FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/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 https://opensource.org/licenses/CDDL-1.0.
     * 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]
     * Copyright (c) 2018 Datto Inc.
     */
    
    /* 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/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/metaslab.h>
    #include <sys/trace_zfs.h>
    #include <sys/abd.h>
    #include <sys/wmsum.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
     * instantiated (e.g. if the dataset is a normal filesystem, 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.
     */
    static uint_t zfs_commit_timeout_pct = 5;
    
    /*
     * Minimal time we care to delay commit waiting for more ZIL records.
     * At least FreeBSD kernel can't sleep for less than 2us at its best.
     * So requests to sleep for less then 5us is a waste of CPU time with
     * a risk of significant log latency increase due to oversleep.
    */
   static uint64_t zil_min_commit_timeout = 5000;
   
   /*
    * See zil.h for more information about these fields.
    */
   static zil_kstat_values_t zil_stats = {
           { "zil_commit_count",                   KSTAT_DATA_UINT64 },
           { "zil_commit_writer_count",            KSTAT_DATA_UINT64 },
           { "zil_itx_count",                      KSTAT_DATA_UINT64 },
           { "zil_itx_indirect_count",             KSTAT_DATA_UINT64 },
           { "zil_itx_indirect_bytes",             KSTAT_DATA_UINT64 },
           { "zil_itx_copied_count",               KSTAT_DATA_UINT64 },
           { "zil_itx_copied_bytes",               KSTAT_DATA_UINT64 },
           { "zil_itx_needcopy_count",             KSTAT_DATA_UINT64 },
           { "zil_itx_needcopy_bytes",             KSTAT_DATA_UINT64 },
           { "zil_itx_metaslab_normal_count",      KSTAT_DATA_UINT64 },
           { "zil_itx_metaslab_normal_bytes",      KSTAT_DATA_UINT64 },
           { "zil_itx_metaslab_slog_count",        KSTAT_DATA_UINT64 },
           { "zil_itx_metaslab_slog_bytes",        KSTAT_DATA_UINT64 },
   };
   
   static zil_sums_t zil_sums_global;
   static kstat_t *zil_kstats_global;
   
   /*
    * Disable intent logging replay.  This global ZIL switch affects all pools.
    */
   int zil_replay_disable = 0;
   
   /*
    * 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.
    */
   static int zil_nocacheflush = 0;
   
   /*
    * 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.
    */
   static uint64_t zil_slog_bulk = 768 * 1024;
   
   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 = TREE_CMP(DVA_GET_VDEV(dva1), DVA_GET_VDEV(dva2));
           if (likely(cmp))
                   return (cmp);
   
           return (TREE_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;
   
           (void) random_get_pseudo_bytes((void *)&zc->zc_word[ZIL_ZC_GUID_0],
               sizeof (zc->zc_word[ZIL_ZC_GUID_0]));
           (void) random_get_pseudo_bytes((void *)&zc->zc_word[ZIL_ZC_GUID_1],
               sizeof (zc->zc_word[ZIL_ZC_GUID_1]));
           zc->zc_word[ZIL_ZC_OBJSET] = dmu_objset_id(zilog->zl_os);
           zc->zc_word[ZIL_ZC_SEQ] = 1ULL;
   }
   
   static int
   zil_kstats_global_update(kstat_t *ksp, int rw)
   {
           zil_kstat_values_t *zs = ksp->ks_data;
           ASSERT3P(&zil_stats, ==, zs);
   
           if (rw == KSTAT_WRITE) {
                   return (SET_ERROR(EACCES));
           }
   
           zil_kstat_values_update(zs, &zil_sums_global);
   
           return (0);
   }
   
   /*
    * Read a log block and make sure it's valid.
    */
   static int
   zil_read_log_block(zilog_t *zilog, boolean_t decrypt, const blkptr_t *bp,
       blkptr_t *nbp, void *dst, char **end)
   {
           zio_flag_t 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;
   
           if (!decrypt)
                   zio_flags |= ZIO_FLAG_RAW;
   
           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 (memcmp(&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);
                                   memcpy(dst, lr, 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 (memcmp(&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);
                                   memcpy(dst, lr, 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)
   {
           zio_flag_t 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)
                           memset(wbuf, 0, 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;
   
           /*
            * If we are not using the resulting data, we are just checking that
            * it hasn't been corrupted so we don't need to waste CPU time
            * decompressing and decrypting it.
            */
           if (wbuf == NULL)
                   zio_flags |= ZIO_FLAG_RAW;
   
           ASSERT3U(BP_GET_LSIZE(bp), !=, 0);
           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)
                           memcpy(wbuf, abuf->b_data, arc_buf_size(abuf));
                   arc_buf_destroy(abuf, &abuf);
           }
   
           return (error);
   }
   
   void
   zil_sums_init(zil_sums_t *zs)
   {
           wmsum_init(&zs->zil_commit_count, 0);
           wmsum_init(&zs->zil_commit_writer_count, 0);
           wmsum_init(&zs->zil_itx_count, 0);
           wmsum_init(&zs->zil_itx_indirect_count, 0);
           wmsum_init(&zs->zil_itx_indirect_bytes, 0);
           wmsum_init(&zs->zil_itx_copied_count, 0);
           wmsum_init(&zs->zil_itx_copied_bytes, 0);
           wmsum_init(&zs->zil_itx_needcopy_count, 0);
           wmsum_init(&zs->zil_itx_needcopy_bytes, 0);
           wmsum_init(&zs->zil_itx_metaslab_normal_count, 0);
           wmsum_init(&zs->zil_itx_metaslab_normal_bytes, 0);
           wmsum_init(&zs->zil_itx_metaslab_slog_count, 0);
           wmsum_init(&zs->zil_itx_metaslab_slog_bytes, 0);
   }
   
   void
   zil_sums_fini(zil_sums_t *zs)
   {
           wmsum_fini(&zs->zil_commit_count);
           wmsum_fini(&zs->zil_commit_writer_count);
           wmsum_fini(&zs->zil_itx_count);
           wmsum_fini(&zs->zil_itx_indirect_count);
           wmsum_fini(&zs->zil_itx_indirect_bytes);
           wmsum_fini(&zs->zil_itx_copied_count);
           wmsum_fini(&zs->zil_itx_copied_bytes);
           wmsum_fini(&zs->zil_itx_needcopy_count);
           wmsum_fini(&zs->zil_itx_needcopy_bytes);
           wmsum_fini(&zs->zil_itx_metaslab_normal_count);
           wmsum_fini(&zs->zil_itx_metaslab_normal_bytes);
           wmsum_fini(&zs->zil_itx_metaslab_slog_count);
           wmsum_fini(&zs->zil_itx_metaslab_slog_bytes);
   }
   
   void
   zil_kstat_values_update(zil_kstat_values_t *zs, zil_sums_t *zil_sums)
   {
           zs->zil_commit_count.value.ui64 =
               wmsum_value(&zil_sums->zil_commit_count);
           zs->zil_commit_writer_count.value.ui64 =
               wmsum_value(&zil_sums->zil_commit_writer_count);
           zs->zil_itx_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_count);
           zs->zil_itx_indirect_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_indirect_count);
           zs->zil_itx_indirect_bytes.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_indirect_bytes);
           zs->zil_itx_copied_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_copied_count);
           zs->zil_itx_copied_bytes.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_copied_bytes);
           zs->zil_itx_needcopy_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_needcopy_count);
           zs->zil_itx_needcopy_bytes.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_needcopy_bytes);
           zs->zil_itx_metaslab_normal_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_metaslab_normal_count);
           zs->zil_itx_metaslab_normal_bytes.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_metaslab_normal_bytes);
           zs->zil_itx_metaslab_slog_count.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_metaslab_slog_count);
           zs->zil_itx_metaslab_slog_bytes.value.ui64 =
               wmsum_value(&zil_sums->zil_itx_metaslab_slog_bytes);
   }
   
   /*
    * 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,
       boolean_t decrypt)
   {
           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 = {{{{0}}}};
           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 = NULL;
   
                   if (blk_seq > claim_blk_seq)
                           break;
   
                   error = parse_blk_func(zilog, &blk, arg, txg);
                   if (error != 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, decrypt, &blk, &next_blk,
                       lrbuf, &end);
                   if (error != 0) {
                           if (claimed) {
                                   char name[ZFS_MAX_DATASET_NAME_LEN];
   
                                   dmu_objset_name(zilog->zl_os, name);
   
                                   cmn_err(CE_WARN, "ZFS read log block error %d, "
                                       "dataset %s, seq 0x%llx\n", error, name,
                                       (u_longlong_t)blk_seq);
                           }
                           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;
   
                           error = parse_lr_func(zilog, lr, arg, txg);
                           if (error != 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;
   
           zil_bp_tree_fini(zilog);
           zio_buf_free(lrbuf, SPA_OLD_MAXBLOCKSIZE);
   
           return (error);
   }
   
   static int
   zil_clear_log_block(zilog_t *zilog, const blkptr_t *bp, void *tx,
       uint64_t first_txg)
   {
           (void) tx;
           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);
   }
   
   static int
   zil_noop_log_record(zilog_t *zilog, const lr_t *lrc, void *tx,
       uint64_t first_txg)
   {
           (void) zilog, (void) lrc, (void) tx, (void) first_txg;
           return (0);
   }
   
   static int
   zil_claim_log_block(zilog_t *zilog, const 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, const 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);
                   if (error != 0)
                           return (error);
           }
   
           return (zil_claim_log_block(zilog, &lr->lr_blkptr, tx, first_txg));
   }
   
   static int
   zil_free_log_block(zilog_t *zilog, const blkptr_t *bp, void *tx,
       uint64_t claim_txg)
   {
           (void) claim_txg;
   
           zio_free(zilog->zl_spa, dmu_tx_get_txg(tx), bp);
   
           return (0);
   }
   
   static int
   zil_free_log_record(zilog_t *zilog, const 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 (TREE_CMP(v1, v2));
   }
   
   static lwb_t *
   zil_alloc_lwb(zilog_t *zilog, blkptr_t *bp, boolean_t slog, uint64_t txg,
       boolean_t fastwrite)
   {
           lwb_t *lwb;
   
           lwb = kmem_cache_alloc(zil_lwb_cache, KM_SLEEP);
           lwb->lwb_zilog = zilog;
           lwb->lwb_blk = *bp;
           lwb->lwb_fastwrite = fastwrite;
           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_issued_timestamp = 0;
           lwb->lwb_issued_txg = 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));
           VERIFY(list_is_empty(&lwb->lwb_itxs));
   
           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));
           VERIFY(list_is_empty(&lwb->lwb_itxs));
           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().
    */
   static 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.
    */
   static boolean_t __maybe_unused
   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().
    */
   static 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);
   }
   
   /*
    * Its called in zil_commit context (zil_process_commit_list()/zil_create()).
    * It activates SPA_FEATURE_ZILSAXATTR feature, if its enabled.
    * Check dsl_dataset_feature_is_active to avoid txg_wait_synced() on every
    * zil_commit.
    */
   static void
   zil_commit_activate_saxattr_feature(zilog_t *zilog)
   {
           dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
           uint64_t txg = 0;
           dmu_tx_t *tx = NULL;
   
           if (spa_feature_is_enabled(zilog->zl_spa, SPA_FEATURE_ZILSAXATTR) &&
               dmu_objset_type(zilog->zl_os) != DMU_OST_ZVOL &&
               !dsl_dataset_feature_is_active(ds, SPA_FEATURE_ZILSAXATTR)) {
                   tx = dmu_tx_create(zilog->zl_os);
                   VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
                   dsl_dataset_dirty(ds, tx);
                   txg = dmu_tx_get_txg(tx);
   
                   mutex_enter(&ds->ds_lock);
                   ds->ds_feature_activation[SPA_FEATURE_ZILSAXATTR] =
                       (void *)B_TRUE;
                   mutex_exit(&ds->ds_lock);
                   dmu_tx_commit(tx);
                   txg_wait_synced(zilog->zl_dmu_pool, txg);
           }
   }
   
   /*
    * 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 fastwrite = FALSE;
           boolean_t slog = FALSE;
           dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
   
   
           /*
            * 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 endianness
            */
           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, txg, &blk,
                       ZIL_MIN_BLKSZ, &slog);
                   fastwrite = TRUE;
   
                   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, fastwrite);
   
           /*
            * If we just allocated the first log block, commit our transaction
            * and wait for zil_sync() to stuff the block pointer into zh_log.
            * (zh is part of the MOS, so we cannot modify it in open context.)
            */
           if (tx != NULL) {
                   /*
                    * If "zilsaxattr" feature is enabled on zpool, then activate
                    * it now when we're creating the ZIL chain. We can't wait with
                    * this until we write the first xattr log record because we
                    * need to wait for the feature activation to sync out.
                    */
                   if (spa_feature_is_enabled(zilog->zl_spa,
                       SPA_FEATURE_ZILSAXATTR) && dmu_objset_type(zilog->zl_os) !=
                       DMU_OST_ZVOL) {
                           mutex_enter(&ds->ds_lock);
                           ds->ds_feature_activation[SPA_FEATURE_ZILSAXATTR] =
                               (void *)B_TRUE;
                           mutex_exit(&ds->ds_lock);
                   }
   
                   dmu_tx_commit(tx);
                   txg_wait_synced(zilog->zl_dmu_pool, txg);
           } else {
                   /*
                    * This branch covers the case where we enable the feature on a
                    * zpool that has existing ZIL headers.
                    */
                   zil_commit_activate_saxattr_feature(zilog);
           }
           IMPLY(spa_feature_is_enabled(zilog->zl_spa, SPA_FEATURE_ZILSAXATTR) &&
               dmu_objset_type(zilog->zl_os) != DMU_OST_ZVOL,
               dsl_dataset_feature_is_active(ds, SPA_FEATURE_ZILSAXATTR));
   
           ASSERT(error != 0 || memcmp(&blk, &zh->zh_log, sizeof (blk)) == 0);
           IMPLY(error == 0, lwb != NULL);
   
           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.
    * Return B_TRUE if there were any entries to replay.
    */
   boolean_t
   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 (B_FALSE);
   
           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) {
                           if (lwb->lwb_fastwrite)
                                   metaslab_fastwrite_unmark(zilog->zl_spa,
                                       &lwb->lwb_blk);
   
                           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);
   
           return (B_TRUE);
   }
   
   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, B_FALSE);
   }
   
   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, 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, B_FALSE);
                  }
                  BP_ZERO(&zh->zh_log);
                  if (os->os_encrypted)
                          os->os_next_write_raw[tx->tx_txg & TXG_MASK] = B_TRUE;
                  dsl_dataset_dirty(dmu_objset_ds(os), tx);
                  dmu_objset_disown(os, B_FALSE, 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, B_FALSE);
                  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;
                  if (os->os_encrypted)
                          os->os_next_write_raw[tx->tx_txg & TXG_MASK] = B_TRUE;
                  dsl_dataset_dirty(dmu_objset_ds(os), tx);
          }
  
          ASSERT3U(first_txg, ==, (spa_last_synced_txg(zilog->zl_spa) + 1));
          dmu_objset_disown(os, B_FALSE, 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.
   */
  int
  zil_check_log_chain(dsl_pool_t *dp, dsl_dataset_t *ds, void *tx)
  {
          (void) dp;
          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), B_FALSE);
  
          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;
          zil_commit_waiter_t *zcw;
          itx_t *itx;
          uint64_t txg;
  
          spa_config_exit(zilog->zl_spa, SCL_STATE, lwb);
  
          zio_buf_free(lwb->lwb_buf, lwb->lwb_sz);
  
          mutex_enter(&zilog->zl_lock);
  
          /*
           * 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;
  
          ASSERT3U(lwb->lwb_issued_timestamp, >, 0);
          zilog->zl_last_lwb_latency = (zilog->zl_last_lwb_latency * 3 +
              gethrtime() - lwb->lwb_issued_timestamp) / 4;
  
          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 ((itx = list_head(&lwb->lwb_itxs)) != NULL) {
                  list_remove(&lwb->lwb_itxs, itx);
                  zil_itx_destroy(itx);
          }
  
          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;
                  /*
                   * We expect any ZIO errors from child ZIOs to have been
                   * propagated "up" to this specific LWB's root ZIO, in
                   * order for this error handling to work correctly. This
                   * includes ZIO errors from either this LWB's write or
                   * flush, as well as any errors from other dependent LWBs
                   * (e.g. a root LWB ZIO that might be a child of this LWB).
                   *
                   * With that said, it's important to note that LWB flush
                   * errors are not propagated up to the LWB root ZIO.
                   * This is incorrect behavior, and results in VDEV flush
                   * errors not being handled correctly here. See the
                   * comment above the call to "zio_flush" for details.
                   */
  
                  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);
  
          mutex_enter(&zilog->zl_lwb_io_lock);
          txg = lwb->lwb_issued_txg;
          ASSERT3U(zilog->zl_lwb_inflight[txg & TXG_MASK], >, 0);
          zilog->zl_lwb_inflight[txg & TXG_MASK]--;
          if (zilog->zl_lwb_inflight[txg & TXG_MASK] == 0)
                  cv_broadcast(&zilog->zl_lwb_io_cv);
          mutex_exit(&zilog->zl_lwb_io_lock);
  }
  
  /*
   * Wait for the completion of all issued write/flush of that txg provided.
   * It guarantees zil_lwb_flush_vdevs_done() is called and returned.
   */
  static void
  zil_lwb_flush_wait_all(zilog_t *zilog, uint64_t txg)
  {
          ASSERT3U(txg, ==, spa_syncing_txg(zilog->zl_spa));
  
          mutex_enter(&zilog->zl_lwb_io_lock);
          while (zilog->zl_lwb_inflight[txg & TXG_MASK] > 0)
                  cv_wait(&zilog->zl_lwb_io_cv, &zilog->zl_lwb_io_lock);
          mutex_exit(&zilog->zl_lwb_io_lock);
  
  #ifdef ZFS_DEBUG
          mutex_enter(&zilog->zl_lock);
          mutex_enter(&zilog->zl_lwb_io_lock);
          lwb_t *lwb = list_head(&zilog->zl_lwb_list);
          while (lwb != NULL && lwb->lwb_max_txg <= txg) {
                  if (lwb->lwb_issued_txg <= txg) {
                          ASSERT(lwb->lwb_state != LWB_STATE_ISSUED);
                          ASSERT(lwb->lwb_state != LWB_STATE_WRITE_DONE);
                          IMPLY(lwb->lwb_issued_txg > 0,
                              lwb->lwb_state == LWB_STATE_FLUSH_DONE);
                  }
                  IMPLY(lwb->lwb_state == LWB_STATE_FLUSH_DONE,
                      lwb->lwb_buf == NULL);
                  lwb = list_next(&zilog->zl_lwb_list, lwb);
          }
          mutex_exit(&zilog->zl_lwb_io_lock);
          mutex_exit(&zilog->zl_lock);
  #endif
  }
  
  /*
   * 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_free(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;
          lwb->lwb_fastwrite = FALSE;
          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.
           *
           * Additionally, we don't perform any further error handling at
           * this point (e.g. setting "zcw_zio_error" appropriately), as
           * we expect that to occur in "zil_lwb_flush_vdevs_done" (thus,
           * we expect any error seen here, to have been propagated to
           * that function).
           */
          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) {
                          /*
                           * The "ZIO_FLAG_DONT_PROPAGATE" is currently
                           * always used within "zio_flush". This means,
                           * any errors when flushing the vdev(s), will
                           * (unfortunately) not be handled correctly,
                           * since these "zio_flush" errors will not be
                           * propagated up to "zil_lwb_flush_vdevs_done".
                           */
                          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 deferred,
                   * 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]);
  
          /* Lock so zil_sync() doesn't fastwrite_unmark after zio is created */
          mutex_enter(&zilog->zl_lock);
          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_fastwrite) {
                          metaslab_fastwrite_mark(zilog->zl_spa, &lwb->lwb_blk);
                          lwb->lwb_fastwrite = 1;
                  }
  
                  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_FASTWRITE, &zb);
                  ASSERT3P(lwb->lwb_write_zio, !=, NULL);
  
                  lwb->lwb_state = LWB_STATE_OPENED;
  
                  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.
   */
  static const struct {
          uint64_t        limit;
          uint64_t        blksz;
  } zil_block_buckets[] = {
          { 4096,         4096 },                 /* non TX_WRITE */
          { 8192 + 4096,  8192 + 4096 },          /* database */
          { 32768 + 4096, 32768 + 4096 },         /* NFS writes */
          { 65536 + 4096, 65536 + 4096 },         /* 64KB writes */
          { 131072,       131072 },               /* < 128KB writes */
          { 131072 +4096, 65536 + 4096 },         /* 128KB writes */
          { UINT64_MAX,   SPA_OLD_MAXBLOCKSIZE},  /* > 128KB writes */
  };
  
  /*
   * Maximum block size used by the ZIL.  This is picked up when the ZIL is
   * initialized.  Otherwise this should not be used directly; see
   * zl_max_block_size instead.
   */
  static uint_t zil_maxblocksize = SPA_OLD_MAXBLOCKSIZE;
  
  /*
   * 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.
           */
  
          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);
  
          mutex_enter(&zilog->zl_lwb_io_lock);
          lwb->lwb_issued_txg = txg;
          zilog->zl_lwb_inflight[txg & TXG_MASK]++;
          zilog->zl_lwb_max_issued_txg = MAX(txg, zilog->zl_lwb_max_issued_txg);
          mutex_exit(&zilog->zl_lwb_io_lock);
  
          /*
           * 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
           *   guessing 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].limit; i++)
                  continue;
          zil_blksz = MIN(zil_block_buckets[i].blksz, zilog->zl_max_block_size);
          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);
          error = zio_alloc_zil(spa, zilog->zl_os, txg, bp, zil_blksz, &slog);
          if (slog) {
                  ZIL_STAT_BUMP(zilog, zil_itx_metaslab_slog_count);
                  ZIL_STAT_INCR(zilog, zil_itx_metaslab_slog_bytes,
                      lwb->lwb_nused);
          } else {
                  ZIL_STAT_BUMP(zilog, zil_itx_metaslab_normal_count);
                  ZIL_STAT_INCR(zilog, zil_itx_metaslab_normal_bytes,
                      lwb->lwb_nused);
          }
          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, TRUE);
          }
  
          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
           */
          memset(lwb->lwb_buf + lwb->lwb_nused, 0, 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);
  
          dmu_tx_commit(tx);
  
          /*
           * If there was an allocation failure then nlwb will be null which
           * forces a txg_wait_synced().
           */
          return (nlwb);
  }
  
  /*
   * Maximum amount of write data that can be put into single log block.
   */
  uint64_t
  zil_max_log_data(zilog_t *zilog)
  {
          return (zilog->zl_max_block_size -
              sizeof (zil_chain_t) - sizeof (lr_write_t));
  }
  
  /*
   * Maximum amount of log space we agree to waste to reduce number of
   * WR_NEED_COPY chunks to reduce zl_get_data() overhead (~12%).
   */
  static inline uint64_t
  zil_max_waste_space(zilog_t *zilog)
  {
          return (zil_max_log_data(zilog) / 8);
  }
  
  /*
   * Maximum amount of write data for WR_COPIED.  For correctness, consumers
   * must fall back to WR_NEED_COPY if we can't fit the entire record into one
   * maximum sized log block, because each WR_COPIED record must fit in a
   * single log block.  For space efficiency, we want to fit two records into a
   * max-sized log block.
   */
  uint64_t
  zil_max_copied_data(zilog_t *zilog)
  {
          return ((zilog->zl_max_block_size - sizeof (zil_chain_t)) / 2 -
              sizeof (lr_write_t));
  }
  
  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, dpad, lwb_sp, reclen, txg, max_log_data;
  
          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);
                  dpad = dlen - lrw->lr_length;
          } else {
                  dlen = dpad = 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;
          max_log_data = zil_max_log_data(zilog);
          if (reclen > lwb_sp || (reclen + dlen > lwb_sp &&
              lwb_sp < zil_max_waste_space(zilog) &&
              (dlen % max_log_data == 0 ||
              lwb_sp < reclen + dlen % 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;
  
                  /*
                   * There must be enough space in the new, empty log block to
                   * hold reclen.  For WR_COPIED, we need to fit the whole
                   * record in one block, and reclen is the header size + the
                   * data size. For WR_NEED_COPY, we can create multiple
                   * records, splitting the data into multiple blocks, so we
                   * only need to fit one word of data per block; in this case
                   * reclen is just the header size (no data).
                   */
                  ASSERT3U(reclen + MIN(dlen, sizeof (uint64_t)), <=, lwb_sp);
          }
  
          dnow = MIN(dlen, lwb_sp - reclen);
          lr_buf = lwb->lwb_buf + lwb->lwb_nused;
          memcpy(lr_buf, lrc, reclen);
          lrcb = (lr_t *)lr_buf;          /* Like lrc, but inside lwb. */
          lrwb = (lr_write_t *)lrcb;      /* Like lrw, but inside lwb. */
  
          ZIL_STAT_BUMP(zilog, zil_itx_count);
  
          /*
           * 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) {
                          ZIL_STAT_BUMP(zilog, zil_itx_copied_count);
                          ZIL_STAT_INCR(zilog, zil_itx_copied_bytes,
                              lrw->lr_length);
                  } else {
                          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;
                                  ZIL_STAT_BUMP(zilog, zil_itx_needcopy_count);
                                  ZIL_STAT_INCR(zilog, zil_itx_needcopy_bytes,
                                      dnow);
                          } else {
                                  ASSERT3S(itx->itx_wr_state, ==, WR_INDIRECT);
                                  dbuf = NULL;
                                  ZIL_STAT_BUMP(zilog, zil_itx_indirect_count);
                                  ZIL_STAT_INCR(zilog, zil_itx_indirect_bytes,
                                      lrw->lr_length);
                          }
  
                          /*
                           * 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,
                              itx->itx_gen, lrwb, dbuf, lwb,
                              lwb->lwb_write_zio);
                          if (dbuf != NULL && error == 0 && dnow == dlen)
                                  /* Zero any padding bytes in the last block. */
                                  memset((char *)dbuf + lrwb->lr_length, 0, dpad);
  
                          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 olrsize)
  {
          size_t itxsize, lrsize;
          itx_t *itx;
  
          lrsize = P2ROUNDUP_TYPED(olrsize, sizeof (uint64_t), size_t);
          itxsize = offsetof(itx_t, itx_lr) + lrsize;
  
          itx = zio_data_buf_alloc(itxsize);
          itx->itx_lr.lrc_txtype = txtype;
          itx->itx_lr.lrc_reclen = lrsize;
          itx->itx_lr.lrc_seq = 0;        /* defensive */
          memset((char *)&itx->itx_lr + olrsize, 0, lrsize - olrsize);
          itx->itx_sync = B_TRUE;         /* default is synchronous */
          itx->itx_callback = NULL;
          itx->itx_callback_data = NULL;
          itx->itx_size = itxsize;
  
          return (itx);
  }
  
  void
  zil_itx_destroy(itx_t *itx)
  {
          IMPLY(itx->itx_lr.lrc_txtype == TX_COMMIT, itx->itx_callback == NULL);
          IMPLY(itx->itx_callback != NULL, itx->itx_lr.lrc_txtype != TX_COMMIT);
  
          if (itx->itx_callback != NULL)
                  itx->itx_callback(itx->itx_callback_data);
  
          zio_data_buf_free(itx, itx->itx_size);
  }
  
  /*
   * Free up the sync and async itxs. The itxs_t has already been detached
   * so no locks are needed.
   */
  static void
  zil_itxg_clean(void *arg)
  {
          itx_t *itx;
          list_t *list;
          avl_tree_t *t;
          void *cookie;
          itxs_t *itxs = arg;
          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 (TREE_CMP(o1, o2));
  }
  
  /*
   * Remove all async itx with the given oid.
   */
  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;
  
          /*
           * 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", (u_longlong_t)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 committed) 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);
          taskqid_t id = taskq_dispatch(zilog->zl_dmu_pool->dp_zil_clean_taskq,
              zil_itxg_clean, clean_me, TQ_NOSLEEP);
          if (id == TASKQID_INVALID)
                  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)) != NULL) {
                  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_itxs;
          list_t nolwb_waiters;
          lwb_t *lwb, *plwb;
          itx_t *itx;
          boolean_t first = B_TRUE;
  
          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_itxs, sizeof (itx_t), offsetof(itx_t, itx_node));
          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 {
                  /*
                   * Activate SPA_FEATURE_ZILSAXATTR for the cases where ZIL will
                   * have already been created (zl_lwb_list not empty).
                   */
                  zil_commit_activate_saxattr_feature(zilog);
                  ASSERT3S(lwb->lwb_state, !=, LWB_STATE_ISSUED);
                  ASSERT3S(lwb->lwb_state, !=, LWB_STATE_WRITE_DONE);
                  ASSERT3S(lwb->lwb_state, !=, LWB_STATE_FLUSH_DONE);
                  first = (lwb->lwb_state != LWB_STATE_OPENED) &&
                      ((plwb = list_prev(&zilog->zl_lwb_list, lwb)) == NULL ||
                      plwb->lwb_state == LWB_STATE_FLUSH_DONE);
          }
  
          while ((itx = list_head(&zilog->zl_itx_commit_list)) != NULL) {
                  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);
                  }
  
                  list_remove(&zilog->zl_itx_commit_list, 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 occurring, 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);
  
                                  if (lwb == NULL)
                                          list_insert_tail(&nolwb_itxs, itx);
                                  else
                                          list_insert_tail(&lwb->lwb_itxs, itx);
                          } else {
                                  if (lrc->lrc_txtype == TX_COMMIT) {
                                          zil_commit_waiter_link_nolwb(
                                              itx->itx_private, &nolwb_waiters);
                                  }
  
                                  list_insert_tail(&nolwb_itxs, itx);
                          }
                  } else {
                          ASSERT3S(lrc->lrc_txtype, !=, TX_COMMIT);
                          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)) != NULL) {
                          zil_commit_waiter_skip(zcw);
                          list_remove(&nolwb_waiters, zcw);
                  }
  
                  /*
                   * And finally, we have to destroy the itx's that
                   * couldn't be committed to an lwb; this will also call
                   * the itx's callback if one exists for the itx.
                   */
                  while ((itx = list_head(&nolwb_itxs)) != NULL) {
                          list_remove(&nolwb_itxs, itx);
                          zil_itx_destroy(itx);
                  }
          } 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 it's "closed", then no itxs have been committed to
                   * it, so there's no point in issuing its zio (i.e. it's
                   * "empty").
                   *
                   * If it's "open", 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 occurring
                   * 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 occurring 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 occurring 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 its 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.
                   *
                   * If we had no already running or open LWBs, it can be
                   * the workload is single-threaded.  And if the ZIL write
                   * latency is very small or if the LWB is almost full, it
                   * may be cheaper to bypass the delay.
                   */
                  if (lwb->lwb_state == LWB_STATE_OPENED && first) {
                          hrtime_t sleep = zilog->zl_last_lwb_latency *
                              zfs_commit_timeout_pct / 100;
                          if (sleep < zil_min_commit_timeout ||
                              lwb->lwb_sz - lwb->lwb_nused < lwb->lwb_sz / 8) {
                                  lwb = zil_lwb_write_issue(zilog, lwb);
                                  zilog->zl_cur_used = 0;
                                  if (lwb == NULL)
                                          zil_commit_writer_stall(zilog);
                          }
                  }
          }
  }
  
  /*
   * 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 committed 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_STAT_BUMP(zilog, zil_commit_writer_count);
  
          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 acquired 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 information
           * 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);
          hrtime_t sleep = (zilog->zl_last_lwb_latency * pct) / 100;
          hrtime_t wakeup = gethrtime() + sleep;
          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.
                           */
                          int rc = cv_timedwait_hires(&zcw->zcw_cv,
                              &zcw->zcw_lock, wakeup, USEC2NSEC(1),
                              CALLOUT_FLAG_ABSOLUTE);
  
                          if (rc != -1 || zcw->zcw_done)
                                  continue;
  
                          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 signaled 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(void)
  {
          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 signaled, 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 itx 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 signaled
   * -- 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 signaled, in the zio completion
   *      callback of the lwb's zio[*].
   *
   *      * Actually, the waiters are signaled 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 dependency 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 semantics 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)
  {
          ZIL_STAT_BUMP(zilog, zil_commit_count);
  
          /*
           * 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;
  
          zil_lwb_flush_wait_all(zilog, txg);
  
          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;
                  dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
  
                  ASSERT(list_head(&zilog->zl_lwb_list) == NULL);
  
                  memset(zh, 0, sizeof (zil_header_t));
                  memset(zilog->zl_replayed_seq, 0,
                      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;
                  } else {
                          /*
                           * A destroyed ZIL chain can't contain any TX_SETSAXATTR
                           * records. So, deactivate the feature for this dataset.
                           * We activate it again when we start a new ZIL chain.
                           */
                          if (dsl_dataset_feature_is_active(ds,
                              SPA_FEATURE_ZILSAXATTR))
                                  dsl_dataset_deactivate_feature(ds,
                                      SPA_FEATURE_ZILSAXATTR, tx);
                  }
          }
  
          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);
          }
  
          /*
           * Remove fastwrite on any blocks that have been pre-allocated for
           * the next commit. This prevents fastwrite counter pollution by
           * unused, long-lived LWBs.
           */
          for (; lwb != NULL; lwb = list_next(&zilog->zl_lwb_list, lwb)) {
                  if (lwb->lwb_fastwrite && !lwb->lwb_write_zio) {
                          metaslab_fastwrite_unmark(zilog->zl_spa, &lwb->lwb_blk);
                          lwb->lwb_fastwrite = 0;
                  }
          }
  
          mutex_exit(&zilog->zl_lock);
  }
  
  static int
  zil_lwb_cons(void *vbuf, void *unused, int kmflag)
  {
          (void) unused, (void) kmflag;
          lwb_t *lwb = vbuf;
          list_create(&lwb->lwb_itxs, sizeof (itx_t), offsetof(itx_t, itx_node));
          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);
  }
  
  static void
  zil_lwb_dest(void *vbuf, void *unused)
  {
          (void) unused;
          lwb_t *lwb = vbuf;
          mutex_destroy(&lwb->lwb_vdev_lock);
          avl_destroy(&lwb->lwb_vdev_tree);
          list_destroy(&lwb->lwb_waiters);
          list_destroy(&lwb->lwb_itxs);
  }
  
  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);
  
          zil_sums_init(&zil_sums_global);
          zil_kstats_global = kstat_create("zfs", 0, "zil", "misc",
              KSTAT_TYPE_NAMED, sizeof (zil_stats) / sizeof (kstat_named_t),
              KSTAT_FLAG_VIRTUAL);
  
          if (zil_kstats_global != NULL) {
                  zil_kstats_global->ks_data = &zil_stats;
                  zil_kstats_global->ks_update = zil_kstats_global_update;
                  zil_kstats_global->ks_private = NULL;
                  kstat_install(zil_kstats_global);
          }
  }
  
  void
  zil_fini(void)
  {
          kmem_cache_destroy(zil_zcw_cache);
          kmem_cache_destroy(zil_lwb_cache);
  
          if (zil_kstats_global != NULL) {
                  kstat_delete(zil_kstats_global);
                  zil_kstats_global = NULL;
          }
  
          zil_sums_fini(&zil_sums_global);
  }
  
  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;
          zilog->zl_max_block_size = zil_maxblocksize;
  
          mutex_init(&zilog->zl_lock, NULL, MUTEX_DEFAULT, NULL);
          mutex_init(&zilog->zl_issuer_lock, NULL, MUTEX_DEFAULT, NULL);
          mutex_init(&zilog->zl_lwb_io_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);
          cv_init(&zilog->zl_lwb_io_cv, NULL, CV_DEFAULT, NULL);
  
          return (zilog);
  }
  
  void
  zil_free(zilog_t *zilog)
  {
          int i;
  
          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 (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);
          mutex_destroy(&zilog->zl_lwb_io_lock);
  
          cv_destroy(&zilog->zl_cv_suspend);
          cv_destroy(&zilog->zl_lwb_io_cv);
  
          kmem_free(zilog, sizeof (zilog_t));
  }
  
  /*
   * Open an intent log.
   */
  zilog_t *
  zil_open(objset_t *os, zil_get_data_t *get_data, zil_sums_t *zil_sums)
  {
          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;
          zilog->zl_sums = zil_sums;
  
          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);
  
          /*
           * zl_lwb_max_issued_txg may be larger than lwb_max_txg. It depends
           * on the time when the dmu_tx transaction is assigned in
           * zil_lwb_write_issue().
           */
          mutex_enter(&zilog->zl_lwb_io_lock);
          txg = MAX(zilog->zl_lwb_max_issued_txg, txg);
          mutex_exit(&zilog->zl_lwb_io_lock);
  
          /*
           * We need to use txg_wait_synced() to wait until that txg is synced.
           * zil_sync() will guarantee all lwbs up to that txg have been
           * written out, flushed, and cleaned.
           */
          if (txg != 0)
                  txg_wait_synced(zilog->zl_dmu_pool, txg);
  
          if (zilog_is_dirty(zilog))
                  zfs_dbgmsg("zil (%px) is dirty, txg %llu", zilog,
                      (u_longlong_t)txg);
          if (txg < spa_freeze_txg(zilog->zl_spa))
                  VERIFY(!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);
  
                  if (lwb->lwb_fastwrite)
                          metaslab_fastwrite_unmark(zilog->zl_spa, &lwb->lwb_blk);
  
                  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 const 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);
          }
  
          /*
           * The ZIL has work to do. Ensure that the associated encryption
           * key will remain mapped while we are committing the log by
           * grabbing a reference to it. If the key isn't loaded we have no
           * choice but to return an error until the wrapping key is loaded.
           */
          if (os->os_encrypted &&
              dsl_dataset_create_key_mapping(dmu_objset_ds(os)) != 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);
                  return (SET_ERROR(EACCES));
          }
  
          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 lwbs 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 (os->os_encrypted)
                  dsl_dataset_remove_key_mapping(dmu_objset_ds(os));
  
          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 *const *zr_replay;
          void            *zr_arg;
          boolean_t       zr_byteswap;
          char            *zr_lr;
  } zil_replay_arg_t;
  
  static int
  zil_replay_error(zilog_t *zilog, const 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, const 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.
           */
          memcpy(zr->zr_lr, 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);
  }
  
  static int
  zil_incr_blks(zilog_t *zilog, const blkptr_t *bp, void *arg, uint64_t claim_txg)
  {
          (void) bp, (void) arg, (void) claim_txg;
  
          zilog->zl_replay_blks++;
  
          return (0);
  }
  
  /*
   * If this dataset has a non-empty intent log, replay it and destroy it.
   * Return B_TRUE if there were any entries to replay.
   */
  boolean_t
  zil_replay(objset_t *os, void *arg,
      zil_replay_func_t *const 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) {
                  return (zil_destroy(zilog, B_TRUE));
          }
  
          zr.zr_replay = replay_func;
          zr.zr_arg = arg;
          zr.zr_byteswap = BP_SHOULD_BYTESWAP(&zh->zh_log);
          zr.zr_lr = vmem_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, B_TRUE);
          vmem_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;
  
          return (B_TRUE);
  }
  
  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);
  }
  
  int
  zil_reset(const char *osname, void *arg)
  {
          (void) arg;
  
          int error = zil_suspend(osname, NULL);
          /* EACCES means crypto key not loaded */
          if ((error == EACCES) || (error == EBUSY))
                  return (SET_ERROR(error));
          if (error != 0)
                  return (SET_ERROR(EEXIST));
          return (0);
  }
  
  EXPORT_SYMBOL(zil_alloc);
  EXPORT_SYMBOL(zil_free);
  EXPORT_SYMBOL(zil_open);
  EXPORT_SYMBOL(zil_close);
  EXPORT_SYMBOL(zil_replay);
  EXPORT_SYMBOL(zil_replaying);
  EXPORT_SYMBOL(zil_destroy);
  EXPORT_SYMBOL(zil_destroy_sync);
  EXPORT_SYMBOL(zil_itx_create);
  EXPORT_SYMBOL(zil_itx_destroy);
  EXPORT_SYMBOL(zil_itx_assign);
  EXPORT_SYMBOL(zil_commit);
  EXPORT_SYMBOL(zil_claim);
  EXPORT_SYMBOL(zil_check_log_chain);
  EXPORT_SYMBOL(zil_sync);
  EXPORT_SYMBOL(zil_clean);
  EXPORT_SYMBOL(zil_suspend);
  EXPORT_SYMBOL(zil_resume);
  EXPORT_SYMBOL(zil_lwb_add_block);
  EXPORT_SYMBOL(zil_bp_tree_add);
  EXPORT_SYMBOL(zil_set_sync);
  EXPORT_SYMBOL(zil_set_logbias);
  EXPORT_SYMBOL(zil_sums_init);
  EXPORT_SYMBOL(zil_sums_fini);
  EXPORT_SYMBOL(zil_kstat_values_update);
  
  ZFS_MODULE_PARAM(zfs, zfs_, commit_timeout_pct, UINT, ZMOD_RW,
          "ZIL block open timeout percentage");
  
  ZFS_MODULE_PARAM(zfs_zil, zil_, min_commit_timeout, U64, ZMOD_RW,
          "Minimum delay we care for ZIL block commit");
  
  ZFS_MODULE_PARAM(zfs_zil, zil_, replay_disable, INT, ZMOD_RW,
          "Disable intent logging replay");
  
  ZFS_MODULE_PARAM(zfs_zil, zil_, nocacheflush, INT, ZMOD_RW,
          "Disable ZIL cache flushes");
  
  ZFS_MODULE_PARAM(zfs_zil, zil_, slog_bulk, U64, ZMOD_RW,
          "Limit in bytes slog sync writes per commit");
  
  ZFS_MODULE_PARAM(zfs_zil, zil_, maxblocksize, UINT, ZMOD_RW,
          "Limit in bytes of ZIL log block size");