BRT should return EOPNOTSUPP

[zfs.git] / module / zfs / brt.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) 2020, 2021, 2022 by Pawel Jakub Dawidek
 */

#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/spa_impl.h>
#include <sys/zio.h>
#include <sys/brt.h>
#include <sys/ddt.h>
#include <sys/bitmap.h>
#include <sys/zap.h>
#include <sys/dmu_tx.h>
#include <sys/arc.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_scan.h>
#include <sys/vdev_impl.h>
#include <sys/kstat.h>
#include <sys/wmsum.h>

/*
 * Block Cloning design.
 *
 * Block Cloning allows to manually clone a file (or a subset of its blocks)
 * into another (or the same) file by just creating additional references to
 * the data blocks without copying the data itself. Those references are kept
 * in the Block Reference Tables (BRTs).
 *
 * In many ways this is similar to the existing deduplication, but there are
 * some important differences:
 *
 * - Deduplication is automatic and Block Cloning is not - one has to use a
 *   dedicated system call(s) to clone the given file/blocks.
 * - Deduplication keeps all data blocks in its table, even those referenced
 *   just once. Block Cloning creates an entry in its tables only when there
 *   are at least two references to the given data block. If the block was
 *   never explicitly cloned or the second to last reference was dropped,
 *   there will be neither space nor performance overhead.
 * - Deduplication needs data to work - one needs to pass real data to the
 *   write(2) syscall, so hash can be calculated. Block Cloning doesn't require
 *   data, just block pointers to the data, so it is extremely fast, as we pay
 *   neither the cost of reading the data, nor the cost of writing the data -
 *   we operate exclusively on metadata.
 * - If the D (dedup) bit is not set in the block pointer, it means that
 *   the block is not in the dedup table (DDT) and we won't consult the DDT
 *   when we need to free the block. Block Cloning must be consulted on every
 *   free, because we cannot modify the source BP (eg. by setting something
 *   similar to the D bit), thus we have no hint if the block is in the
 *   Block Reference Table (BRT), so we need to look into the BRT. There is
 *   an optimization in place that allows us to eliminate the majority of BRT
 *   lookups which is described below in the "Minimizing free penalty" section.
 * - The BRT entry is much smaller than the DDT entry - for BRT we only store
 *   64bit offset and 64bit reference counter.
 * - Dedup keys are cryptographic hashes, so two blocks that are close to each
 *   other on disk are most likely in totally different parts of the DDT.
 *   The BRT entry keys are offsets into a single top-level VDEV, so data blocks
 *   from one file should have BRT entries close to each other.
 * - Scrub will only do a single pass over a block that is referenced multiple
 *   times in the DDT. Unfortunately it is not currently (if at all) possible
 *   with Block Cloning and block referenced multiple times will be scrubbed
 *   multiple times. The new, sorted scrub should be able to eliminate
 *   duplicated reads given enough memory.
 * - Deduplication requires cryptographically strong hash as a checksum or
 *   additional data verification. Block Cloning works with any checksum
 *   algorithm or even with checksumming disabled.
 *
 * As mentioned above, the BRT entries are much smaller than the DDT entries.
 * To uniquely identify a block we just need its vdev id and offset. We also
 * need to maintain a reference counter. The vdev id will often repeat, as there
 * is a small number of top-level VDEVs and a large number of blocks stored in
 * each VDEV. We take advantage of that to reduce the BRT entry size further by
 * maintaining one BRT for each top-level VDEV, so we can then have only offset
 * and counter as the BRT entry.
 *
 * Minimizing free penalty.
 *
 * Block Cloning allows creating additional references to any existing block.
 * When we free a block there is no hint in the block pointer whether the block
 * was cloned or not, so on each free we have to check if there is a
 * corresponding entry in the BRT or not. If there is, we need to decrease
 * the reference counter. Doing BRT lookup on every free can potentially be
 * expensive by requiring additional I/Os if the BRT doesn't fit into memory.
 * This is the main problem with deduplication, so we've learned our lesson and
 * try not to repeat the same mistake here. How do we do that? We divide each
 * top-level VDEV into 16MB regions. For each region we maintain a counter that
 * is a sum of all the BRT entries that have offsets within the region. This
 * creates the entries count array of 16bit numbers for each top-level VDEV.
 * The entries count array is always kept in memory and updated on disk in the
 * same transaction group as the BRT updates to keep everything in-sync. We can
 * keep the array in memory, because it is very small. With 16MB regions and
 * 1TB VDEV the array requires only 128kB of memory (we may decide to decrease
 * the region size even further in the future). Now, when we want to free
 * a block, we first consult the array. If the counter for the whole region is
 * zero, there is no need to look for the BRT entry, as there isn't one for
 * sure. If the counter for the region is greater than zero, only then we will
 * do a BRT lookup and if an entry is found we will decrease the reference
 * counter in the BRT entry and in the entry counters array.
 *
 * The entry counters array is small, but can potentially be larger for very
 * large VDEVs or smaller regions. In this case we don't want to rewrite entire
 * array on every change. We then divide the array into 32kB block and keep
 * a bitmap of dirty blocks within a transaction group. When we sync the
 * transaction group we can only update the parts of the entry counters array
 * that were modified. Note: Keeping track of the dirty parts of the entry
 * counters array is implemented, but updating only parts of the array on disk
 * is not yet implemented - for now we will update entire array if there was
 * any change.
 *
 * The implementation tries to be economic: if BRT is not used, or no longer
 * used, there will be no entries in the MOS and no additional memory used (eg.
 * the entry counters array is only allocated if needed).
 *
 * Interaction between Deduplication and Block Cloning.
 *
 * If both functionalities are in use, we could end up with a block that is
 * referenced multiple times in both DDT and BRT. When we free one of the
 * references we couldn't tell where it belongs, so we would have to decide
 * what table takes the precedence: do we first clear DDT references or BRT
 * references? To avoid this dilemma BRT cooperates with DDT - if a given block
 * is being cloned using BRT and the BP has the D (dedup) bit set, BRT will
 * lookup DDT entry instead and increase the counter there. No BRT entry
 * will be created for a block which has the D (dedup) bit set.
 * BRT may be more efficient for manual deduplication, but if the block is
 * already in the DDT, then creating additional BRT entry would be less
 * efficient. This clever idea was proposed by Allan Jude.
 *
 * Block Cloning across datasets.
 *
 * Block Cloning is not limited to cloning blocks within the same dataset.
 * It is possible (and very useful) to clone blocks between different datasets.
 * One use case is recovering files from snapshots. By cloning the files into
 * dataset we need no additional storage. Without Block Cloning we would need
 * additional space for those files.
 * Another interesting use case is moving the files between datasets
 * (copying the file content to the new dataset and removing the source file).
 * In that case Block Cloning will only be used briefly, because the BRT entries
 * will be removed when the source is removed.
 * Note: currently it is not possible to clone blocks between encrypted
 * datasets, even if those datasets use the same encryption key (this includes
 * snapshots of encrypted datasets). Cloning blocks between datasets that use
 * the same keys should be possible and should be implemented in the future.
 *
 * Block Cloning flow through ZFS layers.
 *
 * Note: Block Cloning can be used both for cloning file system blocks and ZVOL
 * blocks. As of this writing no interface is implemented that allows for block
 * cloning within a ZVOL.
 * FreeBSD and Linux provides copy_file_range(2) system call and we will use it
 * for blocking cloning.
 *
 *      ssize_t
 *      copy_file_range(int infd, off_t *inoffp, int outfd, off_t *outoffp,
 *                      size_t len, unsigned int flags);
 *
 * Even though offsets and length represent bytes, they have to be
 * block-aligned or we will return the EXDEV error so the upper layer can
 * fallback to the generic mechanism that will just copy the data.
 * Using copy_file_range(2) will call OS-independent zfs_clone_range() function.
 * This function was implemented based on zfs_write(), but instead of writing
 * the given data we first read block pointers using the new dmu_read_l0_bps()
 * function from the source file. Once we have BPs from the source file we call
 * the dmu_brt_clone() function on the destination file. This function
 * allocates BPs for us. We iterate over all source BPs. If the given BP is
 * a hole or an embedded block, we just copy BP as-is. If it points to a real
 * data we place this BP on a BRT pending list using the brt_pending_add()
 * function.
 *
 * We use this pending list to keep track of all BPs that got new references
 * within this transaction group.
 *
 * Some special cases to consider and how we address them:
 * - The block we want to clone may have been created within the same
 *   transaction group that we are trying to clone. Such block has no BP
 *   allocated yet, so cannot be immediately cloned. We return EXDEV.
 * - The block we want to clone may have been modified within the same
 *   transaction group. We return EXDEV.
 * - A block may be cloned multiple times during one transaction group (that's
 *   why pending list is actually a tree and not an append-only list - this
 *   way we can figure out faster if this block is cloned for the first time
 *   in this txg or consecutive time).
 * - A block may be cloned and freed within the same transaction group
 *   (see dbuf_undirty()).
 * - A block may be cloned and within the same transaction group the clone
 *   can be cloned again (see dmu_read_l0_bps()).
 * - A file might have been deleted, but the caller still has a file descriptor
 *   open to this file and clones it.
 *
 * When we free a block we have an additional step in the ZIO pipeline where we
 * call the zio_brt_free() function. We then call the brt_entry_decref()
 * that loads the corresponding BRT entry (if one exists) and decreases
 * reference counter. If this is not the last reference we will stop ZIO
 * pipeline here. If this is the last reference or the block is not in the
 * BRT, we continue the pipeline and free the block as usual.
 *
 * At the beginning of spa_sync() where there can be no more block cloning,
 * but before issuing frees we call brt_pending_apply(). This function applies
 * all the new clones to the BRT table - we load BRT entries and update
 * reference counters. To sync new BRT entries to disk, we use brt_sync()
 * function. This function will sync all dirty per-top-level-vdev BRTs,
 * the entry counters arrays, etc.
 *
 * Block Cloning and ZIL.
 *
 * Every clone operation is divided into chunks (similar to write) and each
 * chunk is cloned in a separate transaction. The chunk size is determined by
 * how many BPs we can fit into a single ZIL entry.
 * Replaying clone operation is different from the regular clone operation,
 * as when we log clone operations we cannot use the source object - it may
 * reside on a different dataset, so we log BPs we want to clone.
 * The ZIL is replayed when we mount the given dataset, not when the pool is
 * imported. Taking this into account it is possible that the pool is imported
 * without mounting datasets and the source dataset is destroyed before the
 * destination dataset is mounted and its ZIL replayed.
 * To address this situation we leverage zil_claim() mechanism where ZFS will
 * parse all the ZILs on pool import. When we come across TX_CLONE_RANGE
 * entries, we will bump reference counters for their BPs in the BRT and then
 * on mount and ZIL replay we will just attach BPs to the file without
 * bumping reference counters.
 * Note it is still possible that after zil_claim() we never mount the
 * destination, so we never replay its ZIL and we destroy it. This way we would
 * end up with leaked references in BRT. We address that too as ZFS gives us
 * a chance to clean this up on dataset destroy (see zil_free_clone_range()).
 */

/*
 * BRT - Block Reference Table.
 */
#define BRT_OBJECT_VDEV_PREFIX  "com.fudosecurity:brt:vdev:"

/*
 * We divide each VDEV into 16MB chunks. Each chunk is represented in memory
 * by a 16bit counter, thus 1TB VDEV requires 128kB of memory: (1TB / 16MB) * 2B
 * Each element in this array represents how many BRT entries do we have in this
 * chunk of storage. We always load this entire array into memory and update as
 * needed. By having it in memory we can quickly tell (during zio_free()) if
 * there are any BRT entries that we might need to update.
 *
 * This value cannot be larger than 16MB, at least as long as we support
 * 512 byte block sizes. With 512 byte block size we can have exactly
 * 32768 blocks in 16MB. In 32MB we could have 65536 blocks, which is one too
 * many for a 16bit counter.
 */
#define BRT_RANGESIZE   (16 * 1024 * 1024)
_Static_assert(BRT_RANGESIZE / SPA_MINBLOCKSIZE <= UINT16_MAX,
        "BRT_RANGESIZE is too large.");
/*
 * We don't want to update the whole structure every time. Maintain bitmap
 * of dirty blocks within the regions, so that a single bit represents a
 * block size of entcounts. For example if we have a 1PB vdev then all
 * entcounts take 128MB of memory ((64TB / 16MB) * 2B). We can divide this
 * 128MB array of entcounts into 32kB disk blocks, as we don't want to update
 * the whole 128MB on disk when we have updated only a single entcount.
 * We maintain a bitmap where each 32kB disk block within 128MB entcounts array
 * is represented by a single bit. This gives us 4096 bits. A set bit in the
 * bitmap means that we had a change in at least one of the 16384 entcounts
 * that reside on a 32kB disk block (32kB / sizeof (uint16_t)).
 */
#define BRT_BLOCKSIZE   (32 * 1024)
#define BRT_RANGESIZE_TO_NBLOCKS(size)                                  \
        (((size) - 1) / BRT_BLOCKSIZE / sizeof (uint16_t) + 1)

#define BRT_LITTLE_ENDIAN       0
#define BRT_BIG_ENDIAN          1
#ifdef _ZFS_LITTLE_ENDIAN
#define BRT_NATIVE_BYTEORDER            BRT_LITTLE_ENDIAN
#define BRT_NON_NATIVE_BYTEORDER        BRT_BIG_ENDIAN
#else
#define BRT_NATIVE_BYTEORDER            BRT_BIG_ENDIAN
#define BRT_NON_NATIVE_BYTEORDER        BRT_LITTLE_ENDIAN
#endif

typedef struct brt_vdev_phys {
        uint64_t        bvp_mos_entries;
        uint64_t        bvp_size;
        uint64_t        bvp_byteorder;
        uint64_t        bvp_totalcount;
        uint64_t        bvp_rangesize;
        uint64_t        bvp_usedspace;
        uint64_t        bvp_savedspace;
} brt_vdev_phys_t;

typedef struct brt_vdev {
        /*
         * VDEV id.
         */
        uint64_t        bv_vdevid;
        /*
         * Is the structure initiated?
         * (bv_entcount and bv_bitmap are allocated?)
         */
        boolean_t       bv_initiated;
        /*
         * Object number in the MOS for the entcount array and brt_vdev_phys.
         */
        uint64_t        bv_mos_brtvdev;
        /*
         * Object number in the MOS for the entries table.
         */
        uint64_t        bv_mos_entries;
        /*
         * Entries to sync.
         */
        avl_tree_t      bv_tree;
        /*
         * Does the bv_entcount[] array needs byte swapping?
         */
        boolean_t       bv_need_byteswap;
        /*
         * Number of entries in the bv_entcount[] array.
         */
        uint64_t        bv_size;
        /*
         * This is the array with BRT entry count per BRT_RANGESIZE.
         */
        uint16_t        *bv_entcount;
        /*
         * Sum of all bv_entcount[]s.
         */
        uint64_t        bv_totalcount;
        /*
         * Space on disk occupied by cloned blocks (without compression).
         */
        uint64_t        bv_usedspace;
        /*
         * How much additional space would be occupied without block cloning.
         */
        uint64_t        bv_savedspace;
        /*
         * brt_vdev_phys needs updating on disk.
         */
        boolean_t       bv_meta_dirty;
        /*
         * bv_entcount[] needs updating on disk.
         */
        boolean_t       bv_entcount_dirty;
        /*
         * bv_entcount[] potentially can be a bit too big to sychronize it all
         * when we just changed few entcounts. The fields below allow us to
         * track updates to bv_entcount[] array since the last sync.
         * A single bit in the bv_bitmap represents as many entcounts as can
         * fit into a single BRT_BLOCKSIZE.
         * For example we have 65536 entcounts in the bv_entcount array
         * (so the whole array is 128kB). We updated bv_entcount[2] and
         * bv_entcount[5]. In that case only first bit in the bv_bitmap will
         * be set and we will write only first BRT_BLOCKSIZE out of 128kB.
         */
        ulong_t         *bv_bitmap;
        uint64_t        bv_nblocks;
} brt_vdev_t;

/*
 * In-core brt
 */
typedef struct brt {
        krwlock_t       brt_lock;
        spa_t           *brt_spa;
#define brt_mos         brt_spa->spa_meta_objset
        uint64_t        brt_rangesize;
        uint64_t        brt_usedspace;
        uint64_t        brt_savedspace;
        avl_tree_t      brt_pending_tree[TXG_SIZE];
        kmutex_t        brt_pending_lock[TXG_SIZE];
        /* Sum of all entries across all bv_trees. */
        uint64_t        brt_nentries;
        brt_vdev_t      *brt_vdevs;
        uint64_t        brt_nvdevs;
} brt_t;

/* Size of bre_offset / sizeof (uint64_t). */
#define BRT_KEY_WORDS   (1)

/*
 * In-core brt entry.
 * On-disk we use bre_offset as the key and bre_refcount as the value.
 */
typedef struct brt_entry {
        uint64_t        bre_offset;
        uint64_t        bre_refcount;
        avl_node_t      bre_node;
} brt_entry_t;

typedef struct brt_pending_entry {
        blkptr_t        bpe_bp;
        int             bpe_count;
        avl_node_t      bpe_node;
} brt_pending_entry_t;

static kmem_cache_t *brt_entry_cache;
static kmem_cache_t *brt_pending_entry_cache;

/*
 * Enable/disable prefetching of BRT entries that we are going to modify.
 */
int zfs_brt_prefetch = 1;

#ifdef ZFS_DEBUG
#define BRT_DEBUG(...)  do {                                            \
        if ((zfs_flags & ZFS_DEBUG_BRT) != 0) {                         \
                __dprintf(B_TRUE, __FILE__, __func__, __LINE__, __VA_ARGS__); \
        }                                                               \
} while (0)
#else
#define BRT_DEBUG(...)  do { } while (0)
#endif

int brt_zap_leaf_blockshift = 12;
int brt_zap_indirect_blockshift = 12;

static kstat_t  *brt_ksp;

typedef struct brt_stats {
        kstat_named_t brt_addref_entry_in_memory;
        kstat_named_t brt_addref_entry_not_on_disk;
        kstat_named_t brt_addref_entry_on_disk;
        kstat_named_t brt_addref_entry_read_lost_race;
        kstat_named_t brt_decref_entry_in_memory;
        kstat_named_t brt_decref_entry_loaded_from_disk;
        kstat_named_t brt_decref_entry_not_in_memory;
        kstat_named_t brt_decref_entry_not_on_disk;
        kstat_named_t brt_decref_entry_read_lost_race;
        kstat_named_t brt_decref_entry_still_referenced;
        kstat_named_t brt_decref_free_data_later;
        kstat_named_t brt_decref_free_data_now;
        kstat_named_t brt_decref_no_entry;
} brt_stats_t;

static brt_stats_t brt_stats = {
        { "addref_entry_in_memory",             KSTAT_DATA_UINT64 },
        { "addref_entry_not_on_disk",           KSTAT_DATA_UINT64 },
        { "addref_entry_on_disk",               KSTAT_DATA_UINT64 },
        { "addref_entry_read_lost_race",        KSTAT_DATA_UINT64 },
        { "decref_entry_in_memory",             KSTAT_DATA_UINT64 },
        { "decref_entry_loaded_from_disk",      KSTAT_DATA_UINT64 },
        { "decref_entry_not_in_memory",         KSTAT_DATA_UINT64 },
        { "decref_entry_not_on_disk",           KSTAT_DATA_UINT64 },
        { "decref_entry_read_lost_race",        KSTAT_DATA_UINT64 },
        { "decref_entry_still_referenced",      KSTAT_DATA_UINT64 },
        { "decref_free_data_later",             KSTAT_DATA_UINT64 },
        { "decref_free_data_now",               KSTAT_DATA_UINT64 },
        { "decref_no_entry",                    KSTAT_DATA_UINT64 }
};

struct {
        wmsum_t brt_addref_entry_in_memory;
        wmsum_t brt_addref_entry_not_on_disk;
        wmsum_t brt_addref_entry_on_disk;
        wmsum_t brt_addref_entry_read_lost_race;
        wmsum_t brt_decref_entry_in_memory;
        wmsum_t brt_decref_entry_loaded_from_disk;
        wmsum_t brt_decref_entry_not_in_memory;
        wmsum_t brt_decref_entry_not_on_disk;
        wmsum_t brt_decref_entry_read_lost_race;
        wmsum_t brt_decref_entry_still_referenced;
        wmsum_t brt_decref_free_data_later;
        wmsum_t brt_decref_free_data_now;
        wmsum_t brt_decref_no_entry;
} brt_sums;

#define BRTSTAT_BUMP(stat)      wmsum_add(&brt_sums.stat, 1)

static int brt_entry_compare(const void *x1, const void *x2);
static int brt_pending_entry_compare(const void *x1, const void *x2);

static void
brt_rlock(brt_t *brt)
{
        rw_enter(&brt->brt_lock, RW_READER);
}

static void
brt_wlock(brt_t *brt)
{
        rw_enter(&brt->brt_lock, RW_WRITER);
}

static void
brt_unlock(brt_t *brt)
{
        rw_exit(&brt->brt_lock);
}

static uint16_t
brt_vdev_entcount_get(const brt_vdev_t *brtvd, uint64_t idx)
{

        ASSERT3U(idx, <, brtvd->bv_size);

        if (brtvd->bv_need_byteswap) {
                return (BSWAP_16(brtvd->bv_entcount[idx]));
        } else {
                return (brtvd->bv_entcount[idx]);
        }
}

static void
brt_vdev_entcount_set(brt_vdev_t *brtvd, uint64_t idx, uint16_t entcnt)
{

        ASSERT3U(idx, <, brtvd->bv_size);

        if (brtvd->bv_need_byteswap) {
                brtvd->bv_entcount[idx] = BSWAP_16(entcnt);
        } else {
                brtvd->bv_entcount[idx] = entcnt;
        }
}

static void
brt_vdev_entcount_inc(brt_vdev_t *brtvd, uint64_t idx)
{
        uint16_t entcnt;

        ASSERT3U(idx, <, brtvd->bv_size);

        entcnt = brt_vdev_entcount_get(brtvd, idx);
        ASSERT(entcnt < UINT16_MAX);

        brt_vdev_entcount_set(brtvd, idx, entcnt + 1);
}

static void
brt_vdev_entcount_dec(brt_vdev_t *brtvd, uint64_t idx)
{
        uint16_t entcnt;

        ASSERT3U(idx, <, brtvd->bv_size);

        entcnt = brt_vdev_entcount_get(brtvd, idx);
        ASSERT(entcnt > 0);

        brt_vdev_entcount_set(brtvd, idx, entcnt - 1);
}

#ifdef ZFS_DEBUG
static void
brt_vdev_dump(brt_t *brt)
{
        brt_vdev_t *brtvd;
        uint64_t vdevid;

        if ((zfs_flags & ZFS_DEBUG_BRT) == 0) {
                return;
        }

        if (brt->brt_nvdevs == 0) {
                zfs_dbgmsg("BRT empty");
                return;
        }

        zfs_dbgmsg("BRT vdev dump:");
        for (vdevid = 0; vdevid < brt->brt_nvdevs; vdevid++) {
                uint64_t idx;

                brtvd = &brt->brt_vdevs[vdevid];
                zfs_dbgmsg("  vdevid=%llu/%llu meta_dirty=%d entcount_dirty=%d "
                    "size=%llu totalcount=%llu nblocks=%llu bitmapsize=%zu\n",
                    (u_longlong_t)vdevid, (u_longlong_t)brtvd->bv_vdevid,
                    brtvd->bv_meta_dirty, brtvd->bv_entcount_dirty,
                    (u_longlong_t)brtvd->bv_size,
                    (u_longlong_t)brtvd->bv_totalcount,
                    (u_longlong_t)brtvd->bv_nblocks,
                    (size_t)BT_SIZEOFMAP(brtvd->bv_nblocks));
                if (brtvd->bv_totalcount > 0) {
                        zfs_dbgmsg("    entcounts:");
                        for (idx = 0; idx < brtvd->bv_size; idx++) {
                                if (brt_vdev_entcount_get(brtvd, idx) > 0) {
                                        zfs_dbgmsg("      [%04llu] %hu",
                                            (u_longlong_t)idx,
                                            brt_vdev_entcount_get(brtvd, idx));
                                }
                        }
                }
                if (brtvd->bv_entcount_dirty) {
                        char *bitmap;

                        bitmap = kmem_alloc(brtvd->bv_nblocks + 1, KM_SLEEP);
                        for (idx = 0; idx < brtvd->bv_nblocks; idx++) {
                                bitmap[idx] =
                                    BT_TEST(brtvd->bv_bitmap, idx) ? 'x' : '.';
                        }
                        bitmap[idx] = '\0';
                        zfs_dbgmsg("    bitmap: %s", bitmap);
                        kmem_free(bitmap, brtvd->bv_nblocks + 1);
                }
        }
}
#endif

static brt_vdev_t *
brt_vdev(brt_t *brt, uint64_t vdevid)
{
        brt_vdev_t *brtvd;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));

        if (vdevid < brt->brt_nvdevs) {
                brtvd = &brt->brt_vdevs[vdevid];
        } else {
                brtvd = NULL;
        }

        return (brtvd);
}

static void
brt_vdev_create(brt_t *brt, brt_vdev_t *brtvd, dmu_tx_t *tx)
{
        char name[64];

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT0(brtvd->bv_mos_brtvdev);
        ASSERT0(brtvd->bv_mos_entries);
        ASSERT(brtvd->bv_entcount != NULL);
        ASSERT(brtvd->bv_size > 0);
        ASSERT(brtvd->bv_bitmap != NULL);
        ASSERT(brtvd->bv_nblocks > 0);

        brtvd->bv_mos_entries = zap_create_flags(brt->brt_mos, 0,
            ZAP_FLAG_HASH64 | ZAP_FLAG_UINT64_KEY, DMU_OTN_ZAP_METADATA,
            brt_zap_leaf_blockshift, brt_zap_indirect_blockshift, DMU_OT_NONE,
            0, tx);
        VERIFY(brtvd->bv_mos_entries != 0);
        BRT_DEBUG("MOS entries created, object=%llu",
            (u_longlong_t)brtvd->bv_mos_entries);

        /*
         * We allocate DMU buffer to store the bv_entcount[] array.
         * We will keep array size (bv_size) and cummulative count for all
         * bv_entcount[]s (bv_totalcount) in the bonus buffer.
         */
        brtvd->bv_mos_brtvdev = dmu_object_alloc(brt->brt_mos,
            DMU_OTN_UINT64_METADATA, BRT_BLOCKSIZE,
            DMU_OTN_UINT64_METADATA, sizeof (brt_vdev_phys_t), tx);
        VERIFY(brtvd->bv_mos_brtvdev != 0);
        BRT_DEBUG("MOS BRT VDEV created, object=%llu",
            (u_longlong_t)brtvd->bv_mos_brtvdev);

        snprintf(name, sizeof (name), "%s%llu", BRT_OBJECT_VDEV_PREFIX,
            (u_longlong_t)brtvd->bv_vdevid);
        VERIFY0(zap_add(brt->brt_mos, DMU_POOL_DIRECTORY_OBJECT, name,
            sizeof (uint64_t), 1, &brtvd->bv_mos_brtvdev, tx));
        BRT_DEBUG("Pool directory object created, object=%s", name);

        spa_feature_incr(brt->brt_spa, SPA_FEATURE_BLOCK_CLONING, tx);
}

static void
brt_vdev_realloc(brt_t *brt, brt_vdev_t *brtvd)
{
        vdev_t *vd;
        uint16_t *entcount;
        ulong_t *bitmap;
        uint64_t nblocks, size;

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));

        spa_config_enter(brt->brt_spa, SCL_VDEV, FTAG, RW_READER);
        vd = vdev_lookup_top(brt->brt_spa, brtvd->bv_vdevid);
        size = (vdev_get_min_asize(vd) - 1) / brt->brt_rangesize + 1;
        spa_config_exit(brt->brt_spa, SCL_VDEV, FTAG);

        entcount = vmem_zalloc(sizeof (entcount[0]) * size, KM_SLEEP);
        nblocks = BRT_RANGESIZE_TO_NBLOCKS(size);
        bitmap = kmem_zalloc(BT_SIZEOFMAP(nblocks), KM_SLEEP);

        if (!brtvd->bv_initiated) {
                ASSERT0(brtvd->bv_size);
                ASSERT(brtvd->bv_entcount == NULL);
                ASSERT(brtvd->bv_bitmap == NULL);
                ASSERT0(brtvd->bv_nblocks);

                avl_create(&brtvd->bv_tree, brt_entry_compare,
                    sizeof (brt_entry_t), offsetof(brt_entry_t, bre_node));
        } else {
                ASSERT(brtvd->bv_size > 0);
                ASSERT(brtvd->bv_entcount != NULL);
                ASSERT(brtvd->bv_bitmap != NULL);
                ASSERT(brtvd->bv_nblocks > 0);
                /*
                 * TODO: Allow vdev shrinking. We only need to implement
                 * shrinking the on-disk BRT VDEV object.
                 * dmu_free_range(brt->brt_mos, brtvd->bv_mos_brtvdev, offset,
                 *     size, tx);
                 */
                ASSERT3U(brtvd->bv_size, <=, size);

                memcpy(entcount, brtvd->bv_entcount,
                    sizeof (entcount[0]) * MIN(size, brtvd->bv_size));
                memcpy(bitmap, brtvd->bv_bitmap, MIN(BT_SIZEOFMAP(nblocks),
                    BT_SIZEOFMAP(brtvd->bv_nblocks)));
                vmem_free(brtvd->bv_entcount,
                    sizeof (entcount[0]) * brtvd->bv_size);
                kmem_free(brtvd->bv_bitmap, BT_SIZEOFMAP(brtvd->bv_nblocks));
        }

        brtvd->bv_size = size;
        brtvd->bv_entcount = entcount;
        brtvd->bv_bitmap = bitmap;
        brtvd->bv_nblocks = nblocks;
        if (!brtvd->bv_initiated) {
                brtvd->bv_need_byteswap = FALSE;
                brtvd->bv_initiated = TRUE;
                BRT_DEBUG("BRT VDEV %llu initiated.",
                    (u_longlong_t)brtvd->bv_vdevid);
        }
}

static void
brt_vdev_load(brt_t *brt, brt_vdev_t *brtvd)
{
        char name[64];
        dmu_buf_t *db;
        brt_vdev_phys_t *bvphys;
        int error;

        snprintf(name, sizeof (name), "%s%llu", BRT_OBJECT_VDEV_PREFIX,
            (u_longlong_t)brtvd->bv_vdevid);
        error = zap_lookup(brt->brt_mos, DMU_POOL_DIRECTORY_OBJECT, name,
            sizeof (uint64_t), 1, &brtvd->bv_mos_brtvdev);
        if (error != 0)
                return;
        ASSERT(brtvd->bv_mos_brtvdev != 0);

        error = dmu_bonus_hold(brt->brt_mos, brtvd->bv_mos_brtvdev, FTAG, &db);
        ASSERT0(error);
        if (error != 0)
                return;

        bvphys = db->db_data;
        if (brt->brt_rangesize == 0) {
                brt->brt_rangesize = bvphys->bvp_rangesize;
        } else {
                ASSERT3U(brt->brt_rangesize, ==, bvphys->bvp_rangesize);
        }

        ASSERT(!brtvd->bv_initiated);
        brt_vdev_realloc(brt, brtvd);

        /* TODO: We don't support VDEV shrinking. */
        ASSERT3U(bvphys->bvp_size, <=, brtvd->bv_size);

        /*
         * If VDEV grew, we will leave new bv_entcount[] entries zeroed out.
         */
        error = dmu_read(brt->brt_mos, brtvd->bv_mos_brtvdev, 0,
            MIN(brtvd->bv_size, bvphys->bvp_size) * sizeof (uint16_t),
            brtvd->bv_entcount, DMU_READ_NO_PREFETCH);
        ASSERT0(error);

        brtvd->bv_mos_entries = bvphys->bvp_mos_entries;
        ASSERT(brtvd->bv_mos_entries != 0);
        brtvd->bv_need_byteswap =
            (bvphys->bvp_byteorder != BRT_NATIVE_BYTEORDER);
        brtvd->bv_totalcount = bvphys->bvp_totalcount;
        brtvd->bv_usedspace = bvphys->bvp_usedspace;
        brtvd->bv_savedspace = bvphys->bvp_savedspace;
        brt->brt_usedspace += brtvd->bv_usedspace;
        brt->brt_savedspace += brtvd->bv_savedspace;

        dmu_buf_rele(db, FTAG);

        BRT_DEBUG("MOS BRT VDEV %s loaded: mos_brtvdev=%llu, mos_entries=%llu",
            name, (u_longlong_t)brtvd->bv_mos_brtvdev,
            (u_longlong_t)brtvd->bv_mos_entries);
}

static void
brt_vdev_dealloc(brt_t *brt, brt_vdev_t *brtvd)
{

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT(brtvd->bv_initiated);

        vmem_free(brtvd->bv_entcount, sizeof (uint16_t) * brtvd->bv_size);
        brtvd->bv_entcount = NULL;
        kmem_free(brtvd->bv_bitmap, BT_SIZEOFMAP(brtvd->bv_nblocks));
        brtvd->bv_bitmap = NULL;
        ASSERT0(avl_numnodes(&brtvd->bv_tree));
        avl_destroy(&brtvd->bv_tree);

        brtvd->bv_size = 0;
        brtvd->bv_nblocks = 0;

        brtvd->bv_initiated = FALSE;
        BRT_DEBUG("BRT VDEV %llu deallocated.", (u_longlong_t)brtvd->bv_vdevid);
}

static void
brt_vdev_destroy(brt_t *brt, brt_vdev_t *brtvd, dmu_tx_t *tx)
{
        char name[64];
        uint64_t count;
        dmu_buf_t *db;
        brt_vdev_phys_t *bvphys;

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT(brtvd->bv_mos_brtvdev != 0);
        ASSERT(brtvd->bv_mos_entries != 0);

        VERIFY0(zap_count(brt->brt_mos, brtvd->bv_mos_entries, &count));
        VERIFY0(count);
        VERIFY0(zap_destroy(brt->brt_mos, brtvd->bv_mos_entries, tx));
        BRT_DEBUG("MOS entries destroyed, object=%llu",
            (u_longlong_t)brtvd->bv_mos_entries);
        brtvd->bv_mos_entries = 0;

        VERIFY0(dmu_bonus_hold(brt->brt_mos, brtvd->bv_mos_brtvdev, FTAG, &db));
        bvphys = db->db_data;
        ASSERT0(bvphys->bvp_totalcount);
        ASSERT0(bvphys->bvp_usedspace);
        ASSERT0(bvphys->bvp_savedspace);
        dmu_buf_rele(db, FTAG);

        VERIFY0(dmu_object_free(brt->brt_mos, brtvd->bv_mos_brtvdev, tx));
        BRT_DEBUG("MOS BRT VDEV destroyed, object=%llu",
            (u_longlong_t)brtvd->bv_mos_brtvdev);
        brtvd->bv_mos_brtvdev = 0;

        snprintf(name, sizeof (name), "%s%llu", BRT_OBJECT_VDEV_PREFIX,
            (u_longlong_t)brtvd->bv_vdevid);
        VERIFY0(zap_remove(brt->brt_mos, DMU_POOL_DIRECTORY_OBJECT, name, tx));
        BRT_DEBUG("Pool directory object removed, object=%s", name);

        brt_vdev_dealloc(brt, brtvd);

        spa_feature_decr(brt->brt_spa, SPA_FEATURE_BLOCK_CLONING, tx);
}

static void
brt_vdevs_expand(brt_t *brt, uint64_t nvdevs)
{
        brt_vdev_t *brtvd, *vdevs;
        uint64_t vdevid;

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT3U(nvdevs, >, brt->brt_nvdevs);

        vdevs = kmem_zalloc(sizeof (vdevs[0]) * nvdevs, KM_SLEEP);
        if (brt->brt_nvdevs > 0) {
                ASSERT(brt->brt_vdevs != NULL);

                memcpy(vdevs, brt->brt_vdevs,
                    sizeof (brt_vdev_t) * brt->brt_nvdevs);
                kmem_free(brt->brt_vdevs,
                    sizeof (brt_vdev_t) * brt->brt_nvdevs);
        }
        for (vdevid = brt->brt_nvdevs; vdevid < nvdevs; vdevid++) {
                brtvd = &vdevs[vdevid];

                brtvd->bv_vdevid = vdevid;
                brtvd->bv_initiated = FALSE;
        }

        BRT_DEBUG("BRT VDEVs expanded from %llu to %llu.",
            (u_longlong_t)brt->brt_nvdevs, (u_longlong_t)nvdevs);

        brt->brt_vdevs = vdevs;
        brt->brt_nvdevs = nvdevs;
}

static boolean_t
brt_vdev_lookup(brt_t *brt, brt_vdev_t *brtvd, const brt_entry_t *bre)
{
        uint64_t idx;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));

        idx = bre->bre_offset / brt->brt_rangesize;
        if (brtvd->bv_entcount != NULL && idx < brtvd->bv_size) {
                /* VDEV wasn't expanded. */
                return (brt_vdev_entcount_get(brtvd, idx) > 0);
        }

        return (FALSE);
}

static void
brt_vdev_addref(brt_t *brt, brt_vdev_t *brtvd, const brt_entry_t *bre,
    uint64_t dsize)
{
        uint64_t idx;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));
        ASSERT(brtvd != NULL);
        ASSERT(brtvd->bv_entcount != NULL);

        brt->brt_savedspace += dsize;
        brtvd->bv_savedspace += dsize;
        brtvd->bv_meta_dirty = TRUE;

        if (bre->bre_refcount > 1) {
                return;
        }

        brt->brt_usedspace += dsize;
        brtvd->bv_usedspace += dsize;

        idx = bre->bre_offset / brt->brt_rangesize;
        if (idx >= brtvd->bv_size) {
                /* VDEV has been expanded. */
                brt_vdev_realloc(brt, brtvd);
        }

        ASSERT3U(idx, <, brtvd->bv_size);

        brtvd->bv_totalcount++;
        brt_vdev_entcount_inc(brtvd, idx);
        brtvd->bv_entcount_dirty = TRUE;
        idx = idx / BRT_BLOCKSIZE / 8;
        BT_SET(brtvd->bv_bitmap, idx);

#ifdef ZFS_DEBUG
        brt_vdev_dump(brt);
#endif
}

static void
brt_vdev_decref(brt_t *brt, brt_vdev_t *brtvd, const brt_entry_t *bre,
    uint64_t dsize)
{
        uint64_t idx;

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT(brtvd != NULL);
        ASSERT(brtvd->bv_entcount != NULL);

        brt->brt_savedspace -= dsize;
        brtvd->bv_savedspace -= dsize;
        brtvd->bv_meta_dirty = TRUE;

        if (bre->bre_refcount > 0) {
                return;
        }

        brt->brt_usedspace -= dsize;
        brtvd->bv_usedspace -= dsize;

        idx = bre->bre_offset / brt->brt_rangesize;
        ASSERT3U(idx, <, brtvd->bv_size);

        ASSERT(brtvd->bv_totalcount > 0);
        brtvd->bv_totalcount--;
        brt_vdev_entcount_dec(brtvd, idx);
        brtvd->bv_entcount_dirty = TRUE;
        idx = idx / BRT_BLOCKSIZE / 8;
        BT_SET(brtvd->bv_bitmap, idx);

#ifdef ZFS_DEBUG
        brt_vdev_dump(brt);
#endif
}

static void
brt_vdev_sync(brt_t *brt, brt_vdev_t *brtvd, dmu_tx_t *tx)
{
        dmu_buf_t *db;
        brt_vdev_phys_t *bvphys;

        ASSERT(brtvd->bv_meta_dirty);
        ASSERT(brtvd->bv_mos_brtvdev != 0);
        ASSERT(dmu_tx_is_syncing(tx));

        VERIFY0(dmu_bonus_hold(brt->brt_mos, brtvd->bv_mos_brtvdev, FTAG, &db));

        if (brtvd->bv_entcount_dirty) {
                /*
                 * TODO: Walk brtvd->bv_bitmap and write only the dirty blocks.
                 */
                dmu_write(brt->brt_mos, brtvd->bv_mos_brtvdev, 0,
                    brtvd->bv_size * sizeof (brtvd->bv_entcount[0]),
                    brtvd->bv_entcount, tx);
                memset(brtvd->bv_bitmap, 0, BT_SIZEOFMAP(brtvd->bv_nblocks));
                brtvd->bv_entcount_dirty = FALSE;
        }

        dmu_buf_will_dirty(db, tx);
        bvphys = db->db_data;
        bvphys->bvp_mos_entries = brtvd->bv_mos_entries;
        bvphys->bvp_size = brtvd->bv_size;
        if (brtvd->bv_need_byteswap) {
                bvphys->bvp_byteorder = BRT_NON_NATIVE_BYTEORDER;
        } else {
                bvphys->bvp_byteorder = BRT_NATIVE_BYTEORDER;
        }
        bvphys->bvp_totalcount = brtvd->bv_totalcount;
        bvphys->bvp_rangesize = brt->brt_rangesize;
        bvphys->bvp_usedspace = brtvd->bv_usedspace;
        bvphys->bvp_savedspace = brtvd->bv_savedspace;
        dmu_buf_rele(db, FTAG);

        brtvd->bv_meta_dirty = FALSE;
}

static void
brt_vdevs_alloc(brt_t *brt, boolean_t load)
{
        brt_vdev_t *brtvd;
        uint64_t vdevid;

        brt_wlock(brt);

        brt_vdevs_expand(brt, brt->brt_spa->spa_root_vdev->vdev_children);

        if (load) {
                for (vdevid = 0; vdevid < brt->brt_nvdevs; vdevid++) {
                        brtvd = &brt->brt_vdevs[vdevid];
                        ASSERT(brtvd->bv_entcount == NULL);

                        brt_vdev_load(brt, brtvd);
                }
        }

        if (brt->brt_rangesize == 0) {
                brt->brt_rangesize = BRT_RANGESIZE;
        }

        brt_unlock(brt);
}

static void
brt_vdevs_free(brt_t *brt)
{
        brt_vdev_t *brtvd;
        uint64_t vdevid;

        brt_wlock(brt);

        for (vdevid = 0; vdevid < brt->brt_nvdevs; vdevid++) {
                brtvd = &brt->brt_vdevs[vdevid];
                if (brtvd->bv_initiated)
                        brt_vdev_dealloc(brt, brtvd);
        }
        kmem_free(brt->brt_vdevs, sizeof (brt_vdev_t) * brt->brt_nvdevs);

        brt_unlock(brt);
}

static void
brt_entry_fill(const blkptr_t *bp, brt_entry_t *bre, uint64_t *vdevidp)
{

        bre->bre_offset = DVA_GET_OFFSET(&bp->blk_dva[0]);
        bre->bre_refcount = 0;

        *vdevidp = DVA_GET_VDEV(&bp->blk_dva[0]);
}

static int
brt_entry_compare(const void *x1, const void *x2)
{
        const brt_entry_t *bre1 = x1;
        const brt_entry_t *bre2 = x2;

        return (TREE_CMP(bre1->bre_offset, bre2->bre_offset));
}

static int
brt_entry_lookup(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre)
{
        uint64_t mos_entries;
        uint64_t one, physsize;
        int error;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));

        if (!brt_vdev_lookup(brt, brtvd, bre))
                return (SET_ERROR(ENOENT));

        /*
         * Remember mos_entries object number. After we reacquire the BRT lock,
         * the brtvd pointer may be invalid.
         */
        mos_entries = brtvd->bv_mos_entries;
        if (mos_entries == 0)
                return (SET_ERROR(ENOENT));

        brt_unlock(brt);

        error = zap_length_uint64(brt->brt_mos, mos_entries, &bre->bre_offset,
            BRT_KEY_WORDS, &one, &physsize);
        if (error == 0) {
                ASSERT3U(one, ==, 1);
                ASSERT3U(physsize, ==, sizeof (bre->bre_refcount));

                error = zap_lookup_uint64(brt->brt_mos, mos_entries,
                    &bre->bre_offset, BRT_KEY_WORDS, 1,
                    sizeof (bre->bre_refcount), &bre->bre_refcount);
                BRT_DEBUG("ZAP lookup: object=%llu vdev=%llu offset=%llu "
                    "count=%llu error=%d", (u_longlong_t)mos_entries,
                    (u_longlong_t)brtvd->bv_vdevid,
                    (u_longlong_t)bre->bre_offset,
                    error == 0 ? (u_longlong_t)bre->bre_refcount : 0, error);
        }

        brt_wlock(brt);

        return (error);
}

static void
brt_entry_prefetch(brt_t *brt, uint64_t vdevid, brt_entry_t *bre)
{
        brt_vdev_t *brtvd;
        uint64_t mos_entries = 0;

        brt_rlock(brt);
        brtvd = brt_vdev(brt, vdevid);
        if (brtvd != NULL)
                mos_entries = brtvd->bv_mos_entries;
        brt_unlock(brt);

        if (mos_entries == 0)
                return;

        BRT_DEBUG("ZAP prefetch: object=%llu vdev=%llu offset=%llu",
            (u_longlong_t)mos_entries, (u_longlong_t)vdevid,
            (u_longlong_t)bre->bre_offset);
        (void) zap_prefetch_uint64(brt->brt_mos, mos_entries,
            (uint64_t *)&bre->bre_offset, BRT_KEY_WORDS);
}

static int
brt_entry_update(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre, dmu_tx_t *tx)
{
        int error;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));
        ASSERT(brtvd->bv_mos_entries != 0);
        ASSERT(bre->bre_refcount > 0);

        error = zap_update_uint64(brt->brt_mos, brtvd->bv_mos_entries,
            (uint64_t *)&bre->bre_offset, BRT_KEY_WORDS, 1,
            sizeof (bre->bre_refcount), &bre->bre_refcount, tx);
        BRT_DEBUG("ZAP update: object=%llu vdev=%llu offset=%llu count=%llu "
            "error=%d", (u_longlong_t)brtvd->bv_mos_entries,
            (u_longlong_t)brtvd->bv_vdevid, (u_longlong_t)bre->bre_offset,
            (u_longlong_t)bre->bre_refcount, error);

        return (error);
}

static int
brt_entry_remove(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre, dmu_tx_t *tx)
{
        int error;

        ASSERT(RW_LOCK_HELD(&brt->brt_lock));
        ASSERT(brtvd->bv_mos_entries != 0);
        ASSERT0(bre->bre_refcount);

        error = zap_remove_uint64(brt->brt_mos, brtvd->bv_mos_entries,
            (uint64_t *)&bre->bre_offset, BRT_KEY_WORDS, tx);
        BRT_DEBUG("ZAP remove: object=%llu vdev=%llu offset=%llu count=%llu "
            "error=%d", (u_longlong_t)brtvd->bv_mos_entries,
            (u_longlong_t)brtvd->bv_vdevid, (u_longlong_t)bre->bre_offset,
            (u_longlong_t)bre->bre_refcount, error);

        return (error);
}

/*
 * Return TRUE if we _can_ have BRT entry for this bp. It might be false
 * positive, but gives us quick answer if we should look into BRT, which
 * may require reads and thus will be more expensive.
 */
boolean_t
brt_maybe_exists(spa_t *spa, const blkptr_t *bp)
{
        brt_t *brt = spa->spa_brt;
        brt_vdev_t *brtvd;
        brt_entry_t bre_search;
        boolean_t mayexists = FALSE;
        uint64_t vdevid;

        brt_entry_fill(bp, &bre_search, &vdevid);

        brt_rlock(brt);

        brtvd = brt_vdev(brt, vdevid);
        if (brtvd != NULL && brtvd->bv_initiated) {
                if (!avl_is_empty(&brtvd->bv_tree) ||
                    brt_vdev_lookup(brt, brtvd, &bre_search)) {
                        mayexists = TRUE;
                }
        }

        brt_unlock(brt);

        return (mayexists);
}

uint64_t
brt_get_dspace(spa_t *spa)
{
        brt_t *brt = spa->spa_brt;

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

        return (brt->brt_savedspace);
}

uint64_t
brt_get_used(spa_t *spa)
{
        brt_t *brt = spa->spa_brt;

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

        return (brt->brt_usedspace);
}

uint64_t
brt_get_saved(spa_t *spa)
{
        brt_t *brt = spa->spa_brt;

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

        return (brt->brt_savedspace);
}

uint64_t
brt_get_ratio(spa_t *spa)
{
        brt_t *brt = spa->spa_brt;

        if (brt->brt_usedspace == 0)
                return (100);

        return ((brt->brt_usedspace + brt->brt_savedspace) * 100 /
            brt->brt_usedspace);
}

static int
brt_kstats_update(kstat_t *ksp, int rw)
{
        brt_stats_t *bs = ksp->ks_data;

        if (rw == KSTAT_WRITE)
                return (EACCES);

        bs->brt_addref_entry_in_memory.value.ui64 =
            wmsum_value(&brt_sums.brt_addref_entry_in_memory);
        bs->brt_addref_entry_not_on_disk.value.ui64 =
            wmsum_value(&brt_sums.brt_addref_entry_not_on_disk);
        bs->brt_addref_entry_on_disk.value.ui64 =
            wmsum_value(&brt_sums.brt_addref_entry_on_disk);
        bs->brt_addref_entry_read_lost_race.value.ui64 =
            wmsum_value(&brt_sums.brt_addref_entry_read_lost_race);
        bs->brt_decref_entry_in_memory.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_in_memory);
        bs->brt_decref_entry_loaded_from_disk.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_loaded_from_disk);
        bs->brt_decref_entry_not_in_memory.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_not_in_memory);
        bs->brt_decref_entry_not_on_disk.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_not_on_disk);
        bs->brt_decref_entry_read_lost_race.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_read_lost_race);
        bs->brt_decref_entry_still_referenced.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_entry_still_referenced);
        bs->brt_decref_free_data_later.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_free_data_later);
        bs->brt_decref_free_data_now.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_free_data_now);
        bs->brt_decref_no_entry.value.ui64 =
            wmsum_value(&brt_sums.brt_decref_no_entry);

        return (0);
}

static void
brt_stat_init(void)
{

        wmsum_init(&brt_sums.brt_addref_entry_in_memory, 0);
        wmsum_init(&brt_sums.brt_addref_entry_not_on_disk, 0);
        wmsum_init(&brt_sums.brt_addref_entry_on_disk, 0);
        wmsum_init(&brt_sums.brt_addref_entry_read_lost_race, 0);
        wmsum_init(&brt_sums.brt_decref_entry_in_memory, 0);
        wmsum_init(&brt_sums.brt_decref_entry_loaded_from_disk, 0);
        wmsum_init(&brt_sums.brt_decref_entry_not_in_memory, 0);
        wmsum_init(&brt_sums.brt_decref_entry_not_on_disk, 0);
        wmsum_init(&brt_sums.brt_decref_entry_read_lost_race, 0);
        wmsum_init(&brt_sums.brt_decref_entry_still_referenced, 0);
        wmsum_init(&brt_sums.brt_decref_free_data_later, 0);
        wmsum_init(&brt_sums.brt_decref_free_data_now, 0);
        wmsum_init(&brt_sums.brt_decref_no_entry, 0);

        brt_ksp = kstat_create("zfs", 0, "brtstats", "misc", KSTAT_TYPE_NAMED,
            sizeof (brt_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
        if (brt_ksp != NULL) {
                brt_ksp->ks_data = &brt_stats;
                brt_ksp->ks_update = brt_kstats_update;
                kstat_install(brt_ksp);
        }
}

static void
brt_stat_fini(void)
{
        if (brt_ksp != NULL) {
                kstat_delete(brt_ksp);
                brt_ksp = NULL;
        }

        wmsum_fini(&brt_sums.brt_addref_entry_in_memory);
        wmsum_fini(&brt_sums.brt_addref_entry_not_on_disk);
        wmsum_fini(&brt_sums.brt_addref_entry_on_disk);
        wmsum_fini(&brt_sums.brt_addref_entry_read_lost_race);
        wmsum_fini(&brt_sums.brt_decref_entry_in_memory);
        wmsum_fini(&brt_sums.brt_decref_entry_loaded_from_disk);
        wmsum_fini(&brt_sums.brt_decref_entry_not_in_memory);
        wmsum_fini(&brt_sums.brt_decref_entry_not_on_disk);
        wmsum_fini(&brt_sums.brt_decref_entry_read_lost_race);
        wmsum_fini(&brt_sums.brt_decref_entry_still_referenced);
        wmsum_fini(&brt_sums.brt_decref_free_data_later);
        wmsum_fini(&brt_sums.brt_decref_free_data_now);
        wmsum_fini(&brt_sums.brt_decref_no_entry);
}

void
brt_init(void)
{
        brt_entry_cache = kmem_cache_create("brt_entry_cache",
            sizeof (brt_entry_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
        brt_pending_entry_cache = kmem_cache_create("brt_pending_entry_cache",
            sizeof (brt_pending_entry_t), 0, NULL, NULL, NULL, NULL, NULL, 0);

        brt_stat_init();
}

void
brt_fini(void)
{
        brt_stat_fini();

        kmem_cache_destroy(brt_entry_cache);
        kmem_cache_destroy(brt_pending_entry_cache);
}

static brt_entry_t *
brt_entry_alloc(const brt_entry_t *bre_init)
{
        brt_entry_t *bre;

        bre = kmem_cache_alloc(brt_entry_cache, KM_SLEEP);
        bre->bre_offset = bre_init->bre_offset;
        bre->bre_refcount = bre_init->bre_refcount;

        return (bre);
}

static void
brt_entry_free(brt_entry_t *bre)
{

        kmem_cache_free(brt_entry_cache, bre);
}

static void
brt_entry_addref(brt_t *brt, const blkptr_t *bp)
{
        brt_vdev_t *brtvd;
        brt_entry_t *bre, *racebre;
        brt_entry_t bre_search;
        avl_index_t where;
        uint64_t vdevid;
        int error;

        ASSERT(!RW_WRITE_HELD(&brt->brt_lock));

        brt_entry_fill(bp, &bre_search, &vdevid);

        brt_wlock(brt);

        brtvd = brt_vdev(brt, vdevid);
        if (brtvd == NULL) {
                ASSERT3U(vdevid, >=, brt->brt_nvdevs);

                /* New VDEV was added. */
                brt_vdevs_expand(brt, vdevid + 1);
                brtvd = brt_vdev(brt, vdevid);
        }
        ASSERT(brtvd != NULL);
        if (!brtvd->bv_initiated)
                brt_vdev_realloc(brt, brtvd);

        bre = avl_find(&brtvd->bv_tree, &bre_search, NULL);
        if (bre != NULL) {
                BRTSTAT_BUMP(brt_addref_entry_in_memory);
        } else {
                /*
                 * brt_entry_lookup() may drop the BRT (read) lock and
                 * reacquire it (write).
                 */
                error = brt_entry_lookup(brt, brtvd, &bre_search);
                /* bre_search now contains correct bre_refcount */
                ASSERT(error == 0 || error == ENOENT);
                if (error == 0)
                        BRTSTAT_BUMP(brt_addref_entry_on_disk);
                else
                        BRTSTAT_BUMP(brt_addref_entry_not_on_disk);
                /*
                 * When the BRT lock was dropped, brt_vdevs[] may have been
                 * expanded and reallocated, we need to update brtvd's pointer.
                 */
                brtvd = brt_vdev(brt, vdevid);
                ASSERT(brtvd != NULL);

                racebre = avl_find(&brtvd->bv_tree, &bre_search, &where);
                if (racebre == NULL) {
                        bre = brt_entry_alloc(&bre_search);
                        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
                        avl_insert(&brtvd->bv_tree, bre, where);
                        brt->brt_nentries++;
                } else {
                        /*
                         * The entry was added when the BRT lock was dropped in
                         * brt_entry_lookup().
                         */
                        BRTSTAT_BUMP(brt_addref_entry_read_lost_race);
                        bre = racebre;
                }
        }
        bre->bre_refcount++;
        brt_vdev_addref(brt, brtvd, bre, bp_get_dsize(brt->brt_spa, bp));

        brt_unlock(brt);
}

/* Return TRUE if block should be freed immediately. */
boolean_t
brt_entry_decref(spa_t *spa, const blkptr_t *bp)
{
        brt_t *brt = spa->spa_brt;
        brt_vdev_t *brtvd;
        brt_entry_t *bre, *racebre;
        brt_entry_t bre_search;
        avl_index_t where;
        uint64_t vdevid;
        int error;

        brt_entry_fill(bp, &bre_search, &vdevid);

        brt_wlock(brt);

        brtvd = brt_vdev(brt, vdevid);
        ASSERT(brtvd != NULL);

        bre = avl_find(&brtvd->bv_tree, &bre_search, NULL);
        if (bre != NULL) {
                BRTSTAT_BUMP(brt_decref_entry_in_memory);
                goto out;
        } else {
                BRTSTAT_BUMP(brt_decref_entry_not_in_memory);
        }

        /*
         * brt_entry_lookup() may drop the BRT lock and reacquire it.
         */
        error = brt_entry_lookup(brt, brtvd, &bre_search);
        /* bre_search now contains correct bre_refcount */
        ASSERT(error == 0 || error == ENOENT);
        /*
         * When the BRT lock was dropped, brt_vdevs[] may have been expanded
         * and reallocated, we need to update brtvd's pointer.
         */
        brtvd = brt_vdev(brt, vdevid);
        ASSERT(brtvd != NULL);

        if (error == ENOENT) {
                BRTSTAT_BUMP(brt_decref_entry_not_on_disk);
                bre = NULL;
                goto out;
        }

        racebre = avl_find(&brtvd->bv_tree, &bre_search, &where);
        if (racebre != NULL) {
                /*
                 * The entry was added when the BRT lock was dropped in
                 * brt_entry_lookup().
                 */
                BRTSTAT_BUMP(brt_decref_entry_read_lost_race);
                bre = racebre;
                goto out;
        }

        BRTSTAT_BUMP(brt_decref_entry_loaded_from_disk);
        bre = brt_entry_alloc(&bre_search);
        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        avl_insert(&brtvd->bv_tree, bre, where);
        brt->brt_nentries++;

out:
        if (bre == NULL) {
                /*
                 * This is a free of a regular (not cloned) block.
                 */
                brt_unlock(brt);
                BRTSTAT_BUMP(brt_decref_no_entry);
                return (B_TRUE);
        }
        if (bre->bre_refcount == 0) {
                brt_unlock(brt);
                BRTSTAT_BUMP(brt_decref_free_data_now);
                return (B_TRUE);
        }

        ASSERT(bre->bre_refcount > 0);
        bre->bre_refcount--;
        if (bre->bre_refcount == 0)
                BRTSTAT_BUMP(brt_decref_free_data_later);
        else
                BRTSTAT_BUMP(brt_decref_entry_still_referenced);
        brt_vdev_decref(brt, brtvd, bre, bp_get_dsize(brt->brt_spa, bp));

        brt_unlock(brt);

        return (B_FALSE);
}

static void
brt_prefetch(brt_t *brt, const blkptr_t *bp)
{
        brt_entry_t bre;
        uint64_t vdevid;

        ASSERT(bp != NULL);

        if (!zfs_brt_prefetch)
                return;

        brt_entry_fill(bp, &bre, &vdevid);

        brt_entry_prefetch(brt, vdevid, &bre);
}

static int
brt_pending_entry_compare(const void *x1, const void *x2)
{
        const brt_pending_entry_t *bpe1 = x1, *bpe2 = x2;
        const blkptr_t *bp1 = &bpe1->bpe_bp, *bp2 = &bpe2->bpe_bp;
        int cmp;

        cmp = TREE_CMP(BP_PHYSICAL_BIRTH(bp1), BP_PHYSICAL_BIRTH(bp2));
        if (cmp == 0) {
                cmp = TREE_CMP(DVA_GET_VDEV(&bp1->blk_dva[0]),
                    DVA_GET_VDEV(&bp2->blk_dva[0]));
                if (cmp == 0) {
                        cmp = TREE_CMP(DVA_GET_OFFSET(&bp1->blk_dva[0]),
                            DVA_GET_OFFSET(&bp2->blk_dva[0]));
                }
        }

        return (cmp);
}

void
brt_pending_add(spa_t *spa, const blkptr_t *bp, dmu_tx_t *tx)
{
        brt_t *brt;
        avl_tree_t *pending_tree;
        kmutex_t *pending_lock;
        brt_pending_entry_t *bpe, *newbpe;
        avl_index_t where;
        uint64_t txg;

        brt = spa->spa_brt;
        txg = dmu_tx_get_txg(tx);
        ASSERT3U(txg, !=, 0);
        pending_tree = &brt->brt_pending_tree[txg & TXG_MASK];
        pending_lock = &brt->brt_pending_lock[txg & TXG_MASK];

        newbpe = kmem_cache_alloc(brt_pending_entry_cache, KM_SLEEP);
        newbpe->bpe_bp = *bp;
        newbpe->bpe_count = 1;

        mutex_enter(pending_lock);

        bpe = avl_find(pending_tree, newbpe, &where);
        if (bpe == NULL) {
                avl_insert(pending_tree, newbpe, where);
                newbpe = NULL;
        } else {
                bpe->bpe_count++;
        }

        mutex_exit(pending_lock);

        if (newbpe != NULL) {
                ASSERT(bpe != NULL);
                ASSERT(bpe != newbpe);
                kmem_cache_free(brt_pending_entry_cache, newbpe);
        } else {
                ASSERT(bpe == NULL);
        }

        /* Prefetch BRT entry, as we will need it in the syncing context. */
        brt_prefetch(brt, bp);
}

void
brt_pending_remove(spa_t *spa, const blkptr_t *bp, dmu_tx_t *tx)
{
        brt_t *brt;
        avl_tree_t *pending_tree;
        kmutex_t *pending_lock;
        brt_pending_entry_t *bpe, bpe_search;
        uint64_t txg;

        brt = spa->spa_brt;
        txg = dmu_tx_get_txg(tx);
        ASSERT3U(txg, !=, 0);
        pending_tree = &brt->brt_pending_tree[txg & TXG_MASK];
        pending_lock = &brt->brt_pending_lock[txg & TXG_MASK];

        bpe_search.bpe_bp = *bp;

        mutex_enter(pending_lock);

        bpe = avl_find(pending_tree, &bpe_search, NULL);
        /* I believe we should always find bpe when this function is called. */
        if (bpe != NULL) {
                ASSERT(bpe->bpe_count > 0);

                bpe->bpe_count--;
                if (bpe->bpe_count == 0) {
                        avl_remove(pending_tree, bpe);
                        kmem_cache_free(brt_pending_entry_cache, bpe);
                }
        }

        mutex_exit(pending_lock);
}

void
brt_pending_apply(spa_t *spa, uint64_t txg)
{
        brt_t *brt;
        brt_pending_entry_t *bpe;
        avl_tree_t *pending_tree;
        kmutex_t *pending_lock;
        void *c;

        ASSERT3U(txg, !=, 0);

        brt = spa->spa_brt;
        pending_tree = &brt->brt_pending_tree[txg & TXG_MASK];
        pending_lock = &brt->brt_pending_lock[txg & TXG_MASK];

        mutex_enter(pending_lock);

        c = NULL;
        while ((bpe = avl_destroy_nodes(pending_tree, &c)) != NULL) {
                boolean_t added_to_ddt;

                mutex_exit(pending_lock);

                for (int i = 0; i < bpe->bpe_count; i++) {
                        /*
                         * If the block has DEDUP bit set, it means that it
                         * already exists in the DEDUP table, so we can just
                         * use that instead of creating new entry in
                         * the BRT table.
                         */
                        if (BP_GET_DEDUP(&bpe->bpe_bp)) {
                                added_to_ddt = ddt_addref(spa, &bpe->bpe_bp);
                        } else {
                                added_to_ddt = B_FALSE;
                        }
                        if (!added_to_ddt)
                                brt_entry_addref(brt, &bpe->bpe_bp);
                }

                kmem_cache_free(brt_pending_entry_cache, bpe);
                mutex_enter(pending_lock);
        }

        mutex_exit(pending_lock);
}

static void
brt_sync_entry(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre, dmu_tx_t *tx)
{

        ASSERT(RW_WRITE_HELD(&brt->brt_lock));
        ASSERT(brtvd->bv_mos_entries != 0);

        if (bre->bre_refcount == 0) {
                int error;

                error = brt_entry_remove(brt, brtvd, bre, tx);
                ASSERT(error == 0 || error == ENOENT);
                /*
                 * If error == ENOENT then zfs_clone_range() was done from a
                 * removed (but opened) file (open(), unlink()).
                 */
                ASSERT(brt_entry_lookup(brt, brtvd, bre) == ENOENT);
        } else {
                VERIFY0(brt_entry_update(brt, brtvd, bre, tx));
        }
}

static void
brt_sync_table(brt_t *brt, dmu_tx_t *tx)
{
        brt_vdev_t *brtvd;
        brt_entry_t *bre;
        uint64_t vdevid;
        void *c;

        brt_wlock(brt);

        for (vdevid = 0; vdevid < brt->brt_nvdevs; vdevid++) {
                brtvd = &brt->brt_vdevs[vdevid];

                if (!brtvd->bv_initiated)
                        continue;

                if (!brtvd->bv_meta_dirty) {
                        ASSERT(!brtvd->bv_entcount_dirty);
                        ASSERT0(avl_numnodes(&brtvd->bv_tree));
                        continue;
                }

                ASSERT(!brtvd->bv_entcount_dirty ||
                    avl_numnodes(&brtvd->bv_tree) != 0);

                if (brtvd->bv_mos_brtvdev == 0)
                        brt_vdev_create(brt, brtvd, tx);

                c = NULL;
                while ((bre = avl_destroy_nodes(&brtvd->bv_tree, &c)) != NULL) {
                        brt_sync_entry(brt, brtvd, bre, tx);
                        brt_entry_free(bre);
                        ASSERT(brt->brt_nentries > 0);
                        brt->brt_nentries--;
                }

                brt_vdev_sync(brt, brtvd, tx);

                if (brtvd->bv_totalcount == 0)
                        brt_vdev_destroy(brt, brtvd, tx);
        }

        ASSERT0(brt->brt_nentries);

        brt_unlock(brt);
}

void
brt_sync(spa_t *spa, uint64_t txg)
{
        dmu_tx_t *tx;
        brt_t *brt;

        ASSERT(spa_syncing_txg(spa) == txg);

        brt = spa->spa_brt;
        brt_rlock(brt);
        if (brt->brt_nentries == 0) {
                /* No changes. */
                brt_unlock(brt);
                return;
        }
        brt_unlock(brt);

        tx = dmu_tx_create_assigned(spa->spa_dsl_pool, txg);

        brt_sync_table(brt, tx);

        dmu_tx_commit(tx);
}

static void
brt_table_alloc(brt_t *brt)
{

        for (int i = 0; i < TXG_SIZE; i++) {
                avl_create(&brt->brt_pending_tree[i],
                    brt_pending_entry_compare,
                    sizeof (brt_pending_entry_t),
                    offsetof(brt_pending_entry_t, bpe_node));
                mutex_init(&brt->brt_pending_lock[i], NULL, MUTEX_DEFAULT,
                    NULL);
        }
}

static void
brt_table_free(brt_t *brt)
{

        for (int i = 0; i < TXG_SIZE; i++) {
                ASSERT(avl_is_empty(&brt->brt_pending_tree[i]));

                avl_destroy(&brt->brt_pending_tree[i]);
                mutex_destroy(&brt->brt_pending_lock[i]);
        }
}

static void
brt_alloc(spa_t *spa)
{
        brt_t *brt;

        ASSERT(spa->spa_brt == NULL);

        brt = kmem_zalloc(sizeof (*brt), KM_SLEEP);
        rw_init(&brt->brt_lock, NULL, RW_DEFAULT, NULL);
        brt->brt_spa = spa;
        brt->brt_rangesize = 0;
        brt->brt_nentries = 0;
        brt->brt_vdevs = NULL;
        brt->brt_nvdevs = 0;
        brt_table_alloc(brt);

        spa->spa_brt = brt;
}

void
brt_create(spa_t *spa)
{

        brt_alloc(spa);
        brt_vdevs_alloc(spa->spa_brt, B_FALSE);
}

int
brt_load(spa_t *spa)
{

        brt_alloc(spa);
        brt_vdevs_alloc(spa->spa_brt, B_TRUE);

        return (0);
}

void
brt_unload(spa_t *spa)
{
        brt_t *brt = spa->spa_brt;

        if (brt == NULL)
                return;

        brt_vdevs_free(brt);
        brt_table_free(brt);
        rw_destroy(&brt->brt_lock);
        kmem_free(brt, sizeof (*brt));
        spa->spa_brt = NULL;
}

/* BEGIN CSTYLED */
ZFS_MODULE_PARAM(zfs_brt, zfs_brt_, prefetch, INT, ZMOD_RW,
    "Enable prefetching of BRT entries");
#ifdef ZFS_BRT_DEBUG
ZFS_MODULE_PARAM(zfs_brt, zfs_brt_, debug, INT, ZMOD_RW, "BRT debug");
#endif
/* END CSTYLED */