(void *cookie)
{
        objset_t *os = cookie;
        zilog_t *zilog = dmu_objset_zil(os);

        mutex_enter(&zilog->zl_lock);
        ASSERT(zilog->zl_suspend != 0);
        zilog->zl_suspend--;
        mutex_exit(&zilog->zl_lock);
        dsl_dataset_long_rele(dmu_objset_ds(os), suspend_tag);
        dsl_dataset_rele(dmu_objset_ds(os), suspend_tag);
}

typedef struct zil_replay_arg {
        zil_replay_func_t **zr_replay;
        void            *zr_arg;
        boolean_t       zr_byteswap;
        char            *zr_lr;
} zil_replay_arg_t;

static int
zil_replay_error(zilog_t *zilog, lr_t *lr, int error)
{
        char name[ZFS_MAX_DATASET_NAME_LEN];

        zilog->zl_replaying_seq--;      /* didn't actually replay this one */

        dmu_objset_name(zilog->zl_os, name);

        cmn_err(CE_WARN, "ZFS replay transaction error %d, "
            "dataset %s, seq 0x%llx, txtype %llu %s\n", error, name,
            (u_longlong_t)lr->lrc_seq,
            (u_longlong_t)(lr->lrc_txtype & ~TX_CI),
            (lr->lrc_txtype & TX_CI) ? "CI" : "");

        return (error);
}

static int
zil_replay_log_record(zilog_t *zilog, lr_t *lr, void *zra, uint64_t claim_txg)
{
        zil_replay_arg_t *zr = zra;
        const zil_header_t *zh = zilog->zl_header;
        uint64_t reclen = lr->lrc_reclen;
        uint64_t txtype = lr->lrc_txtype;
        int error = 0;

        zilog->zl_replaying_seq = lr->lrc_seq;

        if (lr->lrc_seq <= zh->zh_replay_seq)   /* already replayed */
                return (0);

        if (lr->lrc_txg < claim_txg)            /* already committed */
                return (0);

        /* Strip case-insensitive bit, still present in log record */
        txtype &= ~TX_CI;

        if (txtype == 0 || txtype >= TX_MAX_TYPE)
                return (zil_replay_error(zilog, lr, EINVAL));

        /*
         * If this record type can be logged out of order, the object
         * (lr_foid) may no longer exist.  That's legitimate, not an error.
         */
        if (TX_OOO(txtype)) {
                error = dmu_object_info(zilog->zl_os,
                    LR_FOID_GET_OBJ(((lr_ooo_t *)lr)->lr_foid), NULL);
                if (error == ENOENT || error == EEXIST)
                        return (0);
        }

        /*
         * Make a copy of the data so we can revise and extend it.
         */
        bcopy(lr, zr->zr_lr, reclen);

        /*
         * If this is a TX_WRITE with a blkptr, suck in the data.
         */
        if (txtype == TX_WRITE && reclen == sizeof (lr_write_t)) {
                error = zil_read_log_data(zilog, (lr_write_t *)lr,
                    zr->zr_lr + reclen);
                if (error != 0)
                        return (zil_replay_error(zilog, lr, error));
        }

        /*
         * The log block containing this lr may have been byteswapped
         * so that we can easily examine common fields like lrc_txtype.
         * However, the log is a mix of different record types, and only the
         * replay vectors know how to byteswap their records.  Therefore, if
         * the lr was byteswapped, undo it before invoking the replay vector.
         */
        if (zr->zr_byteswap)
                byteswap_uint64_array(zr->zr_lr, reclen);

        /*
         * We must now do two things atomically: replay this log record,
         * and update the log header sequence number to reflect the fact that
         * we did so. At the end of each replay function the sequence number
         * is updated if we are in replay mode.
         */
        error = zr->zr_replay[txtype](zr->zr_arg, zr->zr_lr, zr->zr_byteswap);
        if (error != 0) {
                /*
                 * The DMU's dnode layer doesn't see removes until the txg
                 * commits, so a subsequent claim can spuriously fail with
                 * EEXIST. So if we receive any error we try syncing out
                 * any removes then retry the transaction.  Note that we
                 * specify B_FALSE for byteswap now, so we don't do it twice.
                 */
                txg_wait_synced(spa_get_dsl(zilog->zl_spa), 0);
                error = zr->zr_replay[txtype](zr->zr_arg, zr->zr_lr, B_FALSE);
                if (error != 0)
                        return (zil_replay_error(zilog, lr, error));
        }
        return (0);
}

/* ARGSUSED */
static int
zil_incr_blks(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
{
        zilog->zl_replay_blks++;

        return (0);
}

/*
 * If this dataset has a non-empty intent log, replay it and destroy it.
 */
void
zil_replay(objset_t *os, void *arg, zil_replay_func_t *replay_func[TX_MAX_TYPE])
{
        zilog_t *zilog = dmu_objset_zil(os);
        const zil_header_t *zh = zilog->zl_header;
        zil_replay_arg_t zr;

        if ((zh->zh_flags & ZIL_REPLAY_NEEDED) == 0) {
                zil_destroy(zilog, B_TRUE);
                return;
        }

        zr.zr_replay = replay_func;
        zr.zr_arg = arg;
        zr.zr_byteswap = BP_SHOULD_BYTESWAP(&zh->zh_log);
        zr.zr_lr = kmem_alloc(2 * SPA_MAXBLOCKSIZE, KM_SLEEP);

        /*
         * Wait for in-progress removes to sync before starting replay.
         */
        txg_wait_synced(zilog->zl_dmu_pool, 0);

        zilog->zl_replay = B_TRUE;
        zilog->zl_replay_time = ddi_get_lbolt();
        ASSERT(zilog->zl_replay_blks == 0);
        (void) zil_parse(zilog, zil_incr_blks, zil_replay_log_record, &zr,
            zh->zh_claim_txg);
        kmem_free(zr.zr_lr, 2 * SPA_MAXBLOCKSIZE);

        zil_destroy(zilog, B_FALSE);
        txg_wait_synced(zilog->zl_dmu_pool, zilog->zl_destroy_txg);
        zilog->zl_replay = B_FALSE;
}

boolean_t
zil_replaying(zilog_t *zilog, dmu_tx_t *tx)
{
        if (zilog->zl_sync == ZFS_SYNC_DISABLED)
                return (B_TRUE);

        if (zilog->zl_replay) {
                dsl_dataset_dirty(dmu_objset_ds(zilog->zl_os), tx);
                zilog->zl_replayed_seq[dmu_tx_get_txg(tx) & TXG_MASK] =
                    zilog->zl_replaying_seq;
                return (B_TRUE);
        }

        return (B_FALSE);
}

/* ARGSUSED */
int
zil_reset(const char *osname, void *arg)
{
        int error;

        error = zil_suspend(osname, NULL);
        if (error != 0)
                return (SET_ERROR(EEXIST));
        return (0);
}