FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/zio.c

     /*
      * CDDL HEADER START
      *
      * The contents of this file are subject to the terms of the
      * Common Development and Distribution License (the "License").
      * You may not use this file except in compliance with the License.
      *
      * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
      * or 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, 2022 by Delphix. All rights reserved.
     * Copyright (c) 2011 Nexenta Systems, Inc. All rights reserved.
     * Copyright (c) 2017, Intel Corporation.
     * Copyright (c) 2019, Klara Inc.
     * Copyright (c) 2019, Allan Jude
     * Copyright (c) 2021, Datto, Inc.
     */
    
    #include <sys/sysmacros.h>
    #include <sys/zfs_context.h>
    #include <sys/fm/fs/zfs.h>
    #include <sys/spa.h>
    #include <sys/txg.h>
    #include <sys/spa_impl.h>
    #include <sys/vdev_impl.h>
    #include <sys/vdev_trim.h>
    #include <sys/zio_impl.h>
    #include <sys/zio_compress.h>
    #include <sys/zio_checksum.h>
    #include <sys/dmu_objset.h>
    #include <sys/arc.h>
    #include <sys/ddt.h>
    #include <sys/blkptr.h>
    #include <sys/zfeature.h>
    #include <sys/dsl_scan.h>
    #include <sys/metaslab_impl.h>
    #include <sys/time.h>
    #include <sys/trace_zfs.h>
    #include <sys/abd.h>
    #include <sys/dsl_crypt.h>
    #include <cityhash.h>
    
    /*
     * ==========================================================================
     * I/O type descriptions
     * ==========================================================================
     */
    const char *const zio_type_name[ZIO_TYPES] = {
            /*
             * Note: Linux kernel thread name length is limited
             * so these names will differ from upstream open zfs.
             */
            "z_null", "z_rd", "z_wr", "z_fr", "z_cl", "z_ioctl", "z_trim"
    };
    
    int zio_dva_throttle_enabled = B_TRUE;
    static int zio_deadman_log_all = B_FALSE;
    
    /*
     * ==========================================================================
     * I/O kmem caches
     * ==========================================================================
     */
    static kmem_cache_t *zio_cache;
    static kmem_cache_t *zio_link_cache;
    kmem_cache_t *zio_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
    kmem_cache_t *zio_data_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
    #if defined(ZFS_DEBUG) && !defined(_KERNEL)
    static uint64_t zio_buf_cache_allocs[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
    static uint64_t zio_buf_cache_frees[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
    #endif
    
    /* Mark IOs as "slow" if they take longer than 30 seconds */
    static uint_t zio_slow_io_ms = (30 * MILLISEC);
    
    #define BP_SPANB(indblkshift, level) \
            (((uint64_t)1) << ((level) * ((indblkshift) - SPA_BLKPTRSHIFT)))
    #define COMPARE_META_LEVEL      0x80000000ul
    /*
     * The following actions directly effect the spa's sync-to-convergence logic.
     * The values below define the sync pass when we start performing the action.
     * Care should be taken when changing these values as they directly impact
     * spa_sync() performance. Tuning these values may introduce subtle performance
     * pathologies and should only be done in the context of performance analysis.
     * These tunables will eventually be removed and replaced with #defines once
     * enough analysis has been done to determine optimal values.
     *
    * The 'zfs_sync_pass_deferred_free' pass must be greater than 1 to ensure that
    * regular blocks are not deferred.
    *
    * Starting in sync pass 8 (zfs_sync_pass_dont_compress), we disable
    * compression (including of metadata).  In practice, we don't have this
    * many sync passes, so this has no effect.
    *
    * The original intent was that disabling compression would help the sync
    * passes to converge. However, in practice disabling compression increases
    * the average number of sync passes, because when we turn compression off, a
    * lot of block's size will change and thus we have to re-allocate (not
    * overwrite) them. It also increases the number of 128KB allocations (e.g.
    * for indirect blocks and spacemaps) because these will not be compressed.
    * The 128K allocations are especially detrimental to performance on highly
    * fragmented systems, which may have very few free segments of this size,
    * and may need to load new metaslabs to satisfy 128K allocations.
    */
   
   /* defer frees starting in this pass */
   uint_t zfs_sync_pass_deferred_free = 2;
   
   /* don't compress starting in this pass */
   static uint_t zfs_sync_pass_dont_compress = 8;
   
   /* rewrite new bps starting in this pass */
   static uint_t zfs_sync_pass_rewrite = 2;
   
   /*
    * An allocating zio is one that either currently has the DVA allocate
    * stage set or will have it later in its lifetime.
    */
   #define IO_IS_ALLOCATING(zio) ((zio)->io_orig_pipeline & ZIO_STAGE_DVA_ALLOCATE)
   
   /*
    * Enable smaller cores by excluding metadata
    * allocations as well.
    */
   int zio_exclude_metadata = 0;
   static int zio_requeue_io_start_cut_in_line = 1;
   
   #ifdef ZFS_DEBUG
   static const int zio_buf_debug_limit = 16384;
   #else
   static const int zio_buf_debug_limit = 0;
   #endif
   
   static inline void __zio_execute(zio_t *zio);
   
   static void zio_taskq_dispatch(zio_t *, zio_taskq_type_t, boolean_t);
   
   void
   zio_init(void)
   {
           size_t c;
   
           zio_cache = kmem_cache_create("zio_cache",
               sizeof (zio_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
           zio_link_cache = kmem_cache_create("zio_link_cache",
               sizeof (zio_link_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
   
           /*
            * For small buffers, we want a cache for each multiple of
            * SPA_MINBLOCKSIZE.  For larger buffers, we want a cache
            * for each quarter-power of 2.
            */
           for (c = 0; c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; c++) {
                   size_t size = (c + 1) << SPA_MINBLOCKSHIFT;
                   size_t p2 = size;
                   size_t align = 0;
                   size_t data_cflags, cflags;
   
                   data_cflags = KMC_NODEBUG;
                   cflags = (zio_exclude_metadata || size > zio_buf_debug_limit) ?
                       KMC_NODEBUG : 0;
   
                   while (!ISP2(p2))
                           p2 &= p2 - 1;
   
   #ifndef _KERNEL
                   /*
                    * If we are using watchpoints, put each buffer on its own page,
                    * to eliminate the performance overhead of trapping to the
                    * kernel when modifying a non-watched buffer that shares the
                    * page with a watched buffer.
                    */
                   if (arc_watch && !IS_P2ALIGNED(size, PAGESIZE))
                           continue;
                   /*
                    * Here's the problem - on 4K native devices in userland on
                    * Linux using O_DIRECT, buffers must be 4K aligned or I/O
                    * will fail with EINVAL, causing zdb (and others) to coredump.
                    * Since userland probably doesn't need optimized buffer caches,
                    * we just force 4K alignment on everything.
                    */
                   align = 8 * SPA_MINBLOCKSIZE;
   #else
                   if (size < PAGESIZE) {
                           align = SPA_MINBLOCKSIZE;
                   } else if (IS_P2ALIGNED(size, p2 >> 2)) {
                           align = PAGESIZE;
                   }
   #endif
   
                   if (align != 0) {
                           char name[36];
                           if (cflags == data_cflags) {
                                   /*
                                    * Resulting kmem caches would be identical.
                                    * Save memory by creating only one.
                                    */
                                   (void) snprintf(name, sizeof (name),
                                       "zio_buf_comb_%lu", (ulong_t)size);
                                   zio_buf_cache[c] = kmem_cache_create(name,
                                       size, align, NULL, NULL, NULL, NULL, NULL,
                                       cflags);
                                   zio_data_buf_cache[c] = zio_buf_cache[c];
                                   continue;
                           }
                           (void) snprintf(name, sizeof (name), "zio_buf_%lu",
                               (ulong_t)size);
                           zio_buf_cache[c] = kmem_cache_create(name, size,
                               align, NULL, NULL, NULL, NULL, NULL, cflags);
   
                           (void) snprintf(name, sizeof (name), "zio_data_buf_%lu",
                               (ulong_t)size);
                           zio_data_buf_cache[c] = kmem_cache_create(name, size,
                               align, NULL, NULL, NULL, NULL, NULL, data_cflags);
                   }
           }
   
           while (--c != 0) {
                   ASSERT(zio_buf_cache[c] != NULL);
                   if (zio_buf_cache[c - 1] == NULL)
                           zio_buf_cache[c - 1] = zio_buf_cache[c];
   
                   ASSERT(zio_data_buf_cache[c] != NULL);
                   if (zio_data_buf_cache[c - 1] == NULL)
                           zio_data_buf_cache[c - 1] = zio_data_buf_cache[c];
           }
   
           zio_inject_init();
   
           lz4_init();
   }
   
   void
   zio_fini(void)
   {
           size_t n = SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT;
   
   #if defined(ZFS_DEBUG) && !defined(_KERNEL)
           for (size_t i = 0; i < n; i++) {
                   if (zio_buf_cache_allocs[i] != zio_buf_cache_frees[i])
                           (void) printf("zio_fini: [%d] %llu != %llu\n",
                               (int)((i + 1) << SPA_MINBLOCKSHIFT),
                               (long long unsigned)zio_buf_cache_allocs[i],
                               (long long unsigned)zio_buf_cache_frees[i]);
           }
   #endif
   
           /*
            * The same kmem cache can show up multiple times in both zio_buf_cache
            * and zio_data_buf_cache. Do a wasteful but trivially correct scan to
            * sort it out.
            */
           for (size_t i = 0; i < n; i++) {
                   kmem_cache_t *cache = zio_buf_cache[i];
                   if (cache == NULL)
                           continue;
                   for (size_t j = i; j < n; j++) {
                           if (cache == zio_buf_cache[j])
                                   zio_buf_cache[j] = NULL;
                           if (cache == zio_data_buf_cache[j])
                                   zio_data_buf_cache[j] = NULL;
                   }
                   kmem_cache_destroy(cache);
           }
   
           for (size_t i = 0; i < n; i++) {
                   kmem_cache_t *cache = zio_data_buf_cache[i];
                   if (cache == NULL)
                           continue;
                   for (size_t j = i; j < n; j++) {
                           if (cache == zio_data_buf_cache[j])
                                   zio_data_buf_cache[j] = NULL;
                   }
                   kmem_cache_destroy(cache);
           }
   
           for (size_t i = 0; i < n; i++) {
                   VERIFY3P(zio_buf_cache[i], ==, NULL);
                   VERIFY3P(zio_data_buf_cache[i], ==, NULL);
           }
   
           kmem_cache_destroy(zio_link_cache);
           kmem_cache_destroy(zio_cache);
   
           zio_inject_fini();
   
           lz4_fini();
   }
   
   /*
    * ==========================================================================
    * Allocate and free I/O buffers
    * ==========================================================================
    */
   
   /*
    * Use zio_buf_alloc to allocate ZFS metadata.  This data will appear in a
    * crashdump if the kernel panics, so use it judiciously.  Obviously, it's
    * useful to inspect ZFS metadata, but if possible, we should avoid keeping
    * excess / transient data in-core during a crashdump.
    */
   void *
   zio_buf_alloc(size_t size)
   {
           size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
   
           VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
   #if defined(ZFS_DEBUG) && !defined(_KERNEL)
           atomic_add_64(&zio_buf_cache_allocs[c], 1);
   #endif
   
           return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
   }
   
   /*
    * Use zio_data_buf_alloc to allocate data.  The data will not appear in a
    * crashdump if the kernel panics.  This exists so that we will limit the amount
    * of ZFS data that shows up in a kernel crashdump.  (Thus reducing the amount
    * of kernel heap dumped to disk when the kernel panics)
    */
   void *
   zio_data_buf_alloc(size_t size)
   {
           size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
   
           VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
   
           return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
   }
   
   void
   zio_buf_free(void *buf, size_t size)
   {
           size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
   
           VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
   #if defined(ZFS_DEBUG) && !defined(_KERNEL)
           atomic_add_64(&zio_buf_cache_frees[c], 1);
   #endif
   
           kmem_cache_free(zio_buf_cache[c], buf);
   }
   
   void
   zio_data_buf_free(void *buf, size_t size)
   {
           size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
   
           VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
   
           kmem_cache_free(zio_data_buf_cache[c], buf);
   }
   
   static void
   zio_abd_free(void *abd, size_t size)
   {
           (void) size;
           abd_free((abd_t *)abd);
   }
   
   /*
    * ==========================================================================
    * Push and pop I/O transform buffers
    * ==========================================================================
    */
   void
   zio_push_transform(zio_t *zio, abd_t *data, uint64_t size, uint64_t bufsize,
       zio_transform_func_t *transform)
   {
           zio_transform_t *zt = kmem_alloc(sizeof (zio_transform_t), KM_SLEEP);
   
           zt->zt_orig_abd = zio->io_abd;
           zt->zt_orig_size = zio->io_size;
           zt->zt_bufsize = bufsize;
           zt->zt_transform = transform;
   
           zt->zt_next = zio->io_transform_stack;
           zio->io_transform_stack = zt;
   
           zio->io_abd = data;
           zio->io_size = size;
   }
   
   void
   zio_pop_transforms(zio_t *zio)
   {
           zio_transform_t *zt;
   
           while ((zt = zio->io_transform_stack) != NULL) {
                   if (zt->zt_transform != NULL)
                           zt->zt_transform(zio,
                               zt->zt_orig_abd, zt->zt_orig_size);
   
                   if (zt->zt_bufsize != 0)
                           abd_free(zio->io_abd);
   
                   zio->io_abd = zt->zt_orig_abd;
                   zio->io_size = zt->zt_orig_size;
                   zio->io_transform_stack = zt->zt_next;
   
                   kmem_free(zt, sizeof (zio_transform_t));
           }
   }
   
   /*
    * ==========================================================================
    * I/O transform callbacks for subblocks, decompression, and decryption
    * ==========================================================================
    */
   static void
   zio_subblock(zio_t *zio, abd_t *data, uint64_t size)
   {
           ASSERT(zio->io_size > size);
   
           if (zio->io_type == ZIO_TYPE_READ)
                   abd_copy(data, zio->io_abd, size);
   }
   
   static void
   zio_decompress(zio_t *zio, abd_t *data, uint64_t size)
   {
           if (zio->io_error == 0) {
                   void *tmp = abd_borrow_buf(data, size);
                   int ret = zio_decompress_data(BP_GET_COMPRESS(zio->io_bp),
                       zio->io_abd, tmp, zio->io_size, size,
                       &zio->io_prop.zp_complevel);
                   abd_return_buf_copy(data, tmp, size);
   
                   if (zio_injection_enabled && ret == 0)
                           ret = zio_handle_fault_injection(zio, EINVAL);
   
                   if (ret != 0)
                           zio->io_error = SET_ERROR(EIO);
           }
   }
   
   static void
   zio_decrypt(zio_t *zio, abd_t *data, uint64_t size)
   {
           int ret;
           void *tmp;
           blkptr_t *bp = zio->io_bp;
           spa_t *spa = zio->io_spa;
           uint64_t dsobj = zio->io_bookmark.zb_objset;
           uint64_t lsize = BP_GET_LSIZE(bp);
           dmu_object_type_t ot = BP_GET_TYPE(bp);
           uint8_t salt[ZIO_DATA_SALT_LEN];
           uint8_t iv[ZIO_DATA_IV_LEN];
           uint8_t mac[ZIO_DATA_MAC_LEN];
           boolean_t no_crypt = B_FALSE;
   
           ASSERT(BP_USES_CRYPT(bp));
           ASSERT3U(size, !=, 0);
   
           if (zio->io_error != 0)
                   return;
   
           /*
            * Verify the cksum of MACs stored in an indirect bp. It will always
            * be possible to verify this since it does not require an encryption
            * key.
            */
           if (BP_HAS_INDIRECT_MAC_CKSUM(bp)) {
                   zio_crypt_decode_mac_bp(bp, mac);
   
                   if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF) {
                           /*
                            * We haven't decompressed the data yet, but
                            * zio_crypt_do_indirect_mac_checksum() requires
                            * decompressed data to be able to parse out the MACs
                            * from the indirect block. We decompress it now and
                            * throw away the result after we are finished.
                            */
                           tmp = zio_buf_alloc(lsize);
                           ret = zio_decompress_data(BP_GET_COMPRESS(bp),
                               zio->io_abd, tmp, zio->io_size, lsize,
                               &zio->io_prop.zp_complevel);
                           if (ret != 0) {
                                   ret = SET_ERROR(EIO);
                                   goto error;
                           }
                           ret = zio_crypt_do_indirect_mac_checksum(B_FALSE,
                               tmp, lsize, BP_SHOULD_BYTESWAP(bp), mac);
                           zio_buf_free(tmp, lsize);
                   } else {
                           ret = zio_crypt_do_indirect_mac_checksum_abd(B_FALSE,
                               zio->io_abd, size, BP_SHOULD_BYTESWAP(bp), mac);
                   }
                   abd_copy(data, zio->io_abd, size);
   
                   if (zio_injection_enabled && ot != DMU_OT_DNODE && ret == 0) {
                           ret = zio_handle_decrypt_injection(spa,
                               &zio->io_bookmark, ot, ECKSUM);
                   }
                   if (ret != 0)
                           goto error;
   
                   return;
           }
   
           /*
            * If this is an authenticated block, just check the MAC. It would be
            * nice to separate this out into its own flag, but when this was done,
            * we had run out of bits in what is now zio_flag_t. Future cleanup
            * could make this a flag bit.
            */
           if (BP_IS_AUTHENTICATED(bp)) {
                   if (ot == DMU_OT_OBJSET) {
                           ret = spa_do_crypt_objset_mac_abd(B_FALSE, spa,
                               dsobj, zio->io_abd, size, BP_SHOULD_BYTESWAP(bp));
                   } else {
                           zio_crypt_decode_mac_bp(bp, mac);
                           ret = spa_do_crypt_mac_abd(B_FALSE, spa, dsobj,
                               zio->io_abd, size, mac);
                           if (zio_injection_enabled && ret == 0) {
                                   ret = zio_handle_decrypt_injection(spa,
                                       &zio->io_bookmark, ot, ECKSUM);
                           }
                   }
                   abd_copy(data, zio->io_abd, size);
   
                   if (ret != 0)
                           goto error;
   
                   return;
           }
   
           zio_crypt_decode_params_bp(bp, salt, iv);
   
           if (ot == DMU_OT_INTENT_LOG) {
                   tmp = abd_borrow_buf_copy(zio->io_abd, sizeof (zil_chain_t));
                   zio_crypt_decode_mac_zil(tmp, mac);
                   abd_return_buf(zio->io_abd, tmp, sizeof (zil_chain_t));
           } else {
                   zio_crypt_decode_mac_bp(bp, mac);
           }
   
           ret = spa_do_crypt_abd(B_FALSE, spa, &zio->io_bookmark, BP_GET_TYPE(bp),
               BP_GET_DEDUP(bp), BP_SHOULD_BYTESWAP(bp), salt, iv, mac, size, data,
               zio->io_abd, &no_crypt);
           if (no_crypt)
                   abd_copy(data, zio->io_abd, size);
   
           if (ret != 0)
                   goto error;
   
           return;
   
   error:
           /* assert that the key was found unless this was speculative */
           ASSERT(ret != EACCES || (zio->io_flags & ZIO_FLAG_SPECULATIVE));
   
           /*
            * If there was a decryption / authentication error return EIO as
            * the io_error. If this was not a speculative zio, create an ereport.
            */
           if (ret == ECKSUM) {
                   zio->io_error = SET_ERROR(EIO);
                   if ((zio->io_flags & ZIO_FLAG_SPECULATIVE) == 0) {
                           spa_log_error(spa, &zio->io_bookmark);
                           (void) zfs_ereport_post(FM_EREPORT_ZFS_AUTHENTICATION,
                               spa, NULL, &zio->io_bookmark, zio, 0);
                   }
           } else {
                   zio->io_error = ret;
           }
   }
   
   /*
    * ==========================================================================
    * I/O parent/child relationships and pipeline interlocks
    * ==========================================================================
    */
   zio_t *
   zio_walk_parents(zio_t *cio, zio_link_t **zl)
   {
           list_t *pl = &cio->io_parent_list;
   
           *zl = (*zl == NULL) ? list_head(pl) : list_next(pl, *zl);
           if (*zl == NULL)
                   return (NULL);
   
           ASSERT((*zl)->zl_child == cio);
           return ((*zl)->zl_parent);
   }
   
   zio_t *
   zio_walk_children(zio_t *pio, zio_link_t **zl)
   {
           list_t *cl = &pio->io_child_list;
   
           ASSERT(MUTEX_HELD(&pio->io_lock));
   
           *zl = (*zl == NULL) ? list_head(cl) : list_next(cl, *zl);
           if (*zl == NULL)
                   return (NULL);
   
           ASSERT((*zl)->zl_parent == pio);
           return ((*zl)->zl_child);
   }
   
   zio_t *
   zio_unique_parent(zio_t *cio)
   {
           zio_link_t *zl = NULL;
           zio_t *pio = zio_walk_parents(cio, &zl);
   
           VERIFY3P(zio_walk_parents(cio, &zl), ==, NULL);
           return (pio);
   }
   
   void
   zio_add_child(zio_t *pio, zio_t *cio)
   {
           zio_link_t *zl = kmem_cache_alloc(zio_link_cache, KM_SLEEP);
   
           /*
            * Logical I/Os can have logical, gang, or vdev children.
            * Gang I/Os can have gang or vdev children.
            * Vdev I/Os can only have vdev children.
            * The following ASSERT captures all of these constraints.
            */
           ASSERT3S(cio->io_child_type, <=, pio->io_child_type);
   
           zl->zl_parent = pio;
           zl->zl_child = cio;
   
           mutex_enter(&pio->io_lock);
           mutex_enter(&cio->io_lock);
   
           ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);
   
           for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                   pio->io_children[cio->io_child_type][w] += !cio->io_state[w];
   
           list_insert_head(&pio->io_child_list, zl);
           list_insert_head(&cio->io_parent_list, zl);
   
           pio->io_child_count++;
           cio->io_parent_count++;
   
           mutex_exit(&cio->io_lock);
           mutex_exit(&pio->io_lock);
   }
   
   static void
   zio_remove_child(zio_t *pio, zio_t *cio, zio_link_t *zl)
   {
           ASSERT(zl->zl_parent == pio);
           ASSERT(zl->zl_child == cio);
   
           mutex_enter(&pio->io_lock);
           mutex_enter(&cio->io_lock);
   
           list_remove(&pio->io_child_list, zl);
           list_remove(&cio->io_parent_list, zl);
   
           pio->io_child_count--;
           cio->io_parent_count--;
   
           mutex_exit(&cio->io_lock);
           mutex_exit(&pio->io_lock);
           kmem_cache_free(zio_link_cache, zl);
   }
   
   static boolean_t
   zio_wait_for_children(zio_t *zio, uint8_t childbits, enum zio_wait_type wait)
   {
           boolean_t waiting = B_FALSE;
   
           mutex_enter(&zio->io_lock);
           ASSERT(zio->io_stall == NULL);
           for (int c = 0; c < ZIO_CHILD_TYPES; c++) {
                   if (!(ZIO_CHILD_BIT_IS_SET(childbits, c)))
                           continue;
   
                   uint64_t *countp = &zio->io_children[c][wait];
                   if (*countp != 0) {
                           zio->io_stage >>= 1;
                           ASSERT3U(zio->io_stage, !=, ZIO_STAGE_OPEN);
                           zio->io_stall = countp;
                           waiting = B_TRUE;
                           break;
                   }
           }
           mutex_exit(&zio->io_lock);
           return (waiting);
   }
   
   __attribute__((always_inline))
   static inline void
   zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait,
       zio_t **next_to_executep)
   {
           uint64_t *countp = &pio->io_children[zio->io_child_type][wait];
           int *errorp = &pio->io_child_error[zio->io_child_type];
   
           mutex_enter(&pio->io_lock);
           if (zio->io_error && !(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE))
                   *errorp = zio_worst_error(*errorp, zio->io_error);
           pio->io_reexecute |= zio->io_reexecute;
           ASSERT3U(*countp, >, 0);
   
           (*countp)--;
   
           if (*countp == 0 && pio->io_stall == countp) {
                   zio_taskq_type_t type =
                       pio->io_stage < ZIO_STAGE_VDEV_IO_START ? ZIO_TASKQ_ISSUE :
                       ZIO_TASKQ_INTERRUPT;
                   pio->io_stall = NULL;
                   mutex_exit(&pio->io_lock);
   
                   /*
                    * If we can tell the caller to execute this parent next, do
                    * so. We only do this if the parent's zio type matches the
                    * child's type. Otherwise dispatch the parent zio in its
                    * own taskq.
                    *
                    * Having the caller execute the parent when possible reduces
                    * locking on the zio taskq's, reduces context switch
                    * overhead, and has no recursion penalty.  Note that one
                    * read from disk typically causes at least 3 zio's: a
                    * zio_null(), the logical zio_read(), and then a physical
                    * zio.  When the physical ZIO completes, we are able to call
                    * zio_done() on all 3 of these zio's from one invocation of
                    * zio_execute() by returning the parent back to
                    * zio_execute().  Since the parent isn't executed until this
                    * thread returns back to zio_execute(), the caller should do
                    * so promptly.
                    *
                    * In other cases, dispatching the parent prevents
                    * overflowing the stack when we have deeply nested
                    * parent-child relationships, as we do with the "mega zio"
                    * of writes for spa_sync(), and the chain of ZIL blocks.
                    */
                   if (next_to_executep != NULL && *next_to_executep == NULL &&
                       pio->io_type == zio->io_type) {
                           *next_to_executep = pio;
                   } else {
                           zio_taskq_dispatch(pio, type, B_FALSE);
                   }
           } else {
                   mutex_exit(&pio->io_lock);
           }
   }
   
   static void
   zio_inherit_child_errors(zio_t *zio, enum zio_child c)
   {
           if (zio->io_child_error[c] != 0 && zio->io_error == 0)
                   zio->io_error = zio->io_child_error[c];
   }
   
   int
   zio_bookmark_compare(const void *x1, const void *x2)
   {
           const zio_t *z1 = x1;
           const zio_t *z2 = x2;
   
           if (z1->io_bookmark.zb_objset < z2->io_bookmark.zb_objset)
                   return (-1);
           if (z1->io_bookmark.zb_objset > z2->io_bookmark.zb_objset)
                   return (1);
   
           if (z1->io_bookmark.zb_object < z2->io_bookmark.zb_object)
                   return (-1);
           if (z1->io_bookmark.zb_object > z2->io_bookmark.zb_object)
                   return (1);
   
           if (z1->io_bookmark.zb_level < z2->io_bookmark.zb_level)
                   return (-1);
           if (z1->io_bookmark.zb_level > z2->io_bookmark.zb_level)
                   return (1);
   
           if (z1->io_bookmark.zb_blkid < z2->io_bookmark.zb_blkid)
                   return (-1);
           if (z1->io_bookmark.zb_blkid > z2->io_bookmark.zb_blkid)
                   return (1);
   
           if (z1 < z2)
                   return (-1);
           if (z1 > z2)
                   return (1);
   
           return (0);
   }
   
   /*
    * ==========================================================================
    * Create the various types of I/O (read, write, free, etc)
    * ==========================================================================
    */
   static zio_t *
   zio_create(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
       abd_t *data, uint64_t lsize, uint64_t psize, zio_done_func_t *done,
       void *private, zio_type_t type, zio_priority_t priority,
       zio_flag_t flags, vdev_t *vd, uint64_t offset,
       const zbookmark_phys_t *zb, enum zio_stage stage,
       enum zio_stage pipeline)
   {
           zio_t *zio;
   
           IMPLY(type != ZIO_TYPE_TRIM, psize <= SPA_MAXBLOCKSIZE);
           ASSERT(P2PHASE(psize, SPA_MINBLOCKSIZE) == 0);
           ASSERT(P2PHASE(offset, SPA_MINBLOCKSIZE) == 0);
   
           ASSERT(!vd || spa_config_held(spa, SCL_STATE_ALL, RW_READER));
           ASSERT(!bp || !(flags & ZIO_FLAG_CONFIG_WRITER));
           ASSERT(vd || stage == ZIO_STAGE_OPEN);
   
           IMPLY(lsize != psize, (flags & ZIO_FLAG_RAW_COMPRESS) != 0);
   
           zio = kmem_cache_alloc(zio_cache, KM_SLEEP);
           memset(zio, 0, sizeof (zio_t));
   
           mutex_init(&zio->io_lock, NULL, MUTEX_NOLOCKDEP, NULL);
           cv_init(&zio->io_cv, NULL, CV_DEFAULT, NULL);
   
           list_create(&zio->io_parent_list, sizeof (zio_link_t),
               offsetof(zio_link_t, zl_parent_node));
           list_create(&zio->io_child_list, sizeof (zio_link_t),
               offsetof(zio_link_t, zl_child_node));
           metaslab_trace_init(&zio->io_alloc_list);
   
           if (vd != NULL)
                   zio->io_child_type = ZIO_CHILD_VDEV;
           else if (flags & ZIO_FLAG_GANG_CHILD)
                   zio->io_child_type = ZIO_CHILD_GANG;
           else if (flags & ZIO_FLAG_DDT_CHILD)
                   zio->io_child_type = ZIO_CHILD_DDT;
           else
                   zio->io_child_type = ZIO_CHILD_LOGICAL;
   
           if (bp != NULL) {
                   zio->io_bp = (blkptr_t *)bp;
                   zio->io_bp_copy = *bp;
                   zio->io_bp_orig = *bp;
                   if (type != ZIO_TYPE_WRITE ||
                       zio->io_child_type == ZIO_CHILD_DDT)
                           zio->io_bp = &zio->io_bp_copy;  /* so caller can free */
                   if (zio->io_child_type == ZIO_CHILD_LOGICAL)
                           zio->io_logical = zio;
                   if (zio->io_child_type > ZIO_CHILD_GANG && BP_IS_GANG(bp))
                           pipeline |= ZIO_GANG_STAGES;
           }
   
           zio->io_spa = spa;
           zio->io_txg = txg;
           zio->io_done = done;
           zio->io_private = private;
           zio->io_type = type;
           zio->io_priority = priority;
           zio->io_vd = vd;
           zio->io_offset = offset;
           zio->io_orig_abd = zio->io_abd = data;
           zio->io_orig_size = zio->io_size = psize;
           zio->io_lsize = lsize;
           zio->io_orig_flags = zio->io_flags = flags;
           zio->io_orig_stage = zio->io_stage = stage;
           zio->io_orig_pipeline = zio->io_pipeline = pipeline;
           zio->io_pipeline_trace = ZIO_STAGE_OPEN;
   
           zio->io_state[ZIO_WAIT_READY] = (stage >= ZIO_STAGE_READY);
           zio->io_state[ZIO_WAIT_DONE] = (stage >= ZIO_STAGE_DONE);
   
           if (zb != NULL)
                   zio->io_bookmark = *zb;
   
           if (pio != NULL) {
                   zio->io_metaslab_class = pio->io_metaslab_class;
                   if (zio->io_logical == NULL)
                           zio->io_logical = pio->io_logical;
                   if (zio->io_child_type == ZIO_CHILD_GANG)
                           zio->io_gang_leader = pio->io_gang_leader;
                   zio_add_child(pio, zio);
           }
   
           taskq_init_ent(&zio->io_tqent);
   
           return (zio);
   }
   
   void
   zio_destroy(zio_t *zio)
   {
           metaslab_trace_fini(&zio->io_alloc_list);
           list_destroy(&zio->io_parent_list);
           list_destroy(&zio->io_child_list);
           mutex_destroy(&zio->io_lock);
           cv_destroy(&zio->io_cv);
           kmem_cache_free(zio_cache, zio);
   }
   
   zio_t *
   zio_null(zio_t *pio, spa_t *spa, vdev_t *vd, zio_done_func_t *done,
       void *private, zio_flag_t flags)
   {
           zio_t *zio;
   
           zio = zio_create(pio, spa, 0, NULL, NULL, 0, 0, done, private,
               ZIO_TYPE_NULL, ZIO_PRIORITY_NOW, flags, vd, 0, NULL,
               ZIO_STAGE_OPEN, ZIO_INTERLOCK_PIPELINE);
   
           return (zio);
   }
   
   zio_t *
   zio_root(spa_t *spa, zio_done_func_t *done, void *private, zio_flag_t flags)
   {
           return (zio_null(NULL, spa, NULL, done, private, flags));
   }
   
   static int
   zfs_blkptr_verify_log(spa_t *spa, const blkptr_t *bp,
       enum blk_verify_flag blk_verify, const char *fmt, ...)
   {
           va_list adx;
           char buf[256];
   
           va_start(adx, fmt);
           (void) vsnprintf(buf, sizeof (buf), fmt, adx);
           va_end(adx);
   
           switch (blk_verify) {
           case BLK_VERIFY_HALT:
                   dprintf_bp(bp, "blkptr at %p dprintf_bp():", bp);
                   zfs_panic_recover("%s: %s", spa_name(spa), buf);
                   break;
           case BLK_VERIFY_LOG:
                   zfs_dbgmsg("%s: %s", spa_name(spa), buf);
                   break;
           case BLK_VERIFY_ONLY:
                   break;
           }
   
           return (1);
   }
   
   /*
    * Verify the block pointer fields contain reasonable values.  This means
    * it only contains known object types, checksum/compression identifiers,
    * block sizes within the maximum allowed limits, valid DVAs, etc.
    *
    * If everything checks out B_TRUE is returned.  The zfs_blkptr_verify
    * argument controls the behavior when an invalid field is detected.
    *
    * Modes for zfs_blkptr_verify:
    *   1) BLK_VERIFY_ONLY (evaluate the block)
    *   2) BLK_VERIFY_LOG (evaluate the block and log problems)
    *   3) BLK_VERIFY_HALT (call zfs_panic_recover on error)
    */
   boolean_t
   zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp, boolean_t config_held,
       enum blk_verify_flag blk_verify)
   {
           int errors = 0;
   
           if (!DMU_OT_IS_VALID(BP_GET_TYPE(bp))) {
                   errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                       "blkptr at %p has invalid TYPE %llu",
                       bp, (longlong_t)BP_GET_TYPE(bp));
           }
           if (BP_GET_CHECKSUM(bp) >= ZIO_CHECKSUM_FUNCTIONS) {
                   errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                       "blkptr at %p has invalid CHECKSUM %llu",
                       bp, (longlong_t)BP_GET_CHECKSUM(bp));
           }
           if (BP_GET_COMPRESS(bp) >= ZIO_COMPRESS_FUNCTIONS) {
                   errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                       "blkptr at %p has invalid COMPRESS %llu",
                       bp, (longlong_t)BP_GET_COMPRESS(bp));
           }
           if (BP_GET_LSIZE(bp) > SPA_MAXBLOCKSIZE) {
                   errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                       "blkptr at %p has invalid LSIZE %llu",
                       bp, (longlong_t)BP_GET_LSIZE(bp));
           }
           if (BP_GET_PSIZE(bp) > SPA_MAXBLOCKSIZE) {
                   errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                       "blkptr at %p has invalid PSIZE %llu",
                       bp, (longlong_t)BP_GET_PSIZE(bp));
           }
   
           if (BP_IS_EMBEDDED(bp)) {
                   if (BPE_GET_ETYPE(bp) >= NUM_BP_EMBEDDED_TYPES) {
                           errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                               "blkptr at %p has invalid ETYPE %llu",
                              bp, (longlong_t)BPE_GET_ETYPE(bp));
                  }
          }
  
          /*
           * Do not verify individual DVAs if the config is not trusted. This
           * will be done once the zio is executed in vdev_mirror_map_alloc.
           */
          if (!spa->spa_trust_config)
                  return (errors == 0);
  
          if (!config_held)
                  spa_config_enter(spa, SCL_VDEV, bp, RW_READER);
          else
                  ASSERT(spa_config_held(spa, SCL_VDEV, RW_WRITER));
          /*
           * Pool-specific checks.
           *
           * Note: it would be nice to verify that the blk_birth and
           * BP_PHYSICAL_BIRTH() are not too large.  However, spa_freeze()
           * allows the birth time of log blocks (and dmu_sync()-ed blocks
           * that are in the log) to be arbitrarily large.
           */
          for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
                  const dva_t *dva = &bp->blk_dva[i];
                  uint64_t vdevid = DVA_GET_VDEV(dva);
  
                  if (vdevid >= spa->spa_root_vdev->vdev_children) {
                          errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                              "blkptr at %p DVA %u has invalid VDEV %llu",
                              bp, i, (longlong_t)vdevid);
                          continue;
                  }
                  vdev_t *vd = spa->spa_root_vdev->vdev_child[vdevid];
                  if (vd == NULL) {
                          errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                              "blkptr at %p DVA %u has invalid VDEV %llu",
                              bp, i, (longlong_t)vdevid);
                          continue;
                  }
                  if (vd->vdev_ops == &vdev_hole_ops) {
                          errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                              "blkptr at %p DVA %u has hole VDEV %llu",
                              bp, i, (longlong_t)vdevid);
                          continue;
                  }
                  if (vd->vdev_ops == &vdev_missing_ops) {
                          /*
                           * "missing" vdevs are valid during import, but we
                           * don't have their detailed info (e.g. asize), so
                           * we can't perform any more checks on them.
                           */
                          continue;
                  }
                  uint64_t offset = DVA_GET_OFFSET(dva);
                  uint64_t asize = DVA_GET_ASIZE(dva);
                  if (DVA_GET_GANG(dva))
                          asize = vdev_gang_header_asize(vd);
                  if (offset + asize > vd->vdev_asize) {
                          errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
                              "blkptr at %p DVA %u has invalid OFFSET %llu",
                              bp, i, (longlong_t)offset);
                  }
          }
          if (errors > 0)
                  dprintf_bp(bp, "blkptr at %p dprintf_bp():", bp);
          if (!config_held)
                  spa_config_exit(spa, SCL_VDEV, bp);
  
          return (errors == 0);
  }
  
  boolean_t
  zfs_dva_valid(spa_t *spa, const dva_t *dva, const blkptr_t *bp)
  {
          (void) bp;
          uint64_t vdevid = DVA_GET_VDEV(dva);
  
          if (vdevid >= spa->spa_root_vdev->vdev_children)
                  return (B_FALSE);
  
          vdev_t *vd = spa->spa_root_vdev->vdev_child[vdevid];
          if (vd == NULL)
                  return (B_FALSE);
  
          if (vd->vdev_ops == &vdev_hole_ops)
                  return (B_FALSE);
  
          if (vd->vdev_ops == &vdev_missing_ops) {
                  return (B_FALSE);
          }
  
          uint64_t offset = DVA_GET_OFFSET(dva);
          uint64_t asize = DVA_GET_ASIZE(dva);
  
          if (DVA_GET_GANG(dva))
                  asize = vdev_gang_header_asize(vd);
          if (offset + asize > vd->vdev_asize)
                  return (B_FALSE);
  
          return (B_TRUE);
  }
  
  zio_t *
  zio_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
      abd_t *data, uint64_t size, zio_done_func_t *done, void *private,
      zio_priority_t priority, zio_flag_t flags, const zbookmark_phys_t *zb)
  {
          zio_t *zio;
  
          zio = zio_create(pio, spa, BP_PHYSICAL_BIRTH(bp), bp,
              data, size, size, done, private,
              ZIO_TYPE_READ, priority, flags, NULL, 0, zb,
              ZIO_STAGE_OPEN, (flags & ZIO_FLAG_DDT_CHILD) ?
              ZIO_DDT_CHILD_READ_PIPELINE : ZIO_READ_PIPELINE);
  
          return (zio);
  }
  
  zio_t *
  zio_write(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
      abd_t *data, uint64_t lsize, uint64_t psize, const zio_prop_t *zp,
      zio_done_func_t *ready, zio_done_func_t *children_ready,
      zio_done_func_t *physdone, zio_done_func_t *done,
      void *private, zio_priority_t priority, zio_flag_t flags,
      const zbookmark_phys_t *zb)
  {
          zio_t *zio;
  
          ASSERT(zp->zp_checksum >= ZIO_CHECKSUM_OFF &&
              zp->zp_checksum < ZIO_CHECKSUM_FUNCTIONS &&
              zp->zp_compress >= ZIO_COMPRESS_OFF &&
              zp->zp_compress < ZIO_COMPRESS_FUNCTIONS &&
              DMU_OT_IS_VALID(zp->zp_type) &&
              zp->zp_level < 32 &&
              zp->zp_copies > 0 &&
              zp->zp_copies <= spa_max_replication(spa));
  
          zio = zio_create(pio, spa, txg, bp, data, lsize, psize, done, private,
              ZIO_TYPE_WRITE, priority, flags, NULL, 0, zb,
              ZIO_STAGE_OPEN, (flags & ZIO_FLAG_DDT_CHILD) ?
              ZIO_DDT_CHILD_WRITE_PIPELINE : ZIO_WRITE_PIPELINE);
  
          zio->io_ready = ready;
          zio->io_children_ready = children_ready;
          zio->io_physdone = physdone;
          zio->io_prop = *zp;
  
          /*
           * Data can be NULL if we are going to call zio_write_override() to
           * provide the already-allocated BP.  But we may need the data to
           * verify a dedup hit (if requested).  In this case, don't try to
           * dedup (just take the already-allocated BP verbatim). Encrypted
           * dedup blocks need data as well so we also disable dedup in this
           * case.
           */
          if (data == NULL &&
              (zio->io_prop.zp_dedup_verify || zio->io_prop.zp_encrypt)) {
                  zio->io_prop.zp_dedup = zio->io_prop.zp_dedup_verify = B_FALSE;
          }
  
          return (zio);
  }
  
  zio_t *
  zio_rewrite(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp, abd_t *data,
      uint64_t size, zio_done_func_t *done, void *private,
      zio_priority_t priority, zio_flag_t flags, zbookmark_phys_t *zb)
  {
          zio_t *zio;
  
          zio = zio_create(pio, spa, txg, bp, data, size, size, done, private,
              ZIO_TYPE_WRITE, priority, flags | ZIO_FLAG_IO_REWRITE, NULL, 0, zb,
              ZIO_STAGE_OPEN, ZIO_REWRITE_PIPELINE);
  
          return (zio);
  }
  
  void
  zio_write_override(zio_t *zio, blkptr_t *bp, int copies, boolean_t nopwrite)
  {
          ASSERT(zio->io_type == ZIO_TYPE_WRITE);
          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
          ASSERT(zio->io_stage == ZIO_STAGE_OPEN);
          ASSERT(zio->io_txg == spa_syncing_txg(zio->io_spa));
  
          /*
           * We must reset the io_prop to match the values that existed
           * when the bp was first written by dmu_sync() keeping in mind
           * that nopwrite and dedup are mutually exclusive.
           */
          zio->io_prop.zp_dedup = nopwrite ? B_FALSE : zio->io_prop.zp_dedup;
          zio->io_prop.zp_nopwrite = nopwrite;
          zio->io_prop.zp_copies = copies;
          zio->io_bp_override = bp;
  }
  
  void
  zio_free(spa_t *spa, uint64_t txg, const blkptr_t *bp)
  {
  
          (void) zfs_blkptr_verify(spa, bp, B_FALSE, BLK_VERIFY_HALT);
  
          /*
           * The check for EMBEDDED is a performance optimization.  We
           * process the free here (by ignoring it) rather than
           * putting it on the list and then processing it in zio_free_sync().
           */
          if (BP_IS_EMBEDDED(bp))
                  return;
  
          /*
           * Frees that are for the currently-syncing txg, are not going to be
           * deferred, and which will not need to do a read (i.e. not GANG or
           * DEDUP), can be processed immediately.  Otherwise, put them on the
           * in-memory list for later processing.
           *
           * Note that we only defer frees after zfs_sync_pass_deferred_free
           * when the log space map feature is disabled. [see relevant comment
           * in spa_sync_iterate_to_convergence()]
           */
          if (BP_IS_GANG(bp) ||
              BP_GET_DEDUP(bp) ||
              txg != spa->spa_syncing_txg ||
              (spa_sync_pass(spa) >= zfs_sync_pass_deferred_free &&
              !spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP))) {
                  metaslab_check_free(spa, bp);
                  bplist_append(&spa->spa_free_bplist[txg & TXG_MASK], bp);
          } else {
                  VERIFY3P(zio_free_sync(NULL, spa, txg, bp, 0), ==, NULL);
          }
  }
  
  /*
   * To improve performance, this function may return NULL if we were able
   * to do the free immediately.  This avoids the cost of creating a zio
   * (and linking it to the parent, etc).
   */
  zio_t *
  zio_free_sync(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
      zio_flag_t flags)
  {
          ASSERT(!BP_IS_HOLE(bp));
          ASSERT(spa_syncing_txg(spa) == txg);
  
          if (BP_IS_EMBEDDED(bp))
                  return (NULL);
  
          metaslab_check_free(spa, bp);
          arc_freed(spa, bp);
          dsl_scan_freed(spa, bp);
  
          if (BP_IS_GANG(bp) || BP_GET_DEDUP(bp)) {
                  /*
                   * GANG and DEDUP blocks can induce a read (for the gang block
                   * header, or the DDT), so issue them asynchronously so that
                   * this thread is not tied up.
                   */
                  enum zio_stage stage =
                      ZIO_FREE_PIPELINE | ZIO_STAGE_ISSUE_ASYNC;
  
                  return (zio_create(pio, spa, txg, bp, NULL, BP_GET_PSIZE(bp),
                      BP_GET_PSIZE(bp), NULL, NULL,
                      ZIO_TYPE_FREE, ZIO_PRIORITY_NOW,
                      flags, NULL, 0, NULL, ZIO_STAGE_OPEN, stage));
          } else {
                  metaslab_free(spa, bp, txg, B_FALSE);
                  return (NULL);
          }
  }
  
  zio_t *
  zio_claim(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
      zio_done_func_t *done, void *private, zio_flag_t flags)
  {
          zio_t *zio;
  
          (void) zfs_blkptr_verify(spa, bp, flags & ZIO_FLAG_CONFIG_WRITER,
              BLK_VERIFY_HALT);
  
          if (BP_IS_EMBEDDED(bp))
                  return (zio_null(pio, spa, NULL, NULL, NULL, 0));
  
          /*
           * A claim is an allocation of a specific block.  Claims are needed
           * to support immediate writes in the intent log.  The issue is that
           * immediate writes contain committed data, but in a txg that was
           * *not* committed.  Upon opening the pool after an unclean shutdown,
           * the intent log claims all blocks that contain immediate write data
           * so that the SPA knows they're in use.
           *
           * All claims *must* be resolved in the first txg -- before the SPA
           * starts allocating blocks -- so that nothing is allocated twice.
           * If txg == 0 we just verify that the block is claimable.
           */
          ASSERT3U(spa->spa_uberblock.ub_rootbp.blk_birth, <,
              spa_min_claim_txg(spa));
          ASSERT(txg == spa_min_claim_txg(spa) || txg == 0);
          ASSERT(!BP_GET_DEDUP(bp) || !spa_writeable(spa));       /* zdb(8) */
  
          zio = zio_create(pio, spa, txg, bp, NULL, BP_GET_PSIZE(bp),
              BP_GET_PSIZE(bp), done, private, ZIO_TYPE_CLAIM, ZIO_PRIORITY_NOW,
              flags, NULL, 0, NULL, ZIO_STAGE_OPEN, ZIO_CLAIM_PIPELINE);
          ASSERT0(zio->io_queued_timestamp);
  
          return (zio);
  }
  
  zio_t *
  zio_ioctl(zio_t *pio, spa_t *spa, vdev_t *vd, int cmd,
      zio_done_func_t *done, void *private, zio_flag_t flags)
  {
          zio_t *zio;
          int c;
  
          if (vd->vdev_children == 0) {
                  zio = zio_create(pio, spa, 0, NULL, NULL, 0, 0, done, private,
                      ZIO_TYPE_IOCTL, ZIO_PRIORITY_NOW, flags, vd, 0, NULL,
                      ZIO_STAGE_OPEN, ZIO_IOCTL_PIPELINE);
  
                  zio->io_cmd = cmd;
          } else {
                  zio = zio_null(pio, spa, NULL, NULL, NULL, flags);
  
                  for (c = 0; c < vd->vdev_children; c++)
                          zio_nowait(zio_ioctl(zio, spa, vd->vdev_child[c], cmd,
                              done, private, flags));
          }
  
          return (zio);
  }
  
  zio_t *
  zio_trim(zio_t *pio, vdev_t *vd, uint64_t offset, uint64_t size,
      zio_done_func_t *done, void *private, zio_priority_t priority,
      zio_flag_t flags, enum trim_flag trim_flags)
  {
          zio_t *zio;
  
          ASSERT0(vd->vdev_children);
          ASSERT0(P2PHASE(offset, 1ULL << vd->vdev_ashift));
          ASSERT0(P2PHASE(size, 1ULL << vd->vdev_ashift));
          ASSERT3U(size, !=, 0);
  
          zio = zio_create(pio, vd->vdev_spa, 0, NULL, NULL, size, size, done,
              private, ZIO_TYPE_TRIM, priority, flags | ZIO_FLAG_PHYSICAL,
              vd, offset, NULL, ZIO_STAGE_OPEN, ZIO_TRIM_PIPELINE);
          zio->io_trim_flags = trim_flags;
  
          return (zio);
  }
  
  zio_t *
  zio_read_phys(zio_t *pio, vdev_t *vd, uint64_t offset, uint64_t size,
      abd_t *data, int checksum, zio_done_func_t *done, void *private,
      zio_priority_t priority, zio_flag_t flags, boolean_t labels)
  {
          zio_t *zio;
  
          ASSERT(vd->vdev_children == 0);
          ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
              offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
          ASSERT3U(offset + size, <=, vd->vdev_psize);
  
          zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, size, done,
              private, ZIO_TYPE_READ, priority, flags | ZIO_FLAG_PHYSICAL, vd,
              offset, NULL, ZIO_STAGE_OPEN, ZIO_READ_PHYS_PIPELINE);
  
          zio->io_prop.zp_checksum = checksum;
  
          return (zio);
  }
  
  zio_t *
  zio_write_phys(zio_t *pio, vdev_t *vd, uint64_t offset, uint64_t size,
      abd_t *data, int checksum, zio_done_func_t *done, void *private,
      zio_priority_t priority, zio_flag_t flags, boolean_t labels)
  {
          zio_t *zio;
  
          ASSERT(vd->vdev_children == 0);
          ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
              offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
          ASSERT3U(offset + size, <=, vd->vdev_psize);
  
          zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, size, done,
              private, ZIO_TYPE_WRITE, priority, flags | ZIO_FLAG_PHYSICAL, vd,
              offset, NULL, ZIO_STAGE_OPEN, ZIO_WRITE_PHYS_PIPELINE);
  
          zio->io_prop.zp_checksum = checksum;
  
          if (zio_checksum_table[checksum].ci_flags & ZCHECKSUM_FLAG_EMBEDDED) {
                  /*
                   * zec checksums are necessarily destructive -- they modify
                   * the end of the write buffer to hold the verifier/checksum.
                   * Therefore, we must make a local copy in case the data is
                   * being written to multiple places in parallel.
                   */
                  abd_t *wbuf = abd_alloc_sametype(data, size);
                  abd_copy(wbuf, data, size);
  
                  zio_push_transform(zio, wbuf, size, size, NULL);
          }
  
          return (zio);
  }
  
  /*
   * Create a child I/O to do some work for us.
   */
  zio_t *
  zio_vdev_child_io(zio_t *pio, blkptr_t *bp, vdev_t *vd, uint64_t offset,
      abd_t *data, uint64_t size, int type, zio_priority_t priority,
      zio_flag_t flags, zio_done_func_t *done, void *private)
  {
          enum zio_stage pipeline = ZIO_VDEV_CHILD_PIPELINE;
          zio_t *zio;
  
          /*
           * vdev child I/Os do not propagate their error to the parent.
           * Therefore, for correct operation the caller *must* check for
           * and handle the error in the child i/o's done callback.
           * The only exceptions are i/os that we don't care about
           * (OPTIONAL or REPAIR).
           */
          ASSERT((flags & ZIO_FLAG_OPTIONAL) || (flags & ZIO_FLAG_IO_REPAIR) ||
              done != NULL);
  
          if (type == ZIO_TYPE_READ && bp != NULL) {
                  /*
                   * If we have the bp, then the child should perform the
                   * checksum and the parent need not.  This pushes error
                   * detection as close to the leaves as possible and
                   * eliminates redundant checksums in the interior nodes.
                   */
                  pipeline |= ZIO_STAGE_CHECKSUM_VERIFY;
                  pio->io_pipeline &= ~ZIO_STAGE_CHECKSUM_VERIFY;
          }
  
          if (vd->vdev_ops->vdev_op_leaf) {
                  ASSERT0(vd->vdev_children);
                  offset += VDEV_LABEL_START_SIZE;
          }
  
          flags |= ZIO_VDEV_CHILD_FLAGS(pio);
  
          /*
           * If we've decided to do a repair, the write is not speculative --
           * even if the original read was.
           */
          if (flags & ZIO_FLAG_IO_REPAIR)
                  flags &= ~ZIO_FLAG_SPECULATIVE;
  
          /*
           * If we're creating a child I/O that is not associated with a
           * top-level vdev, then the child zio is not an allocating I/O.
           * If this is a retried I/O then we ignore it since we will
           * have already processed the original allocating I/O.
           */
          if (flags & ZIO_FLAG_IO_ALLOCATING &&
              (vd != vd->vdev_top || (flags & ZIO_FLAG_IO_RETRY))) {
                  ASSERT(pio->io_metaslab_class != NULL);
                  ASSERT(pio->io_metaslab_class->mc_alloc_throttle_enabled);
                  ASSERT(type == ZIO_TYPE_WRITE);
                  ASSERT(priority == ZIO_PRIORITY_ASYNC_WRITE);
                  ASSERT(!(flags & ZIO_FLAG_IO_REPAIR));
                  ASSERT(!(pio->io_flags & ZIO_FLAG_IO_REWRITE) ||
                      pio->io_child_type == ZIO_CHILD_GANG);
  
                  flags &= ~ZIO_FLAG_IO_ALLOCATING;
          }
  
  
          zio = zio_create(pio, pio->io_spa, pio->io_txg, bp, data, size, size,
              done, private, type, priority, flags, vd, offset, &pio->io_bookmark,
              ZIO_STAGE_VDEV_IO_START >> 1, pipeline);
          ASSERT3U(zio->io_child_type, ==, ZIO_CHILD_VDEV);
  
          zio->io_physdone = pio->io_physdone;
          if (vd->vdev_ops->vdev_op_leaf && zio->io_logical != NULL)
                  zio->io_logical->io_phys_children++;
  
          return (zio);
  }
  
  zio_t *
  zio_vdev_delegated_io(vdev_t *vd, uint64_t offset, abd_t *data, uint64_t size,
      zio_type_t type, zio_priority_t priority, zio_flag_t flags,
      zio_done_func_t *done, void *private)
  {
          zio_t *zio;
  
          ASSERT(vd->vdev_ops->vdev_op_leaf);
  
          zio = zio_create(NULL, vd->vdev_spa, 0, NULL,
              data, size, size, done, private, type, priority,
              flags | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_RETRY | ZIO_FLAG_DELEGATED,
              vd, offset, NULL,
              ZIO_STAGE_VDEV_IO_START >> 1, ZIO_VDEV_CHILD_PIPELINE);
  
          return (zio);
  }
  
  void
  zio_flush(zio_t *zio, vdev_t *vd)
  {
          zio_nowait(zio_ioctl(zio, zio->io_spa, vd, DKIOCFLUSHWRITECACHE,
              NULL, NULL,
              ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY));
  }
  
  void
  zio_shrink(zio_t *zio, uint64_t size)
  {
          ASSERT3P(zio->io_executor, ==, NULL);
          ASSERT3U(zio->io_orig_size, ==, zio->io_size);
          ASSERT3U(size, <=, zio->io_size);
  
          /*
           * We don't shrink for raidz because of problems with the
           * reconstruction when reading back less than the block size.
           * Note, BP_IS_RAIDZ() assumes no compression.
           */
          ASSERT(BP_GET_COMPRESS(zio->io_bp) == ZIO_COMPRESS_OFF);
          if (!BP_IS_RAIDZ(zio->io_bp)) {
                  /* we are not doing a raw write */
                  ASSERT3U(zio->io_size, ==, zio->io_lsize);
                  zio->io_orig_size = zio->io_size = zio->io_lsize = size;
          }
  }
  
  /*
   * ==========================================================================
   * Prepare to read and write logical blocks
   * ==========================================================================
   */
  
  static zio_t *
  zio_read_bp_init(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
          uint64_t psize =
              BP_IS_EMBEDDED(bp) ? BPE_GET_PSIZE(bp) : BP_GET_PSIZE(bp);
  
          ASSERT3P(zio->io_bp, ==, &zio->io_bp_copy);
  
          if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF &&
              zio->io_child_type == ZIO_CHILD_LOGICAL &&
              !(zio->io_flags & ZIO_FLAG_RAW_COMPRESS)) {
                  zio_push_transform(zio, abd_alloc_sametype(zio->io_abd, psize),
                      psize, psize, zio_decompress);
          }
  
          if (((BP_IS_PROTECTED(bp) && !(zio->io_flags & ZIO_FLAG_RAW_ENCRYPT)) ||
              BP_HAS_INDIRECT_MAC_CKSUM(bp)) &&
              zio->io_child_type == ZIO_CHILD_LOGICAL) {
                  zio_push_transform(zio, abd_alloc_sametype(zio->io_abd, psize),
                      psize, psize, zio_decrypt);
          }
  
          if (BP_IS_EMBEDDED(bp) && BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA) {
                  int psize = BPE_GET_PSIZE(bp);
                  void *data = abd_borrow_buf(zio->io_abd, psize);
  
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
                  decode_embedded_bp_compressed(bp, data);
                  abd_return_buf_copy(zio->io_abd, data, psize);
          } else {
                  ASSERT(!BP_IS_EMBEDDED(bp));
                  ASSERT3P(zio->io_bp, ==, &zio->io_bp_copy);
          }
  
          if (!DMU_OT_IS_METADATA(BP_GET_TYPE(bp)) && BP_GET_LEVEL(bp) == 0)
                  zio->io_flags |= ZIO_FLAG_DONT_CACHE;
  
          if (BP_GET_TYPE(bp) == DMU_OT_DDT_ZAP)
                  zio->io_flags |= ZIO_FLAG_DONT_CACHE;
  
          if (BP_GET_DEDUP(bp) && zio->io_child_type == ZIO_CHILD_LOGICAL)
                  zio->io_pipeline = ZIO_DDT_READ_PIPELINE;
  
          return (zio);
  }
  
  static zio_t *
  zio_write_bp_init(zio_t *zio)
  {
          if (!IO_IS_ALLOCATING(zio))
                  return (zio);
  
          ASSERT(zio->io_child_type != ZIO_CHILD_DDT);
  
          if (zio->io_bp_override) {
                  blkptr_t *bp = zio->io_bp;
                  zio_prop_t *zp = &zio->io_prop;
  
                  ASSERT(bp->blk_birth != zio->io_txg);
                  ASSERT(BP_GET_DEDUP(zio->io_bp_override) == 0);
  
                  *bp = *zio->io_bp_override;
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
  
                  if (BP_IS_EMBEDDED(bp))
                          return (zio);
  
                  /*
                   * If we've been overridden and nopwrite is set then
                   * set the flag accordingly to indicate that a nopwrite
                   * has already occurred.
                   */
                  if (!BP_IS_HOLE(bp) && zp->zp_nopwrite) {
                          ASSERT(!zp->zp_dedup);
                          ASSERT3U(BP_GET_CHECKSUM(bp), ==, zp->zp_checksum);
                          zio->io_flags |= ZIO_FLAG_NOPWRITE;
                          return (zio);
                  }
  
                  ASSERT(!zp->zp_nopwrite);
  
                  if (BP_IS_HOLE(bp) || !zp->zp_dedup)
                          return (zio);
  
                  ASSERT((zio_checksum_table[zp->zp_checksum].ci_flags &
                      ZCHECKSUM_FLAG_DEDUP) || zp->zp_dedup_verify);
  
                  if (BP_GET_CHECKSUM(bp) == zp->zp_checksum &&
                      !zp->zp_encrypt) {
                          BP_SET_DEDUP(bp, 1);
                          zio->io_pipeline |= ZIO_STAGE_DDT_WRITE;
                          return (zio);
                  }
  
                  /*
                   * We were unable to handle this as an override bp, treat
                   * it as a regular write I/O.
                   */
                  zio->io_bp_override = NULL;
                  *bp = zio->io_bp_orig;
                  zio->io_pipeline = zio->io_orig_pipeline;
          }
  
          return (zio);
  }
  
  static zio_t *
  zio_write_compress(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          zio_prop_t *zp = &zio->io_prop;
          enum zio_compress compress = zp->zp_compress;
          blkptr_t *bp = zio->io_bp;
          uint64_t lsize = zio->io_lsize;
          uint64_t psize = zio->io_size;
          uint32_t pass = 1;
  
          /*
           * If our children haven't all reached the ready stage,
           * wait for them and then repeat this pipeline stage.
           */
          if (zio_wait_for_children(zio, ZIO_CHILD_LOGICAL_BIT |
              ZIO_CHILD_GANG_BIT, ZIO_WAIT_READY)) {
                  return (NULL);
          }
  
          if (!IO_IS_ALLOCATING(zio))
                  return (zio);
  
          if (zio->io_children_ready != NULL) {
                  /*
                   * Now that all our children are ready, run the callback
                   * associated with this zio in case it wants to modify the
                   * data to be written.
                   */
                  ASSERT3U(zp->zp_level, >, 0);
                  zio->io_children_ready(zio);
          }
  
          ASSERT(zio->io_child_type != ZIO_CHILD_DDT);
          ASSERT(zio->io_bp_override == NULL);
  
          if (!BP_IS_HOLE(bp) && bp->blk_birth == zio->io_txg) {
                  /*
                   * We're rewriting an existing block, which means we're
                   * working on behalf of spa_sync().  For spa_sync() to
                   * converge, it must eventually be the case that we don't
                   * have to allocate new blocks.  But compression changes
                   * the blocksize, which forces a reallocate, and makes
                   * convergence take longer.  Therefore, after the first
                   * few passes, stop compressing to ensure convergence.
                   */
                  pass = spa_sync_pass(spa);
  
                  ASSERT(zio->io_txg == spa_syncing_txg(spa));
                  ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                  ASSERT(!BP_GET_DEDUP(bp));
  
                  if (pass >= zfs_sync_pass_dont_compress)
                          compress = ZIO_COMPRESS_OFF;
  
                  /* Make sure someone doesn't change their mind on overwrites */
                  ASSERT(BP_IS_EMBEDDED(bp) || MIN(zp->zp_copies + BP_IS_GANG(bp),
                      spa_max_replication(spa)) == BP_GET_NDVAS(bp));
          }
  
          /* If it's a compressed write that is not raw, compress the buffer. */
          if (compress != ZIO_COMPRESS_OFF &&
              !(zio->io_flags & ZIO_FLAG_RAW_COMPRESS)) {
                  void *cbuf = zio_buf_alloc(lsize);
                  psize = zio_compress_data(compress, zio->io_abd, cbuf, lsize,
                      zp->zp_complevel);
                  if (psize == 0 || psize >= lsize) {
                          compress = ZIO_COMPRESS_OFF;
                          zio_buf_free(cbuf, lsize);
                  } else if (!zp->zp_dedup && !zp->zp_encrypt &&
                      psize <= BPE_PAYLOAD_SIZE &&
                      zp->zp_level == 0 && !DMU_OT_HAS_FILL(zp->zp_type) &&
                      spa_feature_is_enabled(spa, SPA_FEATURE_EMBEDDED_DATA)) {
                          encode_embedded_bp_compressed(bp,
                              cbuf, compress, lsize, psize);
                          BPE_SET_ETYPE(bp, BP_EMBEDDED_TYPE_DATA);
                          BP_SET_TYPE(bp, zio->io_prop.zp_type);
                          BP_SET_LEVEL(bp, zio->io_prop.zp_level);
                          zio_buf_free(cbuf, lsize);
                          bp->blk_birth = zio->io_txg;
                          zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
                          ASSERT(spa_feature_is_active(spa,
                              SPA_FEATURE_EMBEDDED_DATA));
                          return (zio);
                  } else {
                          /*
                           * Round compressed size up to the minimum allocation
                           * size of the smallest-ashift device, and zero the
                           * tail. This ensures that the compressed size of the
                           * BP (and thus compressratio property) are correct,
                           * in that we charge for the padding used to fill out
                           * the last sector.
                           */
                          ASSERT3U(spa->spa_min_alloc, >=, SPA_MINBLOCKSHIFT);
                          size_t rounded = (size_t)roundup(psize,
                              spa->spa_min_alloc);
                          if (rounded >= lsize) {
                                  compress = ZIO_COMPRESS_OFF;
                                  zio_buf_free(cbuf, lsize);
                                  psize = lsize;
                          } else {
                                  abd_t *cdata = abd_get_from_buf(cbuf, lsize);
                                  abd_take_ownership_of_buf(cdata, B_TRUE);
                                  abd_zero_off(cdata, psize, rounded - psize);
                                  psize = rounded;
                                  zio_push_transform(zio, cdata,
                                      psize, lsize, NULL);
                          }
                  }
  
                  /*
                   * We were unable to handle this as an override bp, treat
                   * it as a regular write I/O.
                   */
                  zio->io_bp_override = NULL;
                  *bp = zio->io_bp_orig;
                  zio->io_pipeline = zio->io_orig_pipeline;
  
          } else if ((zio->io_flags & ZIO_FLAG_RAW_ENCRYPT) != 0 &&
              zp->zp_type == DMU_OT_DNODE) {
                  /*
                   * The DMU actually relies on the zio layer's compression
                   * to free metadnode blocks that have had all contained
                   * dnodes freed. As a result, even when doing a raw
                   * receive, we must check whether the block can be compressed
                   * to a hole.
                   */
                  psize = zio_compress_data(ZIO_COMPRESS_EMPTY,
                      zio->io_abd, NULL, lsize, zp->zp_complevel);
                  if (psize == 0 || psize >= lsize)
                          compress = ZIO_COMPRESS_OFF;
          } else if (zio->io_flags & ZIO_FLAG_RAW_COMPRESS &&
              !(zio->io_flags & ZIO_FLAG_RAW_ENCRYPT)) {
                  /*
                   * If we are raw receiving an encrypted dataset we should not
                   * take this codepath because it will change the on-disk block
                   * and decryption will fail.
                   */
                  size_t rounded = MIN((size_t)roundup(psize,
                      spa->spa_min_alloc), lsize);
  
                  if (rounded != psize) {
                          abd_t *cdata = abd_alloc_linear(rounded, B_TRUE);
                          abd_zero_off(cdata, psize, rounded - psize);
                          abd_copy_off(cdata, zio->io_abd, 0, 0, psize);
                          psize = rounded;
                          zio_push_transform(zio, cdata,
                              psize, rounded, NULL);
                  }
          } else {
                  ASSERT3U(psize, !=, 0);
          }
  
          /*
           * The final pass of spa_sync() must be all rewrites, but the first
           * few passes offer a trade-off: allocating blocks defers convergence,
           * but newly allocated blocks are sequential, so they can be written
           * to disk faster.  Therefore, we allow the first few passes of
           * spa_sync() to allocate new blocks, but force rewrites after that.
           * There should only be a handful of blocks after pass 1 in any case.
           */
          if (!BP_IS_HOLE(bp) && bp->blk_birth == zio->io_txg &&
              BP_GET_PSIZE(bp) == psize &&
              pass >= zfs_sync_pass_rewrite) {
                  VERIFY3U(psize, !=, 0);
                  enum zio_stage gang_stages = zio->io_pipeline & ZIO_GANG_STAGES;
  
                  zio->io_pipeline = ZIO_REWRITE_PIPELINE | gang_stages;
                  zio->io_flags |= ZIO_FLAG_IO_REWRITE;
          } else {
                  BP_ZERO(bp);
                  zio->io_pipeline = ZIO_WRITE_PIPELINE;
          }
  
          if (psize == 0) {
                  if (zio->io_bp_orig.blk_birth != 0 &&
                      spa_feature_is_active(spa, SPA_FEATURE_HOLE_BIRTH)) {
                          BP_SET_LSIZE(bp, lsize);
                          BP_SET_TYPE(bp, zp->zp_type);
                          BP_SET_LEVEL(bp, zp->zp_level);
                          BP_SET_BIRTH(bp, zio->io_txg, 0);
                  }
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
          } else {
                  ASSERT(zp->zp_checksum != ZIO_CHECKSUM_GANG_HEADER);
                  BP_SET_LSIZE(bp, lsize);
                  BP_SET_TYPE(bp, zp->zp_type);
                  BP_SET_LEVEL(bp, zp->zp_level);
                  BP_SET_PSIZE(bp, psize);
                  BP_SET_COMPRESS(bp, compress);
                  BP_SET_CHECKSUM(bp, zp->zp_checksum);
                  BP_SET_DEDUP(bp, zp->zp_dedup);
                  BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
                  if (zp->zp_dedup) {
                          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REWRITE));
                          ASSERT(!zp->zp_encrypt ||
                              DMU_OT_IS_ENCRYPTED(zp->zp_type));
                          zio->io_pipeline = ZIO_DDT_WRITE_PIPELINE;
                  }
                  if (zp->zp_nopwrite) {
                          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REWRITE));
                          zio->io_pipeline |= ZIO_STAGE_NOP_WRITE;
                  }
          }
          return (zio);
  }
  
  static zio_t *
  zio_free_bp_init(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
          if (zio->io_child_type == ZIO_CHILD_LOGICAL) {
                  if (BP_GET_DEDUP(bp))
                          zio->io_pipeline = ZIO_DDT_FREE_PIPELINE;
          }
  
          ASSERT3P(zio->io_bp, ==, &zio->io_bp_copy);
  
          return (zio);
  }
  
  /*
   * ==========================================================================
   * Execute the I/O pipeline
   * ==========================================================================
   */
  
  static void
  zio_taskq_dispatch(zio_t *zio, zio_taskq_type_t q, boolean_t cutinline)
  {
          spa_t *spa = zio->io_spa;
          zio_type_t t = zio->io_type;
          int flags = (cutinline ? TQ_FRONT : 0);
  
          /*
           * If we're a config writer or a probe, the normal issue and
           * interrupt threads may all be blocked waiting for the config lock.
           * In this case, select the otherwise-unused taskq for ZIO_TYPE_NULL.
           */
          if (zio->io_flags & (ZIO_FLAG_CONFIG_WRITER | ZIO_FLAG_PROBE))
                  t = ZIO_TYPE_NULL;
  
          /*
           * A similar issue exists for the L2ARC write thread until L2ARC 2.0.
           */
          if (t == ZIO_TYPE_WRITE && zio->io_vd && zio->io_vd->vdev_aux)
                  t = ZIO_TYPE_NULL;
  
          /*
           * If this is a high priority I/O, then use the high priority taskq if
           * available.
           */
          if ((zio->io_priority == ZIO_PRIORITY_NOW ||
              zio->io_priority == ZIO_PRIORITY_SYNC_WRITE) &&
              spa->spa_zio_taskq[t][q + 1].stqs_count != 0)
                  q++;
  
          ASSERT3U(q, <, ZIO_TASKQ_TYPES);
  
          /*
           * NB: We are assuming that the zio can only be dispatched
           * to a single taskq at a time.  It would be a grievous error
           * to dispatch the zio to another taskq at the same time.
           */
          ASSERT(taskq_empty_ent(&zio->io_tqent));
          spa_taskq_dispatch_ent(spa, t, q, zio_execute, zio, flags,
              &zio->io_tqent);
  }
  
  static boolean_t
  zio_taskq_member(zio_t *zio, zio_taskq_type_t q)
  {
          spa_t *spa = zio->io_spa;
  
          taskq_t *tq = taskq_of_curthread();
  
          for (zio_type_t t = 0; t < ZIO_TYPES; t++) {
                  spa_taskqs_t *tqs = &spa->spa_zio_taskq[t][q];
                  uint_t i;
                  for (i = 0; i < tqs->stqs_count; i++) {
                          if (tqs->stqs_taskq[i] == tq)
                                  return (B_TRUE);
                  }
          }
  
          return (B_FALSE);
  }
  
  static zio_t *
  zio_issue_async(zio_t *zio)
  {
          zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, B_FALSE);
  
          return (NULL);
  }
  
  void
  zio_interrupt(void *zio)
  {
          zio_taskq_dispatch(zio, ZIO_TASKQ_INTERRUPT, B_FALSE);
  }
  
  void
  zio_delay_interrupt(zio_t *zio)
  {
          /*
           * The timeout_generic() function isn't defined in userspace, so
           * rather than trying to implement the function, the zio delay
           * functionality has been disabled for userspace builds.
           */
  
  #ifdef _KERNEL
          /*
           * If io_target_timestamp is zero, then no delay has been registered
           * for this IO, thus jump to the end of this function and "skip" the
           * delay; issuing it directly to the zio layer.
           */
          if (zio->io_target_timestamp != 0) {
                  hrtime_t now = gethrtime();
  
                  if (now >= zio->io_target_timestamp) {
                          /*
                           * This IO has already taken longer than the target
                           * delay to complete, so we don't want to delay it
                           * any longer; we "miss" the delay and issue it
                           * directly to the zio layer. This is likely due to
                           * the target latency being set to a value less than
                           * the underlying hardware can satisfy (e.g. delay
                           * set to 1ms, but the disks take 10ms to complete an
                           * IO request).
                           */
  
                          DTRACE_PROBE2(zio__delay__miss, zio_t *, zio,
                              hrtime_t, now);
  
                          zio_interrupt(zio);
                  } else {
                          taskqid_t tid;
                          hrtime_t diff = zio->io_target_timestamp - now;
                          clock_t expire_at_tick = ddi_get_lbolt() +
                              NSEC_TO_TICK(diff);
  
                          DTRACE_PROBE3(zio__delay__hit, zio_t *, zio,
                              hrtime_t, now, hrtime_t, diff);
  
                          if (NSEC_TO_TICK(diff) == 0) {
                                  /* Our delay is less than a jiffy - just spin */
                                  zfs_sleep_until(zio->io_target_timestamp);
                                  zio_interrupt(zio);
                          } else {
                                  /*
                                   * Use taskq_dispatch_delay() in the place of
                                   * OpenZFS's timeout_generic().
                                   */
                                  tid = taskq_dispatch_delay(system_taskq,
                                      zio_interrupt, zio, TQ_NOSLEEP,
                                      expire_at_tick);
                                  if (tid == TASKQID_INVALID) {
                                          /*
                                           * Couldn't allocate a task.  Just
                                           * finish the zio without a delay.
                                           */
                                          zio_interrupt(zio);
                                  }
                          }
                  }
                  return;
          }
  #endif
          DTRACE_PROBE1(zio__delay__skip, zio_t *, zio);
          zio_interrupt(zio);
  }
  
  static void
  zio_deadman_impl(zio_t *pio, int ziodepth)
  {
          zio_t *cio, *cio_next;
          zio_link_t *zl = NULL;
          vdev_t *vd = pio->io_vd;
  
          if (zio_deadman_log_all || (vd != NULL && vd->vdev_ops->vdev_op_leaf)) {
                  vdev_queue_t *vq = vd ? &vd->vdev_queue : NULL;
                  zbookmark_phys_t *zb = &pio->io_bookmark;
                  uint64_t delta = gethrtime() - pio->io_timestamp;
                  uint64_t failmode = spa_get_deadman_failmode(pio->io_spa);
  
                  zfs_dbgmsg("slow zio[%d]: zio=%px timestamp=%llu "
                      "delta=%llu queued=%llu io=%llu "
                      "path=%s "
                      "last=%llu type=%d "
                      "priority=%d flags=0x%llx stage=0x%x "
                      "pipeline=0x%x pipeline-trace=0x%x "
                      "objset=%llu object=%llu "
                      "level=%llu blkid=%llu "
                      "offset=%llu size=%llu "
                      "error=%d",
                      ziodepth, pio, pio->io_timestamp,
                      (u_longlong_t)delta, pio->io_delta, pio->io_delay,
                      vd ? vd->vdev_path : "NULL",
                      vq ? vq->vq_io_complete_ts : 0, pio->io_type,
                      pio->io_priority, (u_longlong_t)pio->io_flags,
                      pio->io_stage, pio->io_pipeline, pio->io_pipeline_trace,
                      (u_longlong_t)zb->zb_objset, (u_longlong_t)zb->zb_object,
                      (u_longlong_t)zb->zb_level, (u_longlong_t)zb->zb_blkid,
                      (u_longlong_t)pio->io_offset, (u_longlong_t)pio->io_size,
                      pio->io_error);
                  (void) zfs_ereport_post(FM_EREPORT_ZFS_DEADMAN,
                      pio->io_spa, vd, zb, pio, 0);
  
                  if (failmode == ZIO_FAILURE_MODE_CONTINUE &&
                      taskq_empty_ent(&pio->io_tqent)) {
                          zio_interrupt(pio);
                  }
          }
  
          mutex_enter(&pio->io_lock);
          for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
                  cio_next = zio_walk_children(pio, &zl);
                  zio_deadman_impl(cio, ziodepth + 1);
          }
          mutex_exit(&pio->io_lock);
  }
  
  /*
   * Log the critical information describing this zio and all of its children
   * using the zfs_dbgmsg() interface then post deadman event for the ZED.
   */
  void
  zio_deadman(zio_t *pio, const char *tag)
  {
          spa_t *spa = pio->io_spa;
          char *name = spa_name(spa);
  
          if (!zfs_deadman_enabled || spa_suspended(spa))
                  return;
  
          zio_deadman_impl(pio, 0);
  
          switch (spa_get_deadman_failmode(spa)) {
          case ZIO_FAILURE_MODE_WAIT:
                  zfs_dbgmsg("%s waiting for hung I/O to pool '%s'", tag, name);
                  break;
  
          case ZIO_FAILURE_MODE_CONTINUE:
                  zfs_dbgmsg("%s restarting hung I/O for pool '%s'", tag, name);
                  break;
  
          case ZIO_FAILURE_MODE_PANIC:
                  fm_panic("%s determined I/O to pool '%s' is hung.", tag, name);
                  break;
          }
  }
  
  /*
   * Execute the I/O pipeline until one of the following occurs:
   * (1) the I/O completes; (2) the pipeline stalls waiting for
   * dependent child I/Os; (3) the I/O issues, so we're waiting
   * for an I/O completion interrupt; (4) the I/O is delegated by
   * vdev-level caching or aggregation; (5) the I/O is deferred
   * due to vdev-level queueing; (6) the I/O is handed off to
   * another thread.  In all cases, the pipeline stops whenever
   * there's no CPU work; it never burns a thread in cv_wait_io().
   *
   * There's no locking on io_stage because there's no legitimate way
   * for multiple threads to be attempting to process the same I/O.
   */
  static zio_pipe_stage_t *zio_pipeline[];
  
  /*
   * zio_execute() is a wrapper around the static function
   * __zio_execute() so that we can force  __zio_execute() to be
   * inlined.  This reduces stack overhead which is important
   * because __zio_execute() is called recursively in several zio
   * code paths.  zio_execute() itself cannot be inlined because
   * it is externally visible.
   */
  void
  zio_execute(void *zio)
  {
          fstrans_cookie_t cookie;
  
          cookie = spl_fstrans_mark();
          __zio_execute(zio);
          spl_fstrans_unmark(cookie);
  }
  
  /*
   * Used to determine if in the current context the stack is sized large
   * enough to allow zio_execute() to be called recursively.  A minimum
   * stack size of 16K is required to avoid needing to re-dispatch the zio.
   */
  static boolean_t
  zio_execute_stack_check(zio_t *zio)
  {
  #if !defined(HAVE_LARGE_STACKS)
          dsl_pool_t *dp = spa_get_dsl(zio->io_spa);
  
          /* Executing in txg_sync_thread() context. */
          if (dp && curthread == dp->dp_tx.tx_sync_thread)
                  return (B_TRUE);
  
          /* Pool initialization outside of zio_taskq context. */
          if (dp && spa_is_initializing(dp->dp_spa) &&
              !zio_taskq_member(zio, ZIO_TASKQ_ISSUE) &&
              !zio_taskq_member(zio, ZIO_TASKQ_ISSUE_HIGH))
                  return (B_TRUE);
  #else
          (void) zio;
  #endif /* HAVE_LARGE_STACKS */
  
          return (B_FALSE);
  }
  
  __attribute__((always_inline))
  static inline void
  __zio_execute(zio_t *zio)
  {
          ASSERT3U(zio->io_queued_timestamp, >, 0);
  
          while (zio->io_stage < ZIO_STAGE_DONE) {
                  enum zio_stage pipeline = zio->io_pipeline;
                  enum zio_stage stage = zio->io_stage;
  
                  zio->io_executor = curthread;
  
                  ASSERT(!MUTEX_HELD(&zio->io_lock));
                  ASSERT(ISP2(stage));
                  ASSERT(zio->io_stall == NULL);
  
                  do {
                          stage <<= 1;
                  } while ((stage & pipeline) == 0);
  
                  ASSERT(stage <= ZIO_STAGE_DONE);
  
                  /*
                   * If we are in interrupt context and this pipeline stage
                   * will grab a config lock that is held across I/O,
                   * or may wait for an I/O that needs an interrupt thread
                   * to complete, issue async to avoid deadlock.
                   *
                   * For VDEV_IO_START, we cut in line so that the io will
                   * be sent to disk promptly.
                   */
                  if ((stage & ZIO_BLOCKING_STAGES) && zio->io_vd == NULL &&
                      zio_taskq_member(zio, ZIO_TASKQ_INTERRUPT)) {
                          boolean_t cut = (stage == ZIO_STAGE_VDEV_IO_START) ?
                              zio_requeue_io_start_cut_in_line : B_FALSE;
                          zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, cut);
                          return;
                  }
  
                  /*
                   * If the current context doesn't have large enough stacks
                   * the zio must be issued asynchronously to prevent overflow.
                   */
                  if (zio_execute_stack_check(zio)) {
                          boolean_t cut = (stage == ZIO_STAGE_VDEV_IO_START) ?
                              zio_requeue_io_start_cut_in_line : B_FALSE;
                          zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, cut);
                          return;
                  }
  
                  zio->io_stage = stage;
                  zio->io_pipeline_trace |= zio->io_stage;
  
                  /*
                   * The zio pipeline stage returns the next zio to execute
                   * (typically the same as this one), or NULL if we should
                   * stop.
                   */
                  zio = zio_pipeline[highbit64(stage) - 1](zio);
  
                  if (zio == NULL)
                          return;
          }
  }
  
  
  /*
   * ==========================================================================
   * Initiate I/O, either sync or async
   * ==========================================================================
   */
  int
  zio_wait(zio_t *zio)
  {
          /*
           * Some routines, like zio_free_sync(), may return a NULL zio
           * to avoid the performance overhead of creating and then destroying
           * an unneeded zio.  For the callers' simplicity, we accept a NULL
           * zio and ignore it.
           */
          if (zio == NULL)
                  return (0);
  
          long timeout = MSEC_TO_TICK(zfs_deadman_ziotime_ms);
          int error;
  
          ASSERT3S(zio->io_stage, ==, ZIO_STAGE_OPEN);
          ASSERT3P(zio->io_executor, ==, NULL);
  
          zio->io_waiter = curthread;
          ASSERT0(zio->io_queued_timestamp);
          zio->io_queued_timestamp = gethrtime();
  
          __zio_execute(zio);
  
          mutex_enter(&zio->io_lock);
          while (zio->io_executor != NULL) {
                  error = cv_timedwait_io(&zio->io_cv, &zio->io_lock,
                      ddi_get_lbolt() + timeout);
  
                  if (zfs_deadman_enabled && error == -1 &&
                      gethrtime() - zio->io_queued_timestamp >
                      spa_deadman_ziotime(zio->io_spa)) {
                          mutex_exit(&zio->io_lock);
                          timeout = MSEC_TO_TICK(zfs_deadman_checktime_ms);
                          zio_deadman(zio, FTAG);
                          mutex_enter(&zio->io_lock);
                  }
          }
          mutex_exit(&zio->io_lock);
  
          error = zio->io_error;
          zio_destroy(zio);
  
          return (error);
  }
  
  void
  zio_nowait(zio_t *zio)
  {
          /*
           * See comment in zio_wait().
           */
          if (zio == NULL)
                  return;
  
          ASSERT3P(zio->io_executor, ==, NULL);
  
          if (zio->io_child_type == ZIO_CHILD_LOGICAL &&
              zio_unique_parent(zio) == NULL) {
                  zio_t *pio;
  
                  /*
                   * This is a logical async I/O with no parent to wait for it.
                   * We add it to the spa_async_root_zio "Godfather" I/O which
                   * will ensure they complete prior to unloading the pool.
                   */
                  spa_t *spa = zio->io_spa;
                  pio = spa->spa_async_zio_root[CPU_SEQID_UNSTABLE];
  
                  zio_add_child(pio, zio);
          }
  
          ASSERT0(zio->io_queued_timestamp);
          zio->io_queued_timestamp = gethrtime();
          __zio_execute(zio);
  }
  
  /*
   * ==========================================================================
   * Reexecute, cancel, or suspend/resume failed I/O
   * ==========================================================================
   */
  
  static void
  zio_reexecute(void *arg)
  {
          zio_t *pio = arg;
          zio_t *cio, *cio_next;
  
          ASSERT(pio->io_child_type == ZIO_CHILD_LOGICAL);
          ASSERT(pio->io_orig_stage == ZIO_STAGE_OPEN);
          ASSERT(pio->io_gang_leader == NULL);
          ASSERT(pio->io_gang_tree == NULL);
  
          pio->io_flags = pio->io_orig_flags;
          pio->io_stage = pio->io_orig_stage;
          pio->io_pipeline = pio->io_orig_pipeline;
          pio->io_reexecute = 0;
          pio->io_flags |= ZIO_FLAG_REEXECUTED;
          pio->io_pipeline_trace = 0;
          pio->io_error = 0;
          for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                  pio->io_state[w] = 0;
          for (int c = 0; c < ZIO_CHILD_TYPES; c++)
                  pio->io_child_error[c] = 0;
  
          if (IO_IS_ALLOCATING(pio))
                  BP_ZERO(pio->io_bp);
  
          /*
           * As we reexecute pio's children, new children could be created.
           * New children go to the head of pio's io_child_list, however,
           * so we will (correctly) not reexecute them.  The key is that
           * the remainder of pio's io_child_list, from 'cio_next' onward,
           * cannot be affected by any side effects of reexecuting 'cio'.
           */
          zio_link_t *zl = NULL;
          mutex_enter(&pio->io_lock);
          for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
                  cio_next = zio_walk_children(pio, &zl);
                  for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                          pio->io_children[cio->io_child_type][w]++;
                  mutex_exit(&pio->io_lock);
                  zio_reexecute(cio);
                  mutex_enter(&pio->io_lock);
          }
          mutex_exit(&pio->io_lock);
  
          /*
           * Now that all children have been reexecuted, execute the parent.
           * We don't reexecute "The Godfather" I/O here as it's the
           * responsibility of the caller to wait on it.
           */
          if (!(pio->io_flags & ZIO_FLAG_GODFATHER)) {
                  pio->io_queued_timestamp = gethrtime();
                  __zio_execute(pio);
          }
  }
  
  void
  zio_suspend(spa_t *spa, zio_t *zio, zio_suspend_reason_t reason)
  {
          if (spa_get_failmode(spa) == ZIO_FAILURE_MODE_PANIC)
                  fm_panic("Pool '%s' has encountered an uncorrectable I/O "
                      "failure and the failure mode property for this pool "
                      "is set to panic.", spa_name(spa));
  
          cmn_err(CE_WARN, "Pool '%s' has encountered an uncorrectable I/O "
              "failure and has been suspended.\n", spa_name(spa));
  
          (void) zfs_ereport_post(FM_EREPORT_ZFS_IO_FAILURE, spa, NULL,
              NULL, NULL, 0);
  
          mutex_enter(&spa->spa_suspend_lock);
  
          if (spa->spa_suspend_zio_root == NULL)
                  spa->spa_suspend_zio_root = zio_root(spa, NULL, NULL,
                      ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE |
                      ZIO_FLAG_GODFATHER);
  
          spa->spa_suspended = reason;
  
          if (zio != NULL) {
                  ASSERT(!(zio->io_flags & ZIO_FLAG_GODFATHER));
                  ASSERT(zio != spa->spa_suspend_zio_root);
                  ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                  ASSERT(zio_unique_parent(zio) == NULL);
                  ASSERT(zio->io_stage == ZIO_STAGE_DONE);
                  zio_add_child(spa->spa_suspend_zio_root, zio);
          }
  
          mutex_exit(&spa->spa_suspend_lock);
  }
  
  int
  zio_resume(spa_t *spa)
  {
          zio_t *pio;
  
          /*
           * Reexecute all previously suspended i/o.
           */
          mutex_enter(&spa->spa_suspend_lock);
          spa->spa_suspended = ZIO_SUSPEND_NONE;
          cv_broadcast(&spa->spa_suspend_cv);
          pio = spa->spa_suspend_zio_root;
          spa->spa_suspend_zio_root = NULL;
          mutex_exit(&spa->spa_suspend_lock);
  
          if (pio == NULL)
                  return (0);
  
          zio_reexecute(pio);
          return (zio_wait(pio));
  }
  
  void
  zio_resume_wait(spa_t *spa)
  {
          mutex_enter(&spa->spa_suspend_lock);
          while (spa_suspended(spa))
                  cv_wait(&spa->spa_suspend_cv, &spa->spa_suspend_lock);
          mutex_exit(&spa->spa_suspend_lock);
  }
  
  /*
   * ==========================================================================
   * Gang blocks.
   *
   * A gang block is a collection of small blocks that looks to the DMU
   * like one large block.  When zio_dva_allocate() cannot find a block
   * of the requested size, due to either severe fragmentation or the pool
   * being nearly full, it calls zio_write_gang_block() to construct the
   * block from smaller fragments.
   *
   * A gang block consists of a gang header (zio_gbh_phys_t) and up to
   * three (SPA_GBH_NBLKPTRS) gang members.  The gang header is just like
   * an indirect block: it's an array of block pointers.  It consumes
   * only one sector and hence is allocatable regardless of fragmentation.
   * The gang header's bps point to its gang members, which hold the data.
   *
   * Gang blocks are self-checksumming, using the bp's <vdev, offset, txg>
   * as the verifier to ensure uniqueness of the SHA256 checksum.
   * Critically, the gang block bp's blk_cksum is the checksum of the data,
   * not the gang header.  This ensures that data block signatures (needed for
   * deduplication) are independent of how the block is physically stored.
   *
   * Gang blocks can be nested: a gang member may itself be a gang block.
   * Thus every gang block is a tree in which root and all interior nodes are
   * gang headers, and the leaves are normal blocks that contain user data.
   * The root of the gang tree is called the gang leader.
   *
   * To perform any operation (read, rewrite, free, claim) on a gang block,
   * zio_gang_assemble() first assembles the gang tree (minus data leaves)
   * in the io_gang_tree field of the original logical i/o by recursively
   * reading the gang leader and all gang headers below it.  This yields
   * an in-core tree containing the contents of every gang header and the
   * bps for every constituent of the gang block.
   *
   * With the gang tree now assembled, zio_gang_issue() just walks the gang tree
   * and invokes a callback on each bp.  To free a gang block, zio_gang_issue()
   * calls zio_free_gang() -- a trivial wrapper around zio_free() -- for each bp.
   * zio_claim_gang() provides a similarly trivial wrapper for zio_claim().
   * zio_read_gang() is a wrapper around zio_read() that omits reading gang
   * headers, since we already have those in io_gang_tree.  zio_rewrite_gang()
   * performs a zio_rewrite() of the data or, for gang headers, a zio_rewrite()
   * of the gang header plus zio_checksum_compute() of the data to update the
   * gang header's blk_cksum as described above.
   *
   * The two-phase assemble/issue model solves the problem of partial failure --
   * what if you'd freed part of a gang block but then couldn't read the
   * gang header for another part?  Assembling the entire gang tree first
   * ensures that all the necessary gang header I/O has succeeded before
   * starting the actual work of free, claim, or write.  Once the gang tree
   * is assembled, free and claim are in-memory operations that cannot fail.
   *
   * In the event that a gang write fails, zio_dva_unallocate() walks the
   * gang tree to immediately free (i.e. insert back into the space map)
   * everything we've allocated.  This ensures that we don't get ENOSPC
   * errors during repeated suspend/resume cycles due to a flaky device.
   *
   * Gang rewrites only happen during sync-to-convergence.  If we can't assemble
   * the gang tree, we won't modify the block, so we can safely defer the free
   * (knowing that the block is still intact).  If we *can* assemble the gang
   * tree, then even if some of the rewrites fail, zio_dva_unallocate() will free
   * each constituent bp and we can allocate a new block on the next sync pass.
   *
   * In all cases, the gang tree allows complete recovery from partial failure.
   * ==========================================================================
   */
  
  static void
  zio_gang_issue_func_done(zio_t *zio)
  {
          abd_free(zio->io_abd);
  }
  
  static zio_t *
  zio_read_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, abd_t *data,
      uint64_t offset)
  {
          if (gn != NULL)
                  return (pio);
  
          return (zio_read(pio, pio->io_spa, bp, abd_get_offset(data, offset),
              BP_GET_PSIZE(bp), zio_gang_issue_func_done,
              NULL, pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
              &pio->io_bookmark));
  }
  
  static zio_t *
  zio_rewrite_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, abd_t *data,
      uint64_t offset)
  {
          zio_t *zio;
  
          if (gn != NULL) {
                  abd_t *gbh_abd =
                      abd_get_from_buf(gn->gn_gbh, SPA_GANGBLOCKSIZE);
                  zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
                      gbh_abd, SPA_GANGBLOCKSIZE, zio_gang_issue_func_done, NULL,
                      pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
                      &pio->io_bookmark);
                  /*
                   * As we rewrite each gang header, the pipeline will compute
                   * a new gang block header checksum for it; but no one will
                   * compute a new data checksum, so we do that here.  The one
                   * exception is the gang leader: the pipeline already computed
                   * its data checksum because that stage precedes gang assembly.
                   * (Presently, nothing actually uses interior data checksums;
                   * this is just good hygiene.)
                   */
                  if (gn != pio->io_gang_leader->io_gang_tree) {
                          abd_t *buf = abd_get_offset(data, offset);
  
                          zio_checksum_compute(zio, BP_GET_CHECKSUM(bp),
                              buf, BP_GET_PSIZE(bp));
  
                          abd_free(buf);
                  }
                  /*
                   * If we are here to damage data for testing purposes,
                   * leave the GBH alone so that we can detect the damage.
                   */
                  if (pio->io_gang_leader->io_flags & ZIO_FLAG_INDUCE_DAMAGE)
                          zio->io_pipeline &= ~ZIO_VDEV_IO_STAGES;
          } else {
                  zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
                      abd_get_offset(data, offset), BP_GET_PSIZE(bp),
                      zio_gang_issue_func_done, NULL, pio->io_priority,
                      ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
          }
  
          return (zio);
  }
  
  static zio_t *
  zio_free_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, abd_t *data,
      uint64_t offset)
  {
          (void) gn, (void) data, (void) offset;
  
          zio_t *zio = zio_free_sync(pio, pio->io_spa, pio->io_txg, bp,
              ZIO_GANG_CHILD_FLAGS(pio));
          if (zio == NULL) {
                  zio = zio_null(pio, pio->io_spa,
                      NULL, NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio));
          }
          return (zio);
  }
  
  static zio_t *
  zio_claim_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, abd_t *data,
      uint64_t offset)
  {
          (void) gn, (void) data, (void) offset;
          return (zio_claim(pio, pio->io_spa, pio->io_txg, bp,
              NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio)));
  }
  
  static zio_gang_issue_func_t *zio_gang_issue_func[ZIO_TYPES] = {
          NULL,
          zio_read_gang,
          zio_rewrite_gang,
          zio_free_gang,
          zio_claim_gang,
          NULL
  };
  
  static void zio_gang_tree_assemble_done(zio_t *zio);
  
  static zio_gang_node_t *
  zio_gang_node_alloc(zio_gang_node_t **gnpp)
  {
          zio_gang_node_t *gn;
  
          ASSERT(*gnpp == NULL);
  
          gn = kmem_zalloc(sizeof (*gn), KM_SLEEP);
          gn->gn_gbh = zio_buf_alloc(SPA_GANGBLOCKSIZE);
          *gnpp = gn;
  
          return (gn);
  }
  
  static void
  zio_gang_node_free(zio_gang_node_t **gnpp)
  {
          zio_gang_node_t *gn = *gnpp;
  
          for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
                  ASSERT(gn->gn_child[g] == NULL);
  
          zio_buf_free(gn->gn_gbh, SPA_GANGBLOCKSIZE);
          kmem_free(gn, sizeof (*gn));
          *gnpp = NULL;
  }
  
  static void
  zio_gang_tree_free(zio_gang_node_t **gnpp)
  {
          zio_gang_node_t *gn = *gnpp;
  
          if (gn == NULL)
                  return;
  
          for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
                  zio_gang_tree_free(&gn->gn_child[g]);
  
          zio_gang_node_free(gnpp);
  }
  
  static void
  zio_gang_tree_assemble(zio_t *gio, blkptr_t *bp, zio_gang_node_t **gnpp)
  {
          zio_gang_node_t *gn = zio_gang_node_alloc(gnpp);
          abd_t *gbh_abd = abd_get_from_buf(gn->gn_gbh, SPA_GANGBLOCKSIZE);
  
          ASSERT(gio->io_gang_leader == gio);
          ASSERT(BP_IS_GANG(bp));
  
          zio_nowait(zio_read(gio, gio->io_spa, bp, gbh_abd, SPA_GANGBLOCKSIZE,
              zio_gang_tree_assemble_done, gn, gio->io_priority,
              ZIO_GANG_CHILD_FLAGS(gio), &gio->io_bookmark));
  }
  
  static void
  zio_gang_tree_assemble_done(zio_t *zio)
  {
          zio_t *gio = zio->io_gang_leader;
          zio_gang_node_t *gn = zio->io_private;
          blkptr_t *bp = zio->io_bp;
  
          ASSERT(gio == zio_unique_parent(zio));
          ASSERT(zio->io_child_count == 0);
  
          if (zio->io_error)
                  return;
  
          /* this ABD was created from a linear buf in zio_gang_tree_assemble */
          if (BP_SHOULD_BYTESWAP(bp))
                  byteswap_uint64_array(abd_to_buf(zio->io_abd), zio->io_size);
  
          ASSERT3P(abd_to_buf(zio->io_abd), ==, gn->gn_gbh);
          ASSERT(zio->io_size == SPA_GANGBLOCKSIZE);
          ASSERT(gn->gn_gbh->zg_tail.zec_magic == ZEC_MAGIC);
  
          abd_free(zio->io_abd);
  
          for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                  blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
                  if (!BP_IS_GANG(gbp))
                          continue;
                  zio_gang_tree_assemble(gio, gbp, &gn->gn_child[g]);
          }
  }
  
  static void
  zio_gang_tree_issue(zio_t *pio, zio_gang_node_t *gn, blkptr_t *bp, abd_t *data,
      uint64_t offset)
  {
          zio_t *gio = pio->io_gang_leader;
          zio_t *zio;
  
          ASSERT(BP_IS_GANG(bp) == !!gn);
          ASSERT(BP_GET_CHECKSUM(bp) == BP_GET_CHECKSUM(gio->io_bp));
          ASSERT(BP_GET_LSIZE(bp) == BP_GET_PSIZE(bp) || gn == gio->io_gang_tree);
  
          /*
           * If you're a gang header, your data is in gn->gn_gbh.
           * If you're a gang member, your data is in 'data' and gn == NULL.
           */
          zio = zio_gang_issue_func[gio->io_type](pio, bp, gn, data, offset);
  
          if (gn != NULL) {
                  ASSERT(gn->gn_gbh->zg_tail.zec_magic == ZEC_MAGIC);
  
                  for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                          blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
                          if (BP_IS_HOLE(gbp))
                                  continue;
                          zio_gang_tree_issue(zio, gn->gn_child[g], gbp, data,
                              offset);
                          offset += BP_GET_PSIZE(gbp);
                  }
          }
  
          if (gn == gio->io_gang_tree)
                  ASSERT3U(gio->io_size, ==, offset);
  
          if (zio != pio)
                  zio_nowait(zio);
  }
  
  static zio_t *
  zio_gang_assemble(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
          ASSERT(BP_IS_GANG(bp) && zio->io_gang_leader == NULL);
          ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
  
          zio->io_gang_leader = zio;
  
          zio_gang_tree_assemble(zio, bp, &zio->io_gang_tree);
  
          return (zio);
  }
  
  static zio_t *
  zio_gang_issue(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
          if (zio_wait_for_children(zio, ZIO_CHILD_GANG_BIT, ZIO_WAIT_DONE)) {
                  return (NULL);
          }
  
          ASSERT(BP_IS_GANG(bp) && zio->io_gang_leader == zio);
          ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
  
          if (zio->io_child_error[ZIO_CHILD_GANG] == 0)
                  zio_gang_tree_issue(zio, zio->io_gang_tree, bp, zio->io_abd,
                      0);
          else
                  zio_gang_tree_free(&zio->io_gang_tree);
  
          zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
  
          return (zio);
  }
  
  static void
  zio_write_gang_member_ready(zio_t *zio)
  {
          zio_t *pio = zio_unique_parent(zio);
          dva_t *cdva = zio->io_bp->blk_dva;
          dva_t *pdva = pio->io_bp->blk_dva;
          uint64_t asize;
          zio_t *gio __maybe_unused = zio->io_gang_leader;
  
          if (BP_IS_HOLE(zio->io_bp))
                  return;
  
          ASSERT(BP_IS_HOLE(&zio->io_bp_orig));
  
          ASSERT(zio->io_child_type == ZIO_CHILD_GANG);
          ASSERT3U(zio->io_prop.zp_copies, ==, gio->io_prop.zp_copies);
          ASSERT3U(zio->io_prop.zp_copies, <=, BP_GET_NDVAS(zio->io_bp));
          ASSERT3U(pio->io_prop.zp_copies, <=, BP_GET_NDVAS(pio->io_bp));
          ASSERT3U(BP_GET_NDVAS(zio->io_bp), <=, BP_GET_NDVAS(pio->io_bp));
  
          mutex_enter(&pio->io_lock);
          for (int d = 0; d < BP_GET_NDVAS(zio->io_bp); d++) {
                  ASSERT(DVA_GET_GANG(&pdva[d]));
                  asize = DVA_GET_ASIZE(&pdva[d]);
                  asize += DVA_GET_ASIZE(&cdva[d]);
                  DVA_SET_ASIZE(&pdva[d], asize);
          }
          mutex_exit(&pio->io_lock);
  }
  
  static void
  zio_write_gang_done(zio_t *zio)
  {
          /*
           * The io_abd field will be NULL for a zio with no data.  The io_flags
           * will initially have the ZIO_FLAG_NODATA bit flag set, but we can't
           * check for it here as it is cleared in zio_ready.
           */
          if (zio->io_abd != NULL)
                  abd_free(zio->io_abd);
  }
  
  static zio_t *
  zio_write_gang_block(zio_t *pio, metaslab_class_t *mc)
  {
          spa_t *spa = pio->io_spa;
          blkptr_t *bp = pio->io_bp;
          zio_t *gio = pio->io_gang_leader;
          zio_t *zio;
          zio_gang_node_t *gn, **gnpp;
          zio_gbh_phys_t *gbh;
          abd_t *gbh_abd;
          uint64_t txg = pio->io_txg;
          uint64_t resid = pio->io_size;
          uint64_t lsize;
          int copies = gio->io_prop.zp_copies;
          int gbh_copies;
          zio_prop_t zp;
          int error;
          boolean_t has_data = !(pio->io_flags & ZIO_FLAG_NODATA);
  
          /*
           * encrypted blocks need DVA[2] free so encrypted gang headers can't
           * have a third copy.
           */
          gbh_copies = MIN(copies + 1, spa_max_replication(spa));
          if (BP_IS_ENCRYPTED(bp) && gbh_copies >= SPA_DVAS_PER_BP)
                  gbh_copies = SPA_DVAS_PER_BP - 1;
  
          int flags = METASLAB_HINTBP_FAVOR | METASLAB_GANG_HEADER;
          if (pio->io_flags & ZIO_FLAG_IO_ALLOCATING) {
                  ASSERT(pio->io_priority == ZIO_PRIORITY_ASYNC_WRITE);
                  ASSERT(has_data);
  
                  flags |= METASLAB_ASYNC_ALLOC;
                  VERIFY(zfs_refcount_held(&mc->mc_allocator[pio->io_allocator].
                      mca_alloc_slots, pio));
  
                  /*
                   * The logical zio has already placed a reservation for
                   * 'copies' allocation slots but gang blocks may require
                   * additional copies. These additional copies
                   * (i.e. gbh_copies - copies) are guaranteed to succeed
                   * since metaslab_class_throttle_reserve() always allows
                   * additional reservations for gang blocks.
                   */
                  VERIFY(metaslab_class_throttle_reserve(mc, gbh_copies - copies,
                      pio->io_allocator, pio, flags));
          }
  
          error = metaslab_alloc(spa, mc, SPA_GANGBLOCKSIZE,
              bp, gbh_copies, txg, pio == gio ? NULL : gio->io_bp, flags,
              &pio->io_alloc_list, pio, pio->io_allocator);
          if (error) {
                  if (pio->io_flags & ZIO_FLAG_IO_ALLOCATING) {
                          ASSERT(pio->io_priority == ZIO_PRIORITY_ASYNC_WRITE);
                          ASSERT(has_data);
  
                          /*
                           * If we failed to allocate the gang block header then
                           * we remove any additional allocation reservations that
                           * we placed here. The original reservation will
                           * be removed when the logical I/O goes to the ready
                           * stage.
                           */
                          metaslab_class_throttle_unreserve(mc,
                              gbh_copies - copies, pio->io_allocator, pio);
                  }
  
                  pio->io_error = error;
                  return (pio);
          }
  
          if (pio == gio) {
                  gnpp = &gio->io_gang_tree;
          } else {
                  gnpp = pio->io_private;
                  ASSERT(pio->io_ready == zio_write_gang_member_ready);
          }
  
          gn = zio_gang_node_alloc(gnpp);
          gbh = gn->gn_gbh;
          memset(gbh, 0, SPA_GANGBLOCKSIZE);
          gbh_abd = abd_get_from_buf(gbh, SPA_GANGBLOCKSIZE);
  
          /*
           * Create the gang header.
           */
          zio = zio_rewrite(pio, spa, txg, bp, gbh_abd, SPA_GANGBLOCKSIZE,
              zio_write_gang_done, NULL, pio->io_priority,
              ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
  
          /*
           * Create and nowait the gang children.
           */
          for (int g = 0; resid != 0; resid -= lsize, g++) {
                  lsize = P2ROUNDUP(resid / (SPA_GBH_NBLKPTRS - g),
                      SPA_MINBLOCKSIZE);
                  ASSERT(lsize >= SPA_MINBLOCKSIZE && lsize <= resid);
  
                  zp.zp_checksum = gio->io_prop.zp_checksum;
                  zp.zp_compress = ZIO_COMPRESS_OFF;
                  zp.zp_complevel = gio->io_prop.zp_complevel;
                  zp.zp_type = DMU_OT_NONE;
                  zp.zp_level = 0;
                  zp.zp_copies = gio->io_prop.zp_copies;
                  zp.zp_dedup = B_FALSE;
                  zp.zp_dedup_verify = B_FALSE;
                  zp.zp_nopwrite = B_FALSE;
                  zp.zp_encrypt = gio->io_prop.zp_encrypt;
                  zp.zp_byteorder = gio->io_prop.zp_byteorder;
                  memset(zp.zp_salt, 0, ZIO_DATA_SALT_LEN);
                  memset(zp.zp_iv, 0, ZIO_DATA_IV_LEN);
                  memset(zp.zp_mac, 0, ZIO_DATA_MAC_LEN);
  
                  zio_t *cio = zio_write(zio, spa, txg, &gbh->zg_blkptr[g],
                      has_data ? abd_get_offset(pio->io_abd, pio->io_size -
                      resid) : NULL, lsize, lsize, &zp,
                      zio_write_gang_member_ready, NULL, NULL,
                      zio_write_gang_done, &gn->gn_child[g], pio->io_priority,
                      ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
  
                  if (pio->io_flags & ZIO_FLAG_IO_ALLOCATING) {
                          ASSERT(pio->io_priority == ZIO_PRIORITY_ASYNC_WRITE);
                          ASSERT(has_data);
  
                          /*
                           * Gang children won't throttle but we should
                           * account for their work, so reserve an allocation
                           * slot for them here.
                           */
                          VERIFY(metaslab_class_throttle_reserve(mc,
                              zp.zp_copies, cio->io_allocator, cio, flags));
                  }
                  zio_nowait(cio);
          }
  
          /*
           * Set pio's pipeline to just wait for zio to finish.
           */
          pio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
  
          /*
           * We didn't allocate this bp, so make sure it doesn't get unmarked.
           */
          pio->io_flags &= ~ZIO_FLAG_FASTWRITE;
  
          zio_nowait(zio);
  
          return (pio);
  }
  
  /*
   * The zio_nop_write stage in the pipeline determines if allocating a
   * new bp is necessary.  The nopwrite feature can handle writes in
   * either syncing or open context (i.e. zil writes) and as a result is
   * mutually exclusive with dedup.
   *
   * By leveraging a cryptographically secure checksum, such as SHA256, we
   * can compare the checksums of the new data and the old to determine if
   * allocating a new block is required.  Note that our requirements for
   * cryptographic strength are fairly weak: there can't be any accidental
   * hash collisions, but we don't need to be secure against intentional
   * (malicious) collisions.  To trigger a nopwrite, you have to be able
   * to write the file to begin with, and triggering an incorrect (hash
   * collision) nopwrite is no worse than simply writing to the file.
   * That said, there are no known attacks against the checksum algorithms
   * used for nopwrite, assuming that the salt and the checksums
   * themselves remain secret.
   */
  static zio_t *
  zio_nop_write(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
          blkptr_t *bp_orig = &zio->io_bp_orig;
          zio_prop_t *zp = &zio->io_prop;
  
          ASSERT(BP_IS_HOLE(bp));
          ASSERT(BP_GET_LEVEL(bp) == 0);
          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REWRITE));
          ASSERT(zp->zp_nopwrite);
          ASSERT(!zp->zp_dedup);
          ASSERT(zio->io_bp_override == NULL);
          ASSERT(IO_IS_ALLOCATING(zio));
  
          /*
           * Check to see if the original bp and the new bp have matching
           * characteristics (i.e. same checksum, compression algorithms, etc).
           * If they don't then just continue with the pipeline which will
           * allocate a new bp.
           */
          if (BP_IS_HOLE(bp_orig) ||
              !(zio_checksum_table[BP_GET_CHECKSUM(bp)].ci_flags &
              ZCHECKSUM_FLAG_NOPWRITE) ||
              BP_IS_ENCRYPTED(bp) || BP_IS_ENCRYPTED(bp_orig) ||
              BP_GET_CHECKSUM(bp) != BP_GET_CHECKSUM(bp_orig) ||
              BP_GET_COMPRESS(bp) != BP_GET_COMPRESS(bp_orig) ||
              BP_GET_DEDUP(bp) != BP_GET_DEDUP(bp_orig) ||
              zp->zp_copies != BP_GET_NDVAS(bp_orig))
                  return (zio);
  
          /*
           * If the checksums match then reset the pipeline so that we
           * avoid allocating a new bp and issuing any I/O.
           */
          if (ZIO_CHECKSUM_EQUAL(bp->blk_cksum, bp_orig->blk_cksum)) {
                  ASSERT(zio_checksum_table[zp->zp_checksum].ci_flags &
                      ZCHECKSUM_FLAG_NOPWRITE);
                  ASSERT3U(BP_GET_PSIZE(bp), ==, BP_GET_PSIZE(bp_orig));
                  ASSERT3U(BP_GET_LSIZE(bp), ==, BP_GET_LSIZE(bp_orig));
                  ASSERT(zp->zp_compress != ZIO_COMPRESS_OFF);
                  ASSERT3U(bp->blk_prop, ==, bp_orig->blk_prop);
  
                  /*
                   * If we're overwriting a block that is currently on an
                   * indirect vdev, then ignore the nopwrite request and
                   * allow a new block to be allocated on a concrete vdev.
                   */
                  spa_config_enter(zio->io_spa, SCL_VDEV, FTAG, RW_READER);
                  for (int d = 0; d < BP_GET_NDVAS(bp_orig); d++) {
                          vdev_t *tvd = vdev_lookup_top(zio->io_spa,
                              DVA_GET_VDEV(&bp_orig->blk_dva[d]));
                          if (tvd->vdev_ops == &vdev_indirect_ops) {
                                  spa_config_exit(zio->io_spa, SCL_VDEV, FTAG);
                                  return (zio);
                          }
                  }
                  spa_config_exit(zio->io_spa, SCL_VDEV, FTAG);
  
                  *bp = *bp_orig;
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
                  zio->io_flags |= ZIO_FLAG_NOPWRITE;
          }
  
          return (zio);
  }
  
  /*
   * ==========================================================================
   * Dedup
   * ==========================================================================
   */
  static void
  zio_ddt_child_read_done(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
          ddt_entry_t *dde = zio->io_private;
          ddt_phys_t *ddp;
          zio_t *pio = zio_unique_parent(zio);
  
          mutex_enter(&pio->io_lock);
          ddp = ddt_phys_select(dde, bp);
          if (zio->io_error == 0)
                  ddt_phys_clear(ddp);    /* this ddp doesn't need repair */
  
          if (zio->io_error == 0 && dde->dde_repair_abd == NULL)
                  dde->dde_repair_abd = zio->io_abd;
          else
                  abd_free(zio->io_abd);
          mutex_exit(&pio->io_lock);
  }
  
  static zio_t *
  zio_ddt_read_start(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
          ASSERT(BP_GET_DEDUP(bp));
          ASSERT(BP_GET_PSIZE(bp) == zio->io_size);
          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
  
          if (zio->io_child_error[ZIO_CHILD_DDT]) {
                  ddt_t *ddt = ddt_select(zio->io_spa, bp);
                  ddt_entry_t *dde = ddt_repair_start(ddt, bp);
                  ddt_phys_t *ddp = dde->dde_phys;
                  ddt_phys_t *ddp_self = ddt_phys_select(dde, bp);
                  blkptr_t blk;
  
                  ASSERT(zio->io_vsd == NULL);
                  zio->io_vsd = dde;
  
                  if (ddp_self == NULL)
                          return (zio);
  
                  for (int p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
                          if (ddp->ddp_phys_birth == 0 || ddp == ddp_self)
                                  continue;
                          ddt_bp_create(ddt->ddt_checksum, &dde->dde_key, ddp,
                              &blk);
                          zio_nowait(zio_read(zio, zio->io_spa, &blk,
                              abd_alloc_for_io(zio->io_size, B_TRUE),
                              zio->io_size, zio_ddt_child_read_done, dde,
                              zio->io_priority, ZIO_DDT_CHILD_FLAGS(zio) |
                              ZIO_FLAG_DONT_PROPAGATE, &zio->io_bookmark));
                  }
                  return (zio);
          }
  
          zio_nowait(zio_read(zio, zio->io_spa, bp,
              zio->io_abd, zio->io_size, NULL, NULL, zio->io_priority,
              ZIO_DDT_CHILD_FLAGS(zio), &zio->io_bookmark));
  
          return (zio);
  }
  
  static zio_t *
  zio_ddt_read_done(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
          if (zio_wait_for_children(zio, ZIO_CHILD_DDT_BIT, ZIO_WAIT_DONE)) {
                  return (NULL);
          }
  
          ASSERT(BP_GET_DEDUP(bp));
          ASSERT(BP_GET_PSIZE(bp) == zio->io_size);
          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
  
          if (zio->io_child_error[ZIO_CHILD_DDT]) {
                  ddt_t *ddt = ddt_select(zio->io_spa, bp);
                  ddt_entry_t *dde = zio->io_vsd;
                  if (ddt == NULL) {
                          ASSERT(spa_load_state(zio->io_spa) != SPA_LOAD_NONE);
                          return (zio);
                  }
                  if (dde == NULL) {
                          zio->io_stage = ZIO_STAGE_DDT_READ_START >> 1;
                          zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, B_FALSE);
                          return (NULL);
                  }
                  if (dde->dde_repair_abd != NULL) {
                          abd_copy(zio->io_abd, dde->dde_repair_abd,
                              zio->io_size);
                          zio->io_child_error[ZIO_CHILD_DDT] = 0;
                  }
                  ddt_repair_done(ddt, dde);
                  zio->io_vsd = NULL;
          }
  
          ASSERT(zio->io_vsd == NULL);
  
          return (zio);
  }
  
  static boolean_t
  zio_ddt_collision(zio_t *zio, ddt_t *ddt, ddt_entry_t *dde)
  {
          spa_t *spa = zio->io_spa;
          boolean_t do_raw = !!(zio->io_flags & ZIO_FLAG_RAW);
  
          ASSERT(!(zio->io_bp_override && do_raw));
  
          /*
           * Note: we compare the original data, not the transformed data,
           * because when zio->io_bp is an override bp, we will not have
           * pushed the I/O transforms.  That's an important optimization
           * because otherwise we'd compress/encrypt all dmu_sync() data twice.
           * However, we should never get a raw, override zio so in these
           * cases we can compare the io_abd directly. This is useful because
           * it allows us to do dedup verification even if we don't have access
           * to the original data (for instance, if the encryption keys aren't
           * loaded).
           */
  
          for (int p = DDT_PHYS_SINGLE; p <= DDT_PHYS_TRIPLE; p++) {
                  zio_t *lio = dde->dde_lead_zio[p];
  
                  if (lio != NULL && do_raw) {
                          return (lio->io_size != zio->io_size ||
                              abd_cmp(zio->io_abd, lio->io_abd) != 0);
                  } else if (lio != NULL) {
                          return (lio->io_orig_size != zio->io_orig_size ||
                              abd_cmp(zio->io_orig_abd, lio->io_orig_abd) != 0);
                  }
          }
  
          for (int p = DDT_PHYS_SINGLE; p <= DDT_PHYS_TRIPLE; p++) {
                  ddt_phys_t *ddp = &dde->dde_phys[p];
  
                  if (ddp->ddp_phys_birth != 0 && do_raw) {
                          blkptr_t blk = *zio->io_bp;
                          uint64_t psize;
                          abd_t *tmpabd;
                          int error;
  
                          ddt_bp_fill(ddp, &blk, ddp->ddp_phys_birth);
                          psize = BP_GET_PSIZE(&blk);
  
                          if (psize != zio->io_size)
                                  return (B_TRUE);
  
                          ddt_exit(ddt);
  
                          tmpabd = abd_alloc_for_io(psize, B_TRUE);
  
                          error = zio_wait(zio_read(NULL, spa, &blk, tmpabd,
                              psize, NULL, NULL, ZIO_PRIORITY_SYNC_READ,
                              ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE |
                              ZIO_FLAG_RAW, &zio->io_bookmark));
  
                          if (error == 0) {
                                  if (abd_cmp(tmpabd, zio->io_abd) != 0)
                                          error = SET_ERROR(ENOENT);
                          }
  
                          abd_free(tmpabd);
                          ddt_enter(ddt);
                          return (error != 0);
                  } else if (ddp->ddp_phys_birth != 0) {
                          arc_buf_t *abuf = NULL;
                          arc_flags_t aflags = ARC_FLAG_WAIT;
                          blkptr_t blk = *zio->io_bp;
                          int error;
  
                          ddt_bp_fill(ddp, &blk, ddp->ddp_phys_birth);
  
                          if (BP_GET_LSIZE(&blk) != zio->io_orig_size)
                                  return (B_TRUE);
  
                          ddt_exit(ddt);
  
                          error = arc_read(NULL, spa, &blk,
                              arc_getbuf_func, &abuf, ZIO_PRIORITY_SYNC_READ,
                              ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
                              &aflags, &zio->io_bookmark);
  
                          if (error == 0) {
                                  if (abd_cmp_buf(zio->io_orig_abd, abuf->b_data,
                                      zio->io_orig_size) != 0)
                                          error = SET_ERROR(ENOENT);
                                  arc_buf_destroy(abuf, &abuf);
                          }
  
                          ddt_enter(ddt);
                          return (error != 0);
                  }
          }
  
          return (B_FALSE);
  }
  
  static void
  zio_ddt_child_write_ready(zio_t *zio)
  {
          int p = zio->io_prop.zp_copies;
          ddt_t *ddt = ddt_select(zio->io_spa, zio->io_bp);
          ddt_entry_t *dde = zio->io_private;
          ddt_phys_t *ddp = &dde->dde_phys[p];
          zio_t *pio;
  
          if (zio->io_error)
                  return;
  
          ddt_enter(ddt);
  
          ASSERT(dde->dde_lead_zio[p] == zio);
  
          ddt_phys_fill(ddp, zio->io_bp);
  
          zio_link_t *zl = NULL;
          while ((pio = zio_walk_parents(zio, &zl)) != NULL)
                  ddt_bp_fill(ddp, pio->io_bp, zio->io_txg);
  
          ddt_exit(ddt);
  }
  
  static void
  zio_ddt_child_write_done(zio_t *zio)
  {
          int p = zio->io_prop.zp_copies;
          ddt_t *ddt = ddt_select(zio->io_spa, zio->io_bp);
          ddt_entry_t *dde = zio->io_private;
          ddt_phys_t *ddp = &dde->dde_phys[p];
  
          ddt_enter(ddt);
  
          ASSERT(ddp->ddp_refcnt == 0);
          ASSERT(dde->dde_lead_zio[p] == zio);
          dde->dde_lead_zio[p] = NULL;
  
          if (zio->io_error == 0) {
                  zio_link_t *zl = NULL;
                  while (zio_walk_parents(zio, &zl) != NULL)
                          ddt_phys_addref(ddp);
          } else {
                  ddt_phys_clear(ddp);
          }
  
          ddt_exit(ddt);
  }
  
  static zio_t *
  zio_ddt_write(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          blkptr_t *bp = zio->io_bp;
          uint64_t txg = zio->io_txg;
          zio_prop_t *zp = &zio->io_prop;
          int p = zp->zp_copies;
          zio_t *cio = NULL;
          ddt_t *ddt = ddt_select(spa, bp);
          ddt_entry_t *dde;
          ddt_phys_t *ddp;
  
          ASSERT(BP_GET_DEDUP(bp));
          ASSERT(BP_GET_CHECKSUM(bp) == zp->zp_checksum);
          ASSERT(BP_IS_HOLE(bp) || zio->io_bp_override);
          ASSERT(!(zio->io_bp_override && (zio->io_flags & ZIO_FLAG_RAW)));
  
          ddt_enter(ddt);
          dde = ddt_lookup(ddt, bp, B_TRUE);
          ddp = &dde->dde_phys[p];
  
          if (zp->zp_dedup_verify && zio_ddt_collision(zio, ddt, dde)) {
                  /*
                   * If we're using a weak checksum, upgrade to a strong checksum
                   * and try again.  If we're already using a strong checksum,
                   * we can't resolve it, so just convert to an ordinary write.
                   * (And automatically e-mail a paper to Nature?)
                   */
                  if (!(zio_checksum_table[zp->zp_checksum].ci_flags &
                      ZCHECKSUM_FLAG_DEDUP)) {
                          zp->zp_checksum = spa_dedup_checksum(spa);
                          zio_pop_transforms(zio);
                          zio->io_stage = ZIO_STAGE_OPEN;
                          BP_ZERO(bp);
                  } else {
                          zp->zp_dedup = B_FALSE;
                          BP_SET_DEDUP(bp, B_FALSE);
                  }
                  ASSERT(!BP_GET_DEDUP(bp));
                  zio->io_pipeline = ZIO_WRITE_PIPELINE;
                  ddt_exit(ddt);
                  return (zio);
          }
  
          if (ddp->ddp_phys_birth != 0 || dde->dde_lead_zio[p] != NULL) {
                  if (ddp->ddp_phys_birth != 0)
                          ddt_bp_fill(ddp, bp, txg);
                  if (dde->dde_lead_zio[p] != NULL)
                          zio_add_child(zio, dde->dde_lead_zio[p]);
                  else
                          ddt_phys_addref(ddp);
          } else if (zio->io_bp_override) {
                  ASSERT(bp->blk_birth == txg);
                  ASSERT(BP_EQUAL(bp, zio->io_bp_override));
                  ddt_phys_fill(ddp, bp);
                  ddt_phys_addref(ddp);
          } else {
                  cio = zio_write(zio, spa, txg, bp, zio->io_orig_abd,
                      zio->io_orig_size, zio->io_orig_size, zp,
                      zio_ddt_child_write_ready, NULL, NULL,
                      zio_ddt_child_write_done, dde, zio->io_priority,
                      ZIO_DDT_CHILD_FLAGS(zio), &zio->io_bookmark);
  
                  zio_push_transform(cio, zio->io_abd, zio->io_size, 0, NULL);
                  dde->dde_lead_zio[p] = cio;
          }
  
          ddt_exit(ddt);
  
          zio_nowait(cio);
  
          return (zio);
  }
  
  static ddt_entry_t *freedde; /* for debugging */
  
  static zio_t *
  zio_ddt_free(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          blkptr_t *bp = zio->io_bp;
          ddt_t *ddt = ddt_select(spa, bp);
          ddt_entry_t *dde;
          ddt_phys_t *ddp;
  
          ASSERT(BP_GET_DEDUP(bp));
          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
  
          ddt_enter(ddt);
          freedde = dde = ddt_lookup(ddt, bp, B_TRUE);
          if (dde) {
                  ddp = ddt_phys_select(dde, bp);
                  if (ddp)
                          ddt_phys_decref(ddp);
          }
          ddt_exit(ddt);
  
          return (zio);
  }
  
  /*
   * ==========================================================================
   * Allocate and free blocks
   * ==========================================================================
   */
  
  static zio_t *
  zio_io_to_allocate(spa_t *spa, int allocator)
  {
          zio_t *zio;
  
          ASSERT(MUTEX_HELD(&spa->spa_allocs[allocator].spaa_lock));
  
          zio = avl_first(&spa->spa_allocs[allocator].spaa_tree);
          if (zio == NULL)
                  return (NULL);
  
          ASSERT(IO_IS_ALLOCATING(zio));
  
          /*
           * Try to place a reservation for this zio. If we're unable to
           * reserve then we throttle.
           */
          ASSERT3U(zio->io_allocator, ==, allocator);
          if (!metaslab_class_throttle_reserve(zio->io_metaslab_class,
              zio->io_prop.zp_copies, allocator, zio, 0)) {
                  return (NULL);
          }
  
          avl_remove(&spa->spa_allocs[allocator].spaa_tree, zio);
          ASSERT3U(zio->io_stage, <, ZIO_STAGE_DVA_ALLOCATE);
  
          return (zio);
  }
  
  static zio_t *
  zio_dva_throttle(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          zio_t *nio;
          metaslab_class_t *mc;
  
          /* locate an appropriate allocation class */
          mc = spa_preferred_class(spa, zio->io_size, zio->io_prop.zp_type,
              zio->io_prop.zp_level, zio->io_prop.zp_zpl_smallblk);
  
          if (zio->io_priority == ZIO_PRIORITY_SYNC_WRITE ||
              !mc->mc_alloc_throttle_enabled ||
              zio->io_child_type == ZIO_CHILD_GANG ||
              zio->io_flags & ZIO_FLAG_NODATA) {
                  return (zio);
          }
  
          ASSERT(zio->io_type == ZIO_TYPE_WRITE);
          ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
          ASSERT3U(zio->io_queued_timestamp, >, 0);
          ASSERT(zio->io_stage == ZIO_STAGE_DVA_THROTTLE);
  
          zbookmark_phys_t *bm = &zio->io_bookmark;
          /*
           * We want to try to use as many allocators as possible to help improve
           * performance, but we also want logically adjacent IOs to be physically
           * adjacent to improve sequential read performance. We chunk each object
           * into 2^20 block regions, and then hash based on the objset, object,
           * level, and region to accomplish both of these goals.
           */
          int allocator = (uint_t)cityhash4(bm->zb_objset, bm->zb_object,
              bm->zb_level, bm->zb_blkid >> 20) % spa->spa_alloc_count;
          zio->io_allocator = allocator;
          zio->io_metaslab_class = mc;
          mutex_enter(&spa->spa_allocs[allocator].spaa_lock);
          avl_add(&spa->spa_allocs[allocator].spaa_tree, zio);
          nio = zio_io_to_allocate(spa, allocator);
          mutex_exit(&spa->spa_allocs[allocator].spaa_lock);
          return (nio);
  }
  
  static void
  zio_allocate_dispatch(spa_t *spa, int allocator)
  {
          zio_t *zio;
  
          mutex_enter(&spa->spa_allocs[allocator].spaa_lock);
          zio = zio_io_to_allocate(spa, allocator);
          mutex_exit(&spa->spa_allocs[allocator].spaa_lock);
          if (zio == NULL)
                  return;
  
          ASSERT3U(zio->io_stage, ==, ZIO_STAGE_DVA_THROTTLE);
          ASSERT0(zio->io_error);
          zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, B_TRUE);
  }
  
  static zio_t *
  zio_dva_allocate(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          metaslab_class_t *mc;
          blkptr_t *bp = zio->io_bp;
          int error;
          int flags = 0;
  
          if (zio->io_gang_leader == NULL) {
                  ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
                  zio->io_gang_leader = zio;
          }
  
          ASSERT(BP_IS_HOLE(bp));
          ASSERT0(BP_GET_NDVAS(bp));
          ASSERT3U(zio->io_prop.zp_copies, >, 0);
          ASSERT3U(zio->io_prop.zp_copies, <=, spa_max_replication(spa));
          ASSERT3U(zio->io_size, ==, BP_GET_PSIZE(bp));
  
          flags |= (zio->io_flags & ZIO_FLAG_FASTWRITE) ? METASLAB_FASTWRITE : 0;
          if (zio->io_flags & ZIO_FLAG_NODATA)
                  flags |= METASLAB_DONT_THROTTLE;
          if (zio->io_flags & ZIO_FLAG_GANG_CHILD)
                  flags |= METASLAB_GANG_CHILD;
          if (zio->io_priority == ZIO_PRIORITY_ASYNC_WRITE)
                  flags |= METASLAB_ASYNC_ALLOC;
  
          /*
           * if not already chosen, locate an appropriate allocation class
           */
          mc = zio->io_metaslab_class;
          if (mc == NULL) {
                  mc = spa_preferred_class(spa, zio->io_size,
                      zio->io_prop.zp_type, zio->io_prop.zp_level,
                      zio->io_prop.zp_zpl_smallblk);
                  zio->io_metaslab_class = mc;
          }
  
          /*
           * Try allocating the block in the usual metaslab class.
           * If that's full, allocate it in the normal class.
           * If that's full, allocate as a gang block,
           * and if all are full, the allocation fails (which shouldn't happen).
           *
           * Note that we do not fall back on embedded slog (ZIL) space, to
           * preserve unfragmented slog space, which is critical for decent
           * sync write performance.  If a log allocation fails, we will fall
           * back to spa_sync() which is abysmal for performance.
           */
          error = metaslab_alloc(spa, mc, zio->io_size, bp,
              zio->io_prop.zp_copies, zio->io_txg, NULL, flags,
              &zio->io_alloc_list, zio, zio->io_allocator);
  
          /*
           * Fallback to normal class when an alloc class is full
           */
          if (error == ENOSPC && mc != spa_normal_class(spa)) {
                  /*
                   * If throttling, transfer reservation over to normal class.
                   * The io_allocator slot can remain the same even though we
                   * are switching classes.
                   */
                  if (mc->mc_alloc_throttle_enabled &&
                      (zio->io_flags & ZIO_FLAG_IO_ALLOCATING)) {
                          metaslab_class_throttle_unreserve(mc,
                              zio->io_prop.zp_copies, zio->io_allocator, zio);
                          zio->io_flags &= ~ZIO_FLAG_IO_ALLOCATING;
  
                          VERIFY(metaslab_class_throttle_reserve(
                              spa_normal_class(spa),
                              zio->io_prop.zp_copies, zio->io_allocator, zio,
                              flags | METASLAB_MUST_RESERVE));
                  }
                  zio->io_metaslab_class = mc = spa_normal_class(spa);
                  if (zfs_flags & ZFS_DEBUG_METASLAB_ALLOC) {
                          zfs_dbgmsg("%s: metaslab allocation failure, "
                              "trying normal class: zio %px, size %llu, error %d",
                              spa_name(spa), zio, (u_longlong_t)zio->io_size,
                              error);
                  }
  
                  error = metaslab_alloc(spa, mc, zio->io_size, bp,
                      zio->io_prop.zp_copies, zio->io_txg, NULL, flags,
                      &zio->io_alloc_list, zio, zio->io_allocator);
          }
  
          if (error == ENOSPC && zio->io_size > SPA_MINBLOCKSIZE) {
                  if (zfs_flags & ZFS_DEBUG_METASLAB_ALLOC) {
                          zfs_dbgmsg("%s: metaslab allocation failure, "
                              "trying ganging: zio %px, size %llu, error %d",
                              spa_name(spa), zio, (u_longlong_t)zio->io_size,
                              error);
                  }
                  return (zio_write_gang_block(zio, mc));
          }
          if (error != 0) {
                  if (error != ENOSPC ||
                      (zfs_flags & ZFS_DEBUG_METASLAB_ALLOC)) {
                          zfs_dbgmsg("%s: metaslab allocation failure: zio %px, "
                              "size %llu, error %d",
                              spa_name(spa), zio, (u_longlong_t)zio->io_size,
                              error);
                  }
                  zio->io_error = error;
          }
  
          return (zio);
  }
  
  static zio_t *
  zio_dva_free(zio_t *zio)
  {
          metaslab_free(zio->io_spa, zio->io_bp, zio->io_txg, B_FALSE);
  
          return (zio);
  }
  
  static zio_t *
  zio_dva_claim(zio_t *zio)
  {
          int error;
  
          error = metaslab_claim(zio->io_spa, zio->io_bp, zio->io_txg);
          if (error)
                  zio->io_error = error;
  
          return (zio);
  }
  
  /*
   * Undo an allocation.  This is used by zio_done() when an I/O fails
   * and we want to give back the block we just allocated.
   * This handles both normal blocks and gang blocks.
   */
  static void
  zio_dva_unallocate(zio_t *zio, zio_gang_node_t *gn, blkptr_t *bp)
  {
          ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp));
          ASSERT(zio->io_bp_override == NULL);
  
          if (!BP_IS_HOLE(bp))
                  metaslab_free(zio->io_spa, bp, bp->blk_birth, B_TRUE);
  
          if (gn != NULL) {
                  for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
                          zio_dva_unallocate(zio, gn->gn_child[g],
                              &gn->gn_gbh->zg_blkptr[g]);
                  }
          }
  }
  
  /*
   * Try to allocate an intent log block.  Return 0 on success, errno on failure.
   */
  int
  zio_alloc_zil(spa_t *spa, objset_t *os, uint64_t txg, blkptr_t *new_bp,
      uint64_t size, boolean_t *slog)
  {
          int error = 1;
          zio_alloc_list_t io_alloc_list;
  
          ASSERT(txg > spa_syncing_txg(spa));
  
          metaslab_trace_init(&io_alloc_list);
  
          /*
           * Block pointer fields are useful to metaslabs for stats and debugging.
           * Fill in the obvious ones before calling into metaslab_alloc().
           */
          BP_SET_TYPE(new_bp, DMU_OT_INTENT_LOG);
          BP_SET_PSIZE(new_bp, size);
          BP_SET_LEVEL(new_bp, 0);
  
          /*
           * When allocating a zil block, we don't have information about
           * the final destination of the block except the objset it's part
           * of, so we just hash the objset ID to pick the allocator to get
           * some parallelism.
           */
          int flags = METASLAB_FASTWRITE | METASLAB_ZIL;
          int allocator = (uint_t)cityhash4(0, 0, 0,
              os->os_dsl_dataset->ds_object) % spa->spa_alloc_count;
          error = metaslab_alloc(spa, spa_log_class(spa), size, new_bp, 1,
              txg, NULL, flags, &io_alloc_list, NULL, allocator);
          *slog = (error == 0);
          if (error != 0) {
                  error = metaslab_alloc(spa, spa_embedded_log_class(spa), size,
                      new_bp, 1, txg, NULL, flags,
                      &io_alloc_list, NULL, allocator);
          }
          if (error != 0) {
                  error = metaslab_alloc(spa, spa_normal_class(spa), size,
                      new_bp, 1, txg, NULL, flags,
                      &io_alloc_list, NULL, allocator);
          }
          metaslab_trace_fini(&io_alloc_list);
  
          if (error == 0) {
                  BP_SET_LSIZE(new_bp, size);
                  BP_SET_PSIZE(new_bp, size);
                  BP_SET_COMPRESS(new_bp, ZIO_COMPRESS_OFF);
                  BP_SET_CHECKSUM(new_bp,
                      spa_version(spa) >= SPA_VERSION_SLIM_ZIL
                      ? ZIO_CHECKSUM_ZILOG2 : ZIO_CHECKSUM_ZILOG);
                  BP_SET_TYPE(new_bp, DMU_OT_INTENT_LOG);
                  BP_SET_LEVEL(new_bp, 0);
                  BP_SET_DEDUP(new_bp, 0);
                  BP_SET_BYTEORDER(new_bp, ZFS_HOST_BYTEORDER);
  
                  /*
                   * encrypted blocks will require an IV and salt. We generate
                   * these now since we will not be rewriting the bp at
                   * rewrite time.
                   */
                  if (os->os_encrypted) {
                          uint8_t iv[ZIO_DATA_IV_LEN];
                          uint8_t salt[ZIO_DATA_SALT_LEN];
  
                          BP_SET_CRYPT(new_bp, B_TRUE);
                          VERIFY0(spa_crypt_get_salt(spa,
                              dmu_objset_id(os), salt));
                          VERIFY0(zio_crypt_generate_iv(iv));
  
                          zio_crypt_encode_params_bp(new_bp, salt, iv);
                  }
          } else {
                  zfs_dbgmsg("%s: zil block allocation failure: "
                      "size %llu, error %d", spa_name(spa), (u_longlong_t)size,
                      error);
          }
  
          return (error);
  }
  
  /*
   * ==========================================================================
   * Read and write to physical devices
   * ==========================================================================
   */
  
  /*
   * Issue an I/O to the underlying vdev. Typically the issue pipeline
   * stops after this stage and will resume upon I/O completion.
   * However, there are instances where the vdev layer may need to
   * continue the pipeline when an I/O was not issued. Since the I/O
   * that was sent to the vdev layer might be different than the one
   * currently active in the pipeline (see vdev_queue_io()), we explicitly
   * force the underlying vdev layers to call either zio_execute() or
   * zio_interrupt() to ensure that the pipeline continues with the correct I/O.
   */
  static zio_t *
  zio_vdev_io_start(zio_t *zio)
  {
          vdev_t *vd = zio->io_vd;
          uint64_t align;
          spa_t *spa = zio->io_spa;
  
          zio->io_delay = 0;
  
          ASSERT(zio->io_error == 0);
          ASSERT(zio->io_child_error[ZIO_CHILD_VDEV] == 0);
  
          if (vd == NULL) {
                  if (!(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
                          spa_config_enter(spa, SCL_ZIO, zio, RW_READER);
  
                  /*
                   * The mirror_ops handle multiple DVAs in a single BP.
                   */
                  vdev_mirror_ops.vdev_op_io_start(zio);
                  return (NULL);
          }
  
          ASSERT3P(zio->io_logical, !=, zio);
          if (zio->io_type == ZIO_TYPE_WRITE) {
                  ASSERT(spa->spa_trust_config);
  
                  /*
                   * Note: the code can handle other kinds of writes,
                   * but we don't expect them.
                   */
                  if (zio->io_vd->vdev_noalloc) {
                          ASSERT(zio->io_flags &
                              (ZIO_FLAG_PHYSICAL | ZIO_FLAG_SELF_HEAL |
                              ZIO_FLAG_RESILVER | ZIO_FLAG_INDUCE_DAMAGE));
                  }
          }
  
          align = 1ULL << vd->vdev_top->vdev_ashift;
  
          if (!(zio->io_flags & ZIO_FLAG_PHYSICAL) &&
              P2PHASE(zio->io_size, align) != 0) {
                  /* Transform logical writes to be a full physical block size. */
                  uint64_t asize = P2ROUNDUP(zio->io_size, align);
                  abd_t *abuf = abd_alloc_sametype(zio->io_abd, asize);
                  ASSERT(vd == vd->vdev_top);
                  if (zio->io_type == ZIO_TYPE_WRITE) {
                          abd_copy(abuf, zio->io_abd, zio->io_size);
                          abd_zero_off(abuf, zio->io_size, asize - zio->io_size);
                  }
                  zio_push_transform(zio, abuf, asize, asize, zio_subblock);
          }
  
          /*
           * If this is not a physical io, make sure that it is properly aligned
           * before proceeding.
           */
          if (!(zio->io_flags & ZIO_FLAG_PHYSICAL)) {
                  ASSERT0(P2PHASE(zio->io_offset, align));
                  ASSERT0(P2PHASE(zio->io_size, align));
          } else {
                  /*
                   * For physical writes, we allow 512b aligned writes and assume
                   * the device will perform a read-modify-write as necessary.
                   */
                  ASSERT0(P2PHASE(zio->io_offset, SPA_MINBLOCKSIZE));
                  ASSERT0(P2PHASE(zio->io_size, SPA_MINBLOCKSIZE));
          }
  
          VERIFY(zio->io_type != ZIO_TYPE_WRITE || spa_writeable(spa));
  
          /*
           * If this is a repair I/O, and there's no self-healing involved --
           * that is, we're just resilvering what we expect to resilver --
           * then don't do the I/O unless zio's txg is actually in vd's DTL.
           * This prevents spurious resilvering.
           *
           * There are a few ways that we can end up creating these spurious
           * resilver i/os:
           *
           * 1. A resilver i/o will be issued if any DVA in the BP has a
           * dirty DTL.  The mirror code will issue resilver writes to
           * each DVA, including the one(s) that are not on vdevs with dirty
           * DTLs.
           *
           * 2. With nested replication, which happens when we have a
           * "replacing" or "spare" vdev that's a child of a mirror or raidz.
           * For example, given mirror(replacing(A+B), C), it's likely that
           * only A is out of date (it's the new device). In this case, we'll
           * read from C, then use the data to resilver A+B -- but we don't
           * actually want to resilver B, just A. The top-level mirror has no
           * way to know this, so instead we just discard unnecessary repairs
           * as we work our way down the vdev tree.
           *
           * 3. ZTEST also creates mirrors of mirrors, mirrors of raidz, etc.
           * The same logic applies to any form of nested replication: ditto
           * + mirror, RAID-Z + replacing, etc.
           *
           * However, indirect vdevs point off to other vdevs which may have
           * DTL's, so we never bypass them.  The child i/os on concrete vdevs
           * will be properly bypassed instead.
           *
           * Leaf DTL_PARTIAL can be empty when a legitimate write comes from
           * a dRAID spare vdev. For example, when a dRAID spare is first
           * used, its spare blocks need to be written to but the leaf vdev's
           * of such blocks can have empty DTL_PARTIAL.
           *
           * There seemed no clean way to allow such writes while bypassing
           * spurious ones. At this point, just avoid all bypassing for dRAID
           * for correctness.
           */
          if ((zio->io_flags & ZIO_FLAG_IO_REPAIR) &&
              !(zio->io_flags & ZIO_FLAG_SELF_HEAL) &&
              zio->io_txg != 0 && /* not a delegated i/o */
              vd->vdev_ops != &vdev_indirect_ops &&
              vd->vdev_top->vdev_ops != &vdev_draid_ops &&
              !vdev_dtl_contains(vd, DTL_PARTIAL, zio->io_txg, 1)) {
                  ASSERT(zio->io_type == ZIO_TYPE_WRITE);
                  zio_vdev_io_bypass(zio);
                  return (zio);
          }
  
          /*
           * Select the next best leaf I/O to process.  Distributed spares are
           * excluded since they dispatch the I/O directly to a leaf vdev after
           * applying the dRAID mapping.
           */
          if (vd->vdev_ops->vdev_op_leaf &&
              vd->vdev_ops != &vdev_draid_spare_ops &&
              (zio->io_type == ZIO_TYPE_READ ||
              zio->io_type == ZIO_TYPE_WRITE ||
              zio->io_type == ZIO_TYPE_TRIM)) {
  
                  if (zio->io_type == ZIO_TYPE_READ && vdev_cache_read(zio))
                          return (zio);
  
                  if ((zio = vdev_queue_io(zio)) == NULL)
                          return (NULL);
  
                  if (!vdev_accessible(vd, zio)) {
                          zio->io_error = SET_ERROR(ENXIO);
                          zio_interrupt(zio);
                          return (NULL);
                  }
                  zio->io_delay = gethrtime();
          }
  
          vd->vdev_ops->vdev_op_io_start(zio);
          return (NULL);
  }
  
  static zio_t *
  zio_vdev_io_done(zio_t *zio)
  {
          vdev_t *vd = zio->io_vd;
          vdev_ops_t *ops = vd ? vd->vdev_ops : &vdev_mirror_ops;
          boolean_t unexpected_error = B_FALSE;
  
          if (zio_wait_for_children(zio, ZIO_CHILD_VDEV_BIT, ZIO_WAIT_DONE)) {
                  return (NULL);
          }
  
          ASSERT(zio->io_type == ZIO_TYPE_READ ||
              zio->io_type == ZIO_TYPE_WRITE || zio->io_type == ZIO_TYPE_TRIM);
  
          if (zio->io_delay)
                  zio->io_delay = gethrtime() - zio->io_delay;
  
          if (vd != NULL && vd->vdev_ops->vdev_op_leaf &&
              vd->vdev_ops != &vdev_draid_spare_ops) {
                  vdev_queue_io_done(zio);
  
                  if (zio->io_type == ZIO_TYPE_WRITE)
                          vdev_cache_write(zio);
  
                  if (zio_injection_enabled && zio->io_error == 0)
                          zio->io_error = zio_handle_device_injections(vd, zio,
                              EIO, EILSEQ);
  
                  if (zio_injection_enabled && zio->io_error == 0)
                          zio->io_error = zio_handle_label_injection(zio, EIO);
  
                  if (zio->io_error && zio->io_type != ZIO_TYPE_TRIM) {
                          if (!vdev_accessible(vd, zio)) {
                                  zio->io_error = SET_ERROR(ENXIO);
                          } else {
                                  unexpected_error = B_TRUE;
                          }
                  }
          }
  
          ops->vdev_op_io_done(zio);
  
          if (unexpected_error && vd->vdev_remove_wanted == B_FALSE)
                  VERIFY(vdev_probe(vd, zio) == NULL);
  
          return (zio);
  }
  
  /*
   * This function is used to change the priority of an existing zio that is
   * currently in-flight. This is used by the arc to upgrade priority in the
   * event that a demand read is made for a block that is currently queued
   * as a scrub or async read IO. Otherwise, the high priority read request
   * would end up having to wait for the lower priority IO.
   */
  void
  zio_change_priority(zio_t *pio, zio_priority_t priority)
  {
          zio_t *cio, *cio_next;
          zio_link_t *zl = NULL;
  
          ASSERT3U(priority, <, ZIO_PRIORITY_NUM_QUEUEABLE);
  
          if (pio->io_vd != NULL && pio->io_vd->vdev_ops->vdev_op_leaf) {
                  vdev_queue_change_io_priority(pio, priority);
          } else {
                  pio->io_priority = priority;
          }
  
          mutex_enter(&pio->io_lock);
          for (cio = zio_walk_children(pio, &zl); cio != NULL; cio = cio_next) {
                  cio_next = zio_walk_children(pio, &zl);
                  zio_change_priority(cio, priority);
          }
          mutex_exit(&pio->io_lock);
  }
  
  /*
   * For non-raidz ZIOs, we can just copy aside the bad data read from the
   * disk, and use that to finish the checksum ereport later.
   */
  static void
  zio_vsd_default_cksum_finish(zio_cksum_report_t *zcr,
      const abd_t *good_buf)
  {
          /* no processing needed */
          zfs_ereport_finish_checksum(zcr, good_buf, zcr->zcr_cbdata, B_FALSE);
  }
  
  void
  zio_vsd_default_cksum_report(zio_t *zio, zio_cksum_report_t *zcr)
  {
          void *abd = abd_alloc_sametype(zio->io_abd, zio->io_size);
  
          abd_copy(abd, zio->io_abd, zio->io_size);
  
          zcr->zcr_cbinfo = zio->io_size;
          zcr->zcr_cbdata = abd;
          zcr->zcr_finish = zio_vsd_default_cksum_finish;
          zcr->zcr_free = zio_abd_free;
  }
  
  static zio_t *
  zio_vdev_io_assess(zio_t *zio)
  {
          vdev_t *vd = zio->io_vd;
  
          if (zio_wait_for_children(zio, ZIO_CHILD_VDEV_BIT, ZIO_WAIT_DONE)) {
                  return (NULL);
          }
  
          if (vd == NULL && !(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
                  spa_config_exit(zio->io_spa, SCL_ZIO, zio);
  
          if (zio->io_vsd != NULL) {
                  zio->io_vsd_ops->vsd_free(zio);
                  zio->io_vsd = NULL;
          }
  
          if (zio_injection_enabled && zio->io_error == 0)
                  zio->io_error = zio_handle_fault_injection(zio, EIO);
  
          /*
           * If the I/O failed, determine whether we should attempt to retry it.
           *
           * On retry, we cut in line in the issue queue, since we don't want
           * compression/checksumming/etc. work to prevent our (cheap) IO reissue.
           */
          if (zio->io_error && vd == NULL &&
              !(zio->io_flags & (ZIO_FLAG_DONT_RETRY | ZIO_FLAG_IO_RETRY))) {
                  ASSERT(!(zio->io_flags & ZIO_FLAG_DONT_QUEUE)); /* not a leaf */
                  ASSERT(!(zio->io_flags & ZIO_FLAG_IO_BYPASS));  /* not a leaf */
                  zio->io_error = 0;
                  zio->io_flags |= ZIO_FLAG_IO_RETRY |
                      ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_AGGREGATE;
                  zio->io_stage = ZIO_STAGE_VDEV_IO_START >> 1;
                  zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE,
                      zio_requeue_io_start_cut_in_line);
                  return (NULL);
          }
  
          /*
           * If we got an error on a leaf device, convert it to ENXIO
           * if the device is not accessible at all.
           */
          if (zio->io_error && vd != NULL && vd->vdev_ops->vdev_op_leaf &&
              !vdev_accessible(vd, zio))
                  zio->io_error = SET_ERROR(ENXIO);
  
          /*
           * If we can't write to an interior vdev (mirror or RAID-Z),
           * set vdev_cant_write so that we stop trying to allocate from it.
           */
          if (zio->io_error == ENXIO && zio->io_type == ZIO_TYPE_WRITE &&
              vd != NULL && !vd->vdev_ops->vdev_op_leaf) {
                  vdev_dbgmsg(vd, "zio_vdev_io_assess(zio=%px) setting "
                      "cant_write=TRUE due to write failure with ENXIO",
                      zio);
                  vd->vdev_cant_write = B_TRUE;
          }
  
          /*
           * If a cache flush returns ENOTSUP or ENOTTY, we know that no future
           * attempts will ever succeed. In this case we set a persistent
           * boolean flag so that we don't bother with it in the future.
           */
          if ((zio->io_error == ENOTSUP || zio->io_error == ENOTTY) &&
              zio->io_type == ZIO_TYPE_IOCTL &&
              zio->io_cmd == DKIOCFLUSHWRITECACHE && vd != NULL)
                  vd->vdev_nowritecache = B_TRUE;
  
          if (zio->io_error)
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
  
          if (vd != NULL && vd->vdev_ops->vdev_op_leaf &&
              zio->io_physdone != NULL) {
                  ASSERT(!(zio->io_flags & ZIO_FLAG_DELEGATED));
                  ASSERT(zio->io_child_type == ZIO_CHILD_VDEV);
                  zio->io_physdone(zio->io_logical);
          }
  
          return (zio);
  }
  
  void
  zio_vdev_io_reissue(zio_t *zio)
  {
          ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_START);
          ASSERT(zio->io_error == 0);
  
          zio->io_stage >>= 1;
  }
  
  void
  zio_vdev_io_redone(zio_t *zio)
  {
          ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_DONE);
  
          zio->io_stage >>= 1;
  }
  
  void
  zio_vdev_io_bypass(zio_t *zio)
  {
          ASSERT(zio->io_stage == ZIO_STAGE_VDEV_IO_START);
          ASSERT(zio->io_error == 0);
  
          zio->io_flags |= ZIO_FLAG_IO_BYPASS;
          zio->io_stage = ZIO_STAGE_VDEV_IO_ASSESS >> 1;
  }
  
  /*
   * ==========================================================================
   * Encrypt and store encryption parameters
   * ==========================================================================
   */
  
  
  /*
   * This function is used for ZIO_STAGE_ENCRYPT. It is responsible for
   * managing the storage of encryption parameters and passing them to the
   * lower-level encryption functions.
   */
  static zio_t *
  zio_encrypt(zio_t *zio)
  {
          zio_prop_t *zp = &zio->io_prop;
          spa_t *spa = zio->io_spa;
          blkptr_t *bp = zio->io_bp;
          uint64_t psize = BP_GET_PSIZE(bp);
          uint64_t dsobj = zio->io_bookmark.zb_objset;
          dmu_object_type_t ot = BP_GET_TYPE(bp);
          void *enc_buf = NULL;
          abd_t *eabd = NULL;
          uint8_t salt[ZIO_DATA_SALT_LEN];
          uint8_t iv[ZIO_DATA_IV_LEN];
          uint8_t mac[ZIO_DATA_MAC_LEN];
          boolean_t no_crypt = B_FALSE;
  
          /* the root zio already encrypted the data */
          if (zio->io_child_type == ZIO_CHILD_GANG)
                  return (zio);
  
          /* only ZIL blocks are re-encrypted on rewrite */
          if (!IO_IS_ALLOCATING(zio) && ot != DMU_OT_INTENT_LOG)
                  return (zio);
  
          if (!(zp->zp_encrypt || BP_IS_ENCRYPTED(bp))) {
                  BP_SET_CRYPT(bp, B_FALSE);
                  return (zio);
          }
  
          /* if we are doing raw encryption set the provided encryption params */
          if (zio->io_flags & ZIO_FLAG_RAW_ENCRYPT) {
                  ASSERT0(BP_GET_LEVEL(bp));
                  BP_SET_CRYPT(bp, B_TRUE);
                  BP_SET_BYTEORDER(bp, zp->zp_byteorder);
                  if (ot != DMU_OT_OBJSET)
                          zio_crypt_encode_mac_bp(bp, zp->zp_mac);
  
                  /* dnode blocks must be written out in the provided byteorder */
                  if (zp->zp_byteorder != ZFS_HOST_BYTEORDER &&
                      ot == DMU_OT_DNODE) {
                          void *bswap_buf = zio_buf_alloc(psize);
                          abd_t *babd = abd_get_from_buf(bswap_buf, psize);
  
                          ASSERT3U(BP_GET_COMPRESS(bp), ==, ZIO_COMPRESS_OFF);
                          abd_copy_to_buf(bswap_buf, zio->io_abd, psize);
                          dmu_ot_byteswap[DMU_OT_BYTESWAP(ot)].ob_func(bswap_buf,
                              psize);
  
                          abd_take_ownership_of_buf(babd, B_TRUE);
                          zio_push_transform(zio, babd, psize, psize, NULL);
                  }
  
                  if (DMU_OT_IS_ENCRYPTED(ot))
                          zio_crypt_encode_params_bp(bp, zp->zp_salt, zp->zp_iv);
                  return (zio);
          }
  
          /* indirect blocks only maintain a cksum of the lower level MACs */
          if (BP_GET_LEVEL(bp) > 0) {
                  BP_SET_CRYPT(bp, B_TRUE);
                  VERIFY0(zio_crypt_do_indirect_mac_checksum_abd(B_TRUE,
                      zio->io_orig_abd, BP_GET_LSIZE(bp), BP_SHOULD_BYTESWAP(bp),
                      mac));
                  zio_crypt_encode_mac_bp(bp, mac);
                  return (zio);
          }
  
          /*
           * Objset blocks are a special case since they have 2 256-bit MACs
           * embedded within them.
           */
          if (ot == DMU_OT_OBJSET) {
                  ASSERT0(DMU_OT_IS_ENCRYPTED(ot));
                  ASSERT3U(BP_GET_COMPRESS(bp), ==, ZIO_COMPRESS_OFF);
                  BP_SET_CRYPT(bp, B_TRUE);
                  VERIFY0(spa_do_crypt_objset_mac_abd(B_TRUE, spa, dsobj,
                      zio->io_abd, psize, BP_SHOULD_BYTESWAP(bp)));
                  return (zio);
          }
  
          /* unencrypted object types are only authenticated with a MAC */
          if (!DMU_OT_IS_ENCRYPTED(ot)) {
                  BP_SET_CRYPT(bp, B_TRUE);
                  VERIFY0(spa_do_crypt_mac_abd(B_TRUE, spa, dsobj,
                      zio->io_abd, psize, mac));
                  zio_crypt_encode_mac_bp(bp, mac);
                  return (zio);
          }
  
          /*
           * Later passes of sync-to-convergence may decide to rewrite data
           * in place to avoid more disk reallocations. This presents a problem
           * for encryption because this constitutes rewriting the new data with
           * the same encryption key and IV. However, this only applies to blocks
           * in the MOS (particularly the spacemaps) and we do not encrypt the
           * MOS. We assert that the zio is allocating or an intent log write
           * to enforce this.
           */
          ASSERT(IO_IS_ALLOCATING(zio) || ot == DMU_OT_INTENT_LOG);
          ASSERT(BP_GET_LEVEL(bp) == 0 || ot == DMU_OT_INTENT_LOG);
          ASSERT(spa_feature_is_active(spa, SPA_FEATURE_ENCRYPTION));
          ASSERT3U(psize, !=, 0);
  
          enc_buf = zio_buf_alloc(psize);
          eabd = abd_get_from_buf(enc_buf, psize);
          abd_take_ownership_of_buf(eabd, B_TRUE);
  
          /*
           * For an explanation of what encryption parameters are stored
           * where, see the block comment in zio_crypt.c.
           */
          if (ot == DMU_OT_INTENT_LOG) {
                  zio_crypt_decode_params_bp(bp, salt, iv);
          } else {
                  BP_SET_CRYPT(bp, B_TRUE);
          }
  
          /* Perform the encryption. This should not fail */
          VERIFY0(spa_do_crypt_abd(B_TRUE, spa, &zio->io_bookmark,
              BP_GET_TYPE(bp), BP_GET_DEDUP(bp), BP_SHOULD_BYTESWAP(bp),
              salt, iv, mac, psize, zio->io_abd, eabd, &no_crypt));
  
          /* encode encryption metadata into the bp */
          if (ot == DMU_OT_INTENT_LOG) {
                  /*
                   * ZIL blocks store the MAC in the embedded checksum, so the
                   * transform must always be applied.
                   */
                  zio_crypt_encode_mac_zil(enc_buf, mac);
                  zio_push_transform(zio, eabd, psize, psize, NULL);
          } else {
                  BP_SET_CRYPT(bp, B_TRUE);
                  zio_crypt_encode_params_bp(bp, salt, iv);
                  zio_crypt_encode_mac_bp(bp, mac);
  
                  if (no_crypt) {
                          ASSERT3U(ot, ==, DMU_OT_DNODE);
                          abd_free(eabd);
                  } else {
                          zio_push_transform(zio, eabd, psize, psize, NULL);
                  }
          }
  
          return (zio);
  }
  
  /*
   * ==========================================================================
   * Generate and verify checksums
   * ==========================================================================
   */
  static zio_t *
  zio_checksum_generate(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
          enum zio_checksum checksum;
  
          if (bp == NULL) {
                  /*
                   * This is zio_write_phys().
                   * We're either generating a label checksum, or none at all.
                   */
                  checksum = zio->io_prop.zp_checksum;
  
                  if (checksum == ZIO_CHECKSUM_OFF)
                          return (zio);
  
                  ASSERT(checksum == ZIO_CHECKSUM_LABEL);
          } else {
                  if (BP_IS_GANG(bp) && zio->io_child_type == ZIO_CHILD_GANG) {
                          ASSERT(!IO_IS_ALLOCATING(zio));
                          checksum = ZIO_CHECKSUM_GANG_HEADER;
                  } else {
                          checksum = BP_GET_CHECKSUM(bp);
                  }
          }
  
          zio_checksum_compute(zio, checksum, zio->io_abd, zio->io_size);
  
          return (zio);
  }
  
  static zio_t *
  zio_checksum_verify(zio_t *zio)
  {
          zio_bad_cksum_t info;
          blkptr_t *bp = zio->io_bp;
          int error;
  
          ASSERT(zio->io_vd != NULL);
  
          if (bp == NULL) {
                  /*
                   * This is zio_read_phys().
                   * We're either verifying a label checksum, or nothing at all.
                   */
                  if (zio->io_prop.zp_checksum == ZIO_CHECKSUM_OFF)
                          return (zio);
  
                  ASSERT3U(zio->io_prop.zp_checksum, ==, ZIO_CHECKSUM_LABEL);
          }
  
          if ((error = zio_checksum_error(zio, &info)) != 0) {
                  zio->io_error = error;
                  if (error == ECKSUM &&
                      !(zio->io_flags & ZIO_FLAG_SPECULATIVE)) {
                          mutex_enter(&zio->io_vd->vdev_stat_lock);
                          zio->io_vd->vdev_stat.vs_checksum_errors++;
                          mutex_exit(&zio->io_vd->vdev_stat_lock);
                          (void) zfs_ereport_start_checksum(zio->io_spa,
                              zio->io_vd, &zio->io_bookmark, zio,
                              zio->io_offset, zio->io_size, &info);
                  }
          }
  
          return (zio);
  }
  
  /*
   * Called by RAID-Z to ensure we don't compute the checksum twice.
   */
  void
  zio_checksum_verified(zio_t *zio)
  {
          zio->io_pipeline &= ~ZIO_STAGE_CHECKSUM_VERIFY;
  }
  
  /*
   * ==========================================================================
   * Error rank.  Error are ranked in the order 0, ENXIO, ECKSUM, EIO, other.
   * An error of 0 indicates success.  ENXIO indicates whole-device failure,
   * which may be transient (e.g. unplugged) or permanent.  ECKSUM and EIO
   * indicate errors that are specific to one I/O, and most likely permanent.
   * Any other error is presumed to be worse because we weren't expecting it.
   * ==========================================================================
   */
  int
  zio_worst_error(int e1, int e2)
  {
          static int zio_error_rank[] = { 0, ENXIO, ECKSUM, EIO };
          int r1, r2;
  
          for (r1 = 0; r1 < sizeof (zio_error_rank) / sizeof (int); r1++)
                  if (e1 == zio_error_rank[r1])
                          break;
  
          for (r2 = 0; r2 < sizeof (zio_error_rank) / sizeof (int); r2++)
                  if (e2 == zio_error_rank[r2])
                          break;
  
          return (r1 > r2 ? e1 : e2);
  }
  
  /*
   * ==========================================================================
   * I/O completion
   * ==========================================================================
   */
  static zio_t *
  zio_ready(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
          zio_t *pio, *pio_next;
          zio_link_t *zl = NULL;
  
          if (zio_wait_for_children(zio, ZIO_CHILD_GANG_BIT | ZIO_CHILD_DDT_BIT,
              ZIO_WAIT_READY)) {
                  return (NULL);
          }
  
          if (zio->io_ready) {
                  ASSERT(IO_IS_ALLOCATING(zio));
                  ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp) ||
                      (zio->io_flags & ZIO_FLAG_NOPWRITE));
                  ASSERT(zio->io_children[ZIO_CHILD_GANG][ZIO_WAIT_READY] == 0);
  
                  zio->io_ready(zio);
          }
  
          if (bp != NULL && bp != &zio->io_bp_copy)
                  zio->io_bp_copy = *bp;
  
          if (zio->io_error != 0) {
                  zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
  
                  if (zio->io_flags & ZIO_FLAG_IO_ALLOCATING) {
                          ASSERT(IO_IS_ALLOCATING(zio));
                          ASSERT(zio->io_priority == ZIO_PRIORITY_ASYNC_WRITE);
                          ASSERT(zio->io_metaslab_class != NULL);
  
                          /*
                           * We were unable to allocate anything, unreserve and
                           * issue the next I/O to allocate.
                           */
                          metaslab_class_throttle_unreserve(
                              zio->io_metaslab_class, zio->io_prop.zp_copies,
                              zio->io_allocator, zio);
                          zio_allocate_dispatch(zio->io_spa, zio->io_allocator);
                  }
          }
  
          mutex_enter(&zio->io_lock);
          zio->io_state[ZIO_WAIT_READY] = 1;
          pio = zio_walk_parents(zio, &zl);
          mutex_exit(&zio->io_lock);
  
          /*
           * As we notify zio's parents, new parents could be added.
           * New parents go to the head of zio's io_parent_list, however,
           * so we will (correctly) not notify them.  The remainder of zio's
           * io_parent_list, from 'pio_next' onward, cannot change because
           * all parents must wait for us to be done before they can be done.
           */
          for (; pio != NULL; pio = pio_next) {
                  pio_next = zio_walk_parents(zio, &zl);
                  zio_notify_parent(pio, zio, ZIO_WAIT_READY, NULL);
          }
  
          if (zio->io_flags & ZIO_FLAG_NODATA) {
                  if (BP_IS_GANG(bp)) {
                          zio->io_flags &= ~ZIO_FLAG_NODATA;
                  } else {
                          ASSERT((uintptr_t)zio->io_abd < SPA_MAXBLOCKSIZE);
                          zio->io_pipeline &= ~ZIO_VDEV_IO_STAGES;
                  }
          }
  
          if (zio_injection_enabled &&
              zio->io_spa->spa_syncing_txg == zio->io_txg)
                  zio_handle_ignored_writes(zio);
  
          return (zio);
  }
  
  /*
   * Update the allocation throttle accounting.
   */
  static void
  zio_dva_throttle_done(zio_t *zio)
  {
          zio_t *lio __maybe_unused = zio->io_logical;
          zio_t *pio = zio_unique_parent(zio);
          vdev_t *vd = zio->io_vd;
          int flags = METASLAB_ASYNC_ALLOC;
  
          ASSERT3P(zio->io_bp, !=, NULL);
          ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
          ASSERT3U(zio->io_priority, ==, ZIO_PRIORITY_ASYNC_WRITE);
          ASSERT3U(zio->io_child_type, ==, ZIO_CHILD_VDEV);
          ASSERT(vd != NULL);
          ASSERT3P(vd, ==, vd->vdev_top);
          ASSERT(zio_injection_enabled || !(zio->io_flags & ZIO_FLAG_IO_RETRY));
          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REPAIR));
          ASSERT(zio->io_flags & ZIO_FLAG_IO_ALLOCATING);
          ASSERT(!(lio->io_flags & ZIO_FLAG_IO_REWRITE));
          ASSERT(!(lio->io_orig_flags & ZIO_FLAG_NODATA));
  
          /*
           * Parents of gang children can have two flavors -- ones that
           * allocated the gang header (will have ZIO_FLAG_IO_REWRITE set)
           * and ones that allocated the constituent blocks. The allocation
           * throttle needs to know the allocating parent zio so we must find
           * it here.
           */
          if (pio->io_child_type == ZIO_CHILD_GANG) {
                  /*
                   * If our parent is a rewrite gang child then our grandparent
                   * would have been the one that performed the allocation.
                   */
                  if (pio->io_flags & ZIO_FLAG_IO_REWRITE)
                          pio = zio_unique_parent(pio);
                  flags |= METASLAB_GANG_CHILD;
          }
  
          ASSERT(IO_IS_ALLOCATING(pio));
          ASSERT3P(zio, !=, zio->io_logical);
          ASSERT(zio->io_logical != NULL);
          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REPAIR));
          ASSERT0(zio->io_flags & ZIO_FLAG_NOPWRITE);
          ASSERT(zio->io_metaslab_class != NULL);
  
          mutex_enter(&pio->io_lock);
          metaslab_group_alloc_decrement(zio->io_spa, vd->vdev_id, pio, flags,
              pio->io_allocator, B_TRUE);
          mutex_exit(&pio->io_lock);
  
          metaslab_class_throttle_unreserve(zio->io_metaslab_class, 1,
              pio->io_allocator, pio);
  
          /*
           * Call into the pipeline to see if there is more work that
           * needs to be done. If there is work to be done it will be
           * dispatched to another taskq thread.
           */
          zio_allocate_dispatch(zio->io_spa, pio->io_allocator);
  }
  
  static zio_t *
  zio_done(zio_t *zio)
  {
          /*
           * Always attempt to keep stack usage minimal here since
           * we can be called recursively up to 19 levels deep.
           */
          const uint64_t psize = zio->io_size;
          zio_t *pio, *pio_next;
          zio_link_t *zl = NULL;
  
          /*
           * If our children haven't all completed,
           * wait for them and then repeat this pipeline stage.
           */
          if (zio_wait_for_children(zio, ZIO_CHILD_ALL_BITS, ZIO_WAIT_DONE)) {
                  return (NULL);
          }
  
          /*
           * If the allocation throttle is enabled, then update the accounting.
           * We only track child I/Os that are part of an allocating async
           * write. We must do this since the allocation is performed
           * by the logical I/O but the actual write is done by child I/Os.
           */
          if (zio->io_flags & ZIO_FLAG_IO_ALLOCATING &&
              zio->io_child_type == ZIO_CHILD_VDEV) {
                  ASSERT(zio->io_metaslab_class != NULL);
                  ASSERT(zio->io_metaslab_class->mc_alloc_throttle_enabled);
                  zio_dva_throttle_done(zio);
          }
  
          /*
           * If the allocation throttle is enabled, verify that
           * we have decremented the refcounts for every I/O that was throttled.
           */
          if (zio->io_flags & ZIO_FLAG_IO_ALLOCATING) {
                  ASSERT(zio->io_type == ZIO_TYPE_WRITE);
                  ASSERT(zio->io_priority == ZIO_PRIORITY_ASYNC_WRITE);
                  ASSERT(zio->io_bp != NULL);
  
                  metaslab_group_alloc_verify(zio->io_spa, zio->io_bp, zio,
                      zio->io_allocator);
                  VERIFY(zfs_refcount_not_held(&zio->io_metaslab_class->
                      mc_allocator[zio->io_allocator].mca_alloc_slots, zio));
          }
  
  
          for (int c = 0; c < ZIO_CHILD_TYPES; c++)
                  for (int w = 0; w < ZIO_WAIT_TYPES; w++)
                          ASSERT(zio->io_children[c][w] == 0);
  
          if (zio->io_bp != NULL && !BP_IS_EMBEDDED(zio->io_bp)) {
                  ASSERT(zio->io_bp->blk_pad[0] == 0);
                  ASSERT(zio->io_bp->blk_pad[1] == 0);
                  ASSERT(memcmp(zio->io_bp, &zio->io_bp_copy,
                      sizeof (blkptr_t)) == 0 ||
                      (zio->io_bp == zio_unique_parent(zio)->io_bp));
                  if (zio->io_type == ZIO_TYPE_WRITE && !BP_IS_HOLE(zio->io_bp) &&
                      zio->io_bp_override == NULL &&
                      !(zio->io_flags & ZIO_FLAG_IO_REPAIR)) {
                          ASSERT3U(zio->io_prop.zp_copies, <=,
                              BP_GET_NDVAS(zio->io_bp));
                          ASSERT(BP_COUNT_GANG(zio->io_bp) == 0 ||
                              (BP_COUNT_GANG(zio->io_bp) ==
                              BP_GET_NDVAS(zio->io_bp)));
                  }
                  if (zio->io_flags & ZIO_FLAG_NOPWRITE)
                          VERIFY(BP_EQUAL(zio->io_bp, &zio->io_bp_orig));
          }
  
          /*
           * If there were child vdev/gang/ddt errors, they apply to us now.
           */
          zio_inherit_child_errors(zio, ZIO_CHILD_VDEV);
          zio_inherit_child_errors(zio, ZIO_CHILD_GANG);
          zio_inherit_child_errors(zio, ZIO_CHILD_DDT);
  
          /*
           * If the I/O on the transformed data was successful, generate any
           * checksum reports now while we still have the transformed data.
           */
          if (zio->io_error == 0) {
                  while (zio->io_cksum_report != NULL) {
                          zio_cksum_report_t *zcr = zio->io_cksum_report;
                          uint64_t align = zcr->zcr_align;
                          uint64_t asize = P2ROUNDUP(psize, align);
                          abd_t *adata = zio->io_abd;
  
                          if (adata != NULL && asize != psize) {
                                  adata = abd_alloc(asize, B_TRUE);
                                  abd_copy(adata, zio->io_abd, psize);
                                  abd_zero_off(adata, psize, asize - psize);
                          }
  
                          zio->io_cksum_report = zcr->zcr_next;
                          zcr->zcr_next = NULL;
                          zcr->zcr_finish(zcr, adata);
                          zfs_ereport_free_checksum(zcr);
  
                          if (adata != NULL && asize != psize)
                                  abd_free(adata);
                  }
          }
  
          zio_pop_transforms(zio);        /* note: may set zio->io_error */
  
          vdev_stat_update(zio, psize);
  
          /*
           * If this I/O is attached to a particular vdev is slow, exceeding
           * 30 seconds to complete, post an error described the I/O delay.
           * We ignore these errors if the device is currently unavailable.
           */
          if (zio->io_delay >= MSEC2NSEC(zio_slow_io_ms)) {
                  if (zio->io_vd != NULL && !vdev_is_dead(zio->io_vd)) {
                          /*
                           * We want to only increment our slow IO counters if
                           * the IO is valid (i.e. not if the drive is removed).
                           *
                           * zfs_ereport_post() will also do these checks, but
                           * it can also ratelimit and have other failures, so we
                           * need to increment the slow_io counters independent
                           * of it.
                           */
                          if (zfs_ereport_is_valid(FM_EREPORT_ZFS_DELAY,
                              zio->io_spa, zio->io_vd, zio)) {
                                  mutex_enter(&zio->io_vd->vdev_stat_lock);
                                  zio->io_vd->vdev_stat.vs_slow_ios++;
                                  mutex_exit(&zio->io_vd->vdev_stat_lock);
  
                                  (void) zfs_ereport_post(FM_EREPORT_ZFS_DELAY,
                                      zio->io_spa, zio->io_vd, &zio->io_bookmark,
                                      zio, 0);
                          }
                  }
          }
  
          if (zio->io_error) {
                  /*
                   * If this I/O is attached to a particular vdev,
                   * generate an error message describing the I/O failure
                   * at the block level.  We ignore these errors if the
                   * device is currently unavailable.
                   */
                  if (zio->io_error != ECKSUM && zio->io_vd != NULL &&
                      !vdev_is_dead(zio->io_vd)) {
                          int ret = zfs_ereport_post(FM_EREPORT_ZFS_IO,
                              zio->io_spa, zio->io_vd, &zio->io_bookmark, zio, 0);
                          if (ret != EALREADY) {
                                  mutex_enter(&zio->io_vd->vdev_stat_lock);
                                  if (zio->io_type == ZIO_TYPE_READ)
                                          zio->io_vd->vdev_stat.vs_read_errors++;
                                  else if (zio->io_type == ZIO_TYPE_WRITE)
                                          zio->io_vd->vdev_stat.vs_write_errors++;
                                  mutex_exit(&zio->io_vd->vdev_stat_lock);
                          }
                  }
  
                  if ((zio->io_error == EIO || !(zio->io_flags &
                      (ZIO_FLAG_SPECULATIVE | ZIO_FLAG_DONT_PROPAGATE))) &&
                      zio == zio->io_logical) {
                          /*
                           * For logical I/O requests, tell the SPA to log the
                           * error and generate a logical data ereport.
                           */
                          spa_log_error(zio->io_spa, &zio->io_bookmark);
                          (void) zfs_ereport_post(FM_EREPORT_ZFS_DATA,
                              zio->io_spa, NULL, &zio->io_bookmark, zio, 0);
                  }
          }
  
          if (zio->io_error && zio == zio->io_logical) {
                  /*
                   * Determine whether zio should be reexecuted.  This will
                   * propagate all the way to the root via zio_notify_parent().
                   */
                  ASSERT(zio->io_vd == NULL && zio->io_bp != NULL);
                  ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
  
                  if (IO_IS_ALLOCATING(zio) &&
                      !(zio->io_flags & ZIO_FLAG_CANFAIL)) {
                          if (zio->io_error != ENOSPC)
                                  zio->io_reexecute |= ZIO_REEXECUTE_NOW;
                          else
                                  zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;
                  }
  
                  if ((zio->io_type == ZIO_TYPE_READ ||
                      zio->io_type == ZIO_TYPE_FREE) &&
                      !(zio->io_flags & ZIO_FLAG_SCAN_THREAD) &&
                      zio->io_error == ENXIO &&
                      spa_load_state(zio->io_spa) == SPA_LOAD_NONE &&
                      spa_get_failmode(zio->io_spa) != ZIO_FAILURE_MODE_CONTINUE)
                          zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;
  
                  if (!(zio->io_flags & ZIO_FLAG_CANFAIL) && !zio->io_reexecute)
                          zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;
  
                  /*
                   * Here is a possibly good place to attempt to do
                   * either combinatorial reconstruction or error correction
                   * based on checksums.  It also might be a good place
                   * to send out preliminary ereports before we suspend
                   * processing.
                   */
          }
  
          /*
           * If there were logical child errors, they apply to us now.
           * We defer this until now to avoid conflating logical child
           * errors with errors that happened to the zio itself when
           * updating vdev stats and reporting FMA events above.
           */
          zio_inherit_child_errors(zio, ZIO_CHILD_LOGICAL);
  
          if ((zio->io_error || zio->io_reexecute) &&
              IO_IS_ALLOCATING(zio) && zio->io_gang_leader == zio &&
              !(zio->io_flags & (ZIO_FLAG_IO_REWRITE | ZIO_FLAG_NOPWRITE)))
                  zio_dva_unallocate(zio, zio->io_gang_tree, zio->io_bp);
  
          zio_gang_tree_free(&zio->io_gang_tree);
  
          /*
           * Godfather I/Os should never suspend.
           */
          if ((zio->io_flags & ZIO_FLAG_GODFATHER) &&
              (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND))
                  zio->io_reexecute &= ~ZIO_REEXECUTE_SUSPEND;
  
          if (zio->io_reexecute) {
                  /*
                   * This is a logical I/O that wants to reexecute.
                   *
                   * Reexecute is top-down.  When an i/o fails, if it's not
                   * the root, it simply notifies its parent and sticks around.
                   * The parent, seeing that it still has children in zio_done(),
                   * does the same.  This percolates all the way up to the root.
                   * The root i/o will reexecute or suspend the entire tree.
                   *
                   * This approach ensures that zio_reexecute() honors
                   * all the original i/o dependency relationships, e.g.
                   * parents not executing until children are ready.
                   */
                  ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
  
                  zio->io_gang_leader = NULL;
  
                  mutex_enter(&zio->io_lock);
                  zio->io_state[ZIO_WAIT_DONE] = 1;
                  mutex_exit(&zio->io_lock);
  
                  /*
                   * "The Godfather" I/O monitors its children but is
                   * not a true parent to them. It will track them through
                   * the pipeline but severs its ties whenever they get into
                   * trouble (e.g. suspended). This allows "The Godfather"
                   * I/O to return status without blocking.
                   */
                  zl = NULL;
                  for (pio = zio_walk_parents(zio, &zl); pio != NULL;
                      pio = pio_next) {
                          zio_link_t *remove_zl = zl;
                          pio_next = zio_walk_parents(zio, &zl);
  
                          if ((pio->io_flags & ZIO_FLAG_GODFATHER) &&
                              (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND)) {
                                  zio_remove_child(pio, zio, remove_zl);
                                  /*
                                   * This is a rare code path, so we don't
                                   * bother with "next_to_execute".
                                   */
                                  zio_notify_parent(pio, zio, ZIO_WAIT_DONE,
                                      NULL);
                          }
                  }
  
                  if ((pio = zio_unique_parent(zio)) != NULL) {
                          /*
                           * We're not a root i/o, so there's nothing to do
                           * but notify our parent.  Don't propagate errors
                           * upward since we haven't permanently failed yet.
                           */
                          ASSERT(!(zio->io_flags & ZIO_FLAG_GODFATHER));
                          zio->io_flags |= ZIO_FLAG_DONT_PROPAGATE;
                          /*
                           * This is a rare code path, so we don't bother with
                           * "next_to_execute".
                           */
                          zio_notify_parent(pio, zio, ZIO_WAIT_DONE, NULL);
                  } else if (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND) {
                          /*
                           * We'd fail again if we reexecuted now, so suspend
                           * until conditions improve (e.g. device comes online).
                           */
                          zio_suspend(zio->io_spa, zio, ZIO_SUSPEND_IOERR);
                  } else {
                          /*
                           * Reexecution is potentially a huge amount of work.
                           * Hand it off to the otherwise-unused claim taskq.
                           */
                          ASSERT(taskq_empty_ent(&zio->io_tqent));
                          spa_taskq_dispatch_ent(zio->io_spa,
                              ZIO_TYPE_CLAIM, ZIO_TASKQ_ISSUE,
                              zio_reexecute, zio, 0, &zio->io_tqent);
                  }
                  return (NULL);
          }
  
          ASSERT(zio->io_child_count == 0);
          ASSERT(zio->io_reexecute == 0);
          ASSERT(zio->io_error == 0 || (zio->io_flags & ZIO_FLAG_CANFAIL));
  
          /*
           * Report any checksum errors, since the I/O is complete.
           */
          while (zio->io_cksum_report != NULL) {
                  zio_cksum_report_t *zcr = zio->io_cksum_report;
                  zio->io_cksum_report = zcr->zcr_next;
                  zcr->zcr_next = NULL;
                  zcr->zcr_finish(zcr, NULL);
                  zfs_ereport_free_checksum(zcr);
          }
  
          if (zio->io_flags & ZIO_FLAG_FASTWRITE && zio->io_bp &&
              !BP_IS_HOLE(zio->io_bp) && !BP_IS_EMBEDDED(zio->io_bp) &&
              !(zio->io_flags & ZIO_FLAG_NOPWRITE)) {
                  metaslab_fastwrite_unmark(zio->io_spa, zio->io_bp);
          }
  
          /*
           * It is the responsibility of the done callback to ensure that this
           * particular zio is no longer discoverable for adoption, and as
           * such, cannot acquire any new parents.
           */
          if (zio->io_done)
                  zio->io_done(zio);
  
          mutex_enter(&zio->io_lock);
          zio->io_state[ZIO_WAIT_DONE] = 1;
          mutex_exit(&zio->io_lock);
  
          /*
           * We are done executing this zio.  We may want to execute a parent
           * next.  See the comment in zio_notify_parent().
           */
          zio_t *next_to_execute = NULL;
          zl = NULL;
          for (pio = zio_walk_parents(zio, &zl); pio != NULL; pio = pio_next) {
                  zio_link_t *remove_zl = zl;
                  pio_next = zio_walk_parents(zio, &zl);
                  zio_remove_child(pio, zio, remove_zl);
                  zio_notify_parent(pio, zio, ZIO_WAIT_DONE, &next_to_execute);
          }
  
          if (zio->io_waiter != NULL) {
                  mutex_enter(&zio->io_lock);
                  zio->io_executor = NULL;
                  cv_broadcast(&zio->io_cv);
                  mutex_exit(&zio->io_lock);
          } else {
                  zio_destroy(zio);
          }
  
          return (next_to_execute);
  }
  
  /*
   * ==========================================================================
   * I/O pipeline definition
   * ==========================================================================
   */
  static zio_pipe_stage_t *zio_pipeline[] = {
          NULL,
          zio_read_bp_init,
          zio_write_bp_init,
          zio_free_bp_init,
          zio_issue_async,
          zio_write_compress,
          zio_encrypt,
          zio_checksum_generate,
          zio_nop_write,
          zio_ddt_read_start,
          zio_ddt_read_done,
          zio_ddt_write,
          zio_ddt_free,
          zio_gang_assemble,
          zio_gang_issue,
          zio_dva_throttle,
          zio_dva_allocate,
          zio_dva_free,
          zio_dva_claim,
          zio_ready,
          zio_vdev_io_start,
          zio_vdev_io_done,
          zio_vdev_io_assess,
          zio_checksum_verify,
          zio_done
  };
  
  
  
  
  /*
   * Compare two zbookmark_phys_t's to see which we would reach first in a
   * pre-order traversal of the object tree.
   *
   * This is simple in every case aside from the meta-dnode object. For all other
   * objects, we traverse them in order (object 1 before object 2, and so on).
   * However, all of these objects are traversed while traversing object 0, since
   * the data it points to is the list of objects.  Thus, we need to convert to a
   * canonical representation so we can compare meta-dnode bookmarks to
   * non-meta-dnode bookmarks.
   *
   * We do this by calculating "equivalents" for each field of the zbookmark.
   * zbookmarks outside of the meta-dnode use their own object and level, and
   * calculate the level 0 equivalent (the first L0 blkid that is contained in the
   * blocks this bookmark refers to) by multiplying their blkid by their span
   * (the number of L0 blocks contained within one block at their level).
   * zbookmarks inside the meta-dnode calculate their object equivalent
   * (which is L0equiv * dnodes per data block), use 0 for their L0equiv, and use
   * level + 1<<31 (any value larger than a level could ever be) for their level.
   * This causes them to always compare before a bookmark in their object
   * equivalent, compare appropriately to bookmarks in other objects, and to
   * compare appropriately to other bookmarks in the meta-dnode.
   */
  int
  zbookmark_compare(uint16_t dbss1, uint8_t ibs1, uint16_t dbss2, uint8_t ibs2,
      const zbookmark_phys_t *zb1, const zbookmark_phys_t *zb2)
  {
          /*
           * These variables represent the "equivalent" values for the zbookmark,
           * after converting zbookmarks inside the meta dnode to their
           * normal-object equivalents.
           */
          uint64_t zb1obj, zb2obj;
          uint64_t zb1L0, zb2L0;
          uint64_t zb1level, zb2level;
  
          if (zb1->zb_object == zb2->zb_object &&
              zb1->zb_level == zb2->zb_level &&
              zb1->zb_blkid == zb2->zb_blkid)
                  return (0);
  
          IMPLY(zb1->zb_level > 0, ibs1 >= SPA_MINBLOCKSHIFT);
          IMPLY(zb2->zb_level > 0, ibs2 >= SPA_MINBLOCKSHIFT);
  
          /*
           * BP_SPANB calculates the span in blocks.
           */
          zb1L0 = (zb1->zb_blkid) * BP_SPANB(ibs1, zb1->zb_level);
          zb2L0 = (zb2->zb_blkid) * BP_SPANB(ibs2, zb2->zb_level);
  
          if (zb1->zb_object == DMU_META_DNODE_OBJECT) {
                  zb1obj = zb1L0 * (dbss1 << (SPA_MINBLOCKSHIFT - DNODE_SHIFT));
                  zb1L0 = 0;
                  zb1level = zb1->zb_level + COMPARE_META_LEVEL;
          } else {
                  zb1obj = zb1->zb_object;
                  zb1level = zb1->zb_level;
          }
  
          if (zb2->zb_object == DMU_META_DNODE_OBJECT) {
                  zb2obj = zb2L0 * (dbss2 << (SPA_MINBLOCKSHIFT - DNODE_SHIFT));
                  zb2L0 = 0;
                  zb2level = zb2->zb_level + COMPARE_META_LEVEL;
          } else {
                  zb2obj = zb2->zb_object;
                  zb2level = zb2->zb_level;
          }
  
          /* Now that we have a canonical representation, do the comparison. */
          if (zb1obj != zb2obj)
                  return (zb1obj < zb2obj ? -1 : 1);
          else if (zb1L0 != zb2L0)
                  return (zb1L0 < zb2L0 ? -1 : 1);
          else if (zb1level != zb2level)
                  return (zb1level > zb2level ? -1 : 1);
          /*
           * This can (theoretically) happen if the bookmarks have the same object
           * and level, but different blkids, if the block sizes are not the same.
           * There is presently no way to change the indirect block sizes
           */
          return (0);
  }
  
  /*
   *  This function checks the following: given that last_block is the place that
   *  our traversal stopped last time, does that guarantee that we've visited
   *  every node under subtree_root?  Therefore, we can't just use the raw output
   *  of zbookmark_compare.  We have to pass in a modified version of
   *  subtree_root; by incrementing the block id, and then checking whether
   *  last_block is before or equal to that, we can tell whether or not having
   *  visited last_block implies that all of subtree_root's children have been
   *  visited.
   */
  boolean_t
  zbookmark_subtree_completed(const dnode_phys_t *dnp,
      const zbookmark_phys_t *subtree_root, const zbookmark_phys_t *last_block)
  {
          zbookmark_phys_t mod_zb = *subtree_root;
          mod_zb.zb_blkid++;
          ASSERT0(last_block->zb_level);
  
          /* The objset_phys_t isn't before anything. */
          if (dnp == NULL)
                  return (B_FALSE);
  
          /*
           * We pass in 1ULL << (DNODE_BLOCK_SHIFT - SPA_MINBLOCKSHIFT) for the
           * data block size in sectors, because that variable is only used if
           * the bookmark refers to a block in the meta-dnode.  Since we don't
           * know without examining it what object it refers to, and there's no
           * harm in passing in this value in other cases, we always pass it in.
           *
           * We pass in 0 for the indirect block size shift because zb2 must be
           * level 0.  The indirect block size is only used to calculate the span
           * of the bookmark, but since the bookmark must be level 0, the span is
           * always 1, so the math works out.
           *
           * If you make changes to how the zbookmark_compare code works, be sure
           * to make sure that this code still works afterwards.
           */
          return (zbookmark_compare(dnp->dn_datablkszsec, dnp->dn_indblkshift,
              1ULL << (DNODE_BLOCK_SHIFT - SPA_MINBLOCKSHIFT), 0, &mod_zb,
              last_block) <= 0);
  }
  
  /*
   * This function is similar to zbookmark_subtree_completed(), but returns true
   * if subtree_root is equal or ahead of last_block, i.e. still to be done.
   */
  boolean_t
  zbookmark_subtree_tbd(const dnode_phys_t *dnp,
      const zbookmark_phys_t *subtree_root, const zbookmark_phys_t *last_block)
  {
          ASSERT0(last_block->zb_level);
          if (dnp == NULL)
                  return (B_FALSE);
          return (zbookmark_compare(dnp->dn_datablkszsec, dnp->dn_indblkshift,
              1ULL << (DNODE_BLOCK_SHIFT - SPA_MINBLOCKSHIFT), 0, subtree_root,
              last_block) >= 0);
  }
  
  EXPORT_SYMBOL(zio_type_name);
  EXPORT_SYMBOL(zio_buf_alloc);
  EXPORT_SYMBOL(zio_data_buf_alloc);
  EXPORT_SYMBOL(zio_buf_free);
  EXPORT_SYMBOL(zio_data_buf_free);
  
  ZFS_MODULE_PARAM(zfs_zio, zio_, slow_io_ms, INT, ZMOD_RW,
          "Max I/O completion time (milliseconds) before marking it as slow");
  
  ZFS_MODULE_PARAM(zfs_zio, zio_, requeue_io_start_cut_in_line, INT, ZMOD_RW,
          "Prioritize requeued I/O");
  
  ZFS_MODULE_PARAM(zfs, zfs_, sync_pass_deferred_free,  UINT, ZMOD_RW,
          "Defer frees starting in this pass");
  
  ZFS_MODULE_PARAM(zfs, zfs_, sync_pass_dont_compress, UINT, ZMOD_RW,
          "Don't compress starting in this pass");
  
  ZFS_MODULE_PARAM(zfs, zfs_, sync_pass_rewrite, UINT, ZMOD_RW,
          "Rewrite new bps starting in this pass");
  
  ZFS_MODULE_PARAM(zfs_zio, zio_, dva_throttle_enabled, INT, ZMOD_RW,
          "Throttle block allocations in the ZIO pipeline");
  
  ZFS_MODULE_PARAM(zfs_zio, zio_, deadman_log_all, INT, ZMOD_RW,
          "Log all slow ZIOs, not just those with vdevs");