fs/zfs/zio.c

	zio.c revision fa94a07fd0519b8abfd871ad8fe60e6bebe1e2bb
/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma ident   "%Z%%M% %I% %E% SMI"

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

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

/*
 * ==========================================================================
 * I/O type descriptions
 * ==========================================================================
 */
char *zio_type_name[ZIO_TYPES] = {
    "null", "read", "write", "free", "claim", "ioctl" };

/* At or above this size, force gang blocking - for testing */
uint64_t zio_gang_bang = SPA_MAXBLOCKSIZE + 1;

/* Force an allocation failure when non-zero */
uint16_t zio_zil_fail_shift = 0;
uint16_t zio_io_fail_shift = 0;

/* Enable/disable the write-retry logic */
int zio_write_retry = 1;

/* Taskq to handle reissuing of I/Os */
taskq_t *zio_taskq;
int zio_resume_threads = 4;

typedef struct zio_sync_pass {
    int zp_defer_free;      /* defer frees after this pass */
    int zp_dontcompress;    /* don't compress after this pass */
    int zp_rewrite;     /* rewrite new bps after this pass */
} zio_sync_pass_t;

zio_sync_pass_t zio_sync_pass = {
    1,  /* zp_defer_free */
    4,  /* zp_dontcompress */
    1,  /* zp_rewrite */
};

static boolean_t zio_io_should_fail(uint16_t);

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

#ifdef _KERNEL
extern vmem_t *zio_alloc_arena;
#endif

/*
 * Determine if we are allowed to issue the IO based on the
 * pool state. If we must wait then block until we are told
 * that we may continue.
 */
#define ZIO_ENTER(spa) {                        \
    if (spa->spa_state == POOL_STATE_IO_FAILURE) {          \
        mutex_enter(&spa->spa_zio_lock);            \
        while (spa->spa_state == POOL_STATE_IO_FAILURE)     \
            cv_wait(&spa->spa_zio_cv, &spa->spa_zio_lock);  \
        mutex_exit(&spa->spa_zio_lock);             \
    }                               \
}

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

void
zio_init(void)
{
    size_t c;
    vmem_t *data_alloc_arena = NULL;

#ifdef _KERNEL
    data_alloc_arena = zio_alloc_arena;
#endif

    zio_cache = kmem_cache_create("zio_cache", sizeof (zio_t), 0,
        NULL, NULL, NULL, NULL, NULL, 0);

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

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

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

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

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

            dprintf("creating cache for size %5lx align %5lx\n",
                size, align);
        }
    }

    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_taskq = taskq_create("zio_taskq", zio_resume_threads,
        maxclsyspri, 50, INT_MAX, TASKQ_PREPOPULATE);

    zio_inject_init();
}

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

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

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

    taskq_destroy(zio_taskq);

    kmem_cache_destroy(zio_cache);

    zio_inject_fini();
}

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

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

    ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

    return (kmem_cache_alloc(zio_buf_cache[c], KM_SLEEP));
}

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

    ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

    return (kmem_cache_alloc(zio_data_buf_cache[c], KM_SLEEP));
}

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

    ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

    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;

    ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

    kmem_cache_free(zio_data_buf_cache[c], buf);
}

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

    zt->zt_data = data;
    zt->zt_size = size;
    zt->zt_bufsize = bufsize;

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

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

static void
zio_pop_transform(zio_t *zio, void **data, uint64_t *size, uint64_t *bufsize)
{
    zio_transform_t *zt = zio->io_transform_stack;

    *data = zt->zt_data;
    *size = zt->zt_size;
    *bufsize = zt->zt_bufsize;

    zio->io_transform_stack = zt->zt_next;
    kmem_free(zt, sizeof (zio_transform_t));

    if ((zt = zio->io_transform_stack) != NULL) {
        zio->io_data = zt->zt_data;
        zio->io_size = zt->zt_size;
    }
}

static void
zio_clear_transform_stack(zio_t *zio)
{
    void *data;
    uint64_t size, bufsize;

    ASSERT(zio->io_transform_stack != NULL);

    zio_pop_transform(zio, &data, &size, &bufsize);
    while (zio->io_transform_stack != NULL) {
        zio_buf_free(data, bufsize);
        zio_pop_transform(zio, &data, &size, &bufsize);
    }
}

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

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

    zio = kmem_cache_alloc(zio_cache, KM_SLEEP);
    bzero(zio, sizeof (zio_t));
    zio->io_parent = pio;
    zio->io_spa = spa;
    zio->io_txg = txg;
    zio->io_flags = flags;
    if (bp != NULL) {
        zio->io_bp = bp;
        zio->io_bp_copy = *bp;
        zio->io_bp_orig = *bp;
        if (dmu_ot[BP_GET_TYPE(bp)].ot_metadata ||
            BP_GET_LEVEL(bp) != 0)
            zio->io_flags |= ZIO_FLAG_METADATA;
    }
    zio->io_done = done;
    zio->io_private = private;
    zio->io_type = type;
    zio->io_priority = priority;
    zio->io_stage = stage;
    zio->io_pipeline = pipeline;
    zio->io_async_stages = ZIO_ASYNC_PIPELINE_STAGES;
    zio->io_timestamp = lbolt64;
    if (pio != NULL)
        zio->io_flags |= (pio->io_flags & ZIO_FLAG_METADATA);
    mutex_init(&zio->io_lock, NULL, MUTEX_DEFAULT, NULL);
    cv_init(&zio->io_cv, NULL, CV_DEFAULT, NULL);
    zio_push_transform(zio, data, size, size);

    /*
     * Note on config lock:
     *
     * If CONFIG_HELD is set, then the caller already has the config
     * lock, so we don't need it for this io.
     *
     * We set CONFIG_GRABBED to indicate that we have grabbed the
     * config lock on behalf of this io, so it should be released
     * in zio_done.
     *
     * Unless CONFIG_HELD is set, we will grab the config lock for
     * any top-level (parent-less) io, *except* NULL top-level ios.
     * The NULL top-level ios rarely have any children, so we delay
     * grabbing the lock until the first child is added (but it is
     * still grabbed on behalf of the top-level i/o, so additional
     * children don't need to also grab it).  This greatly reduces
     * contention on the config lock.
     */
    if (pio == NULL) {
        if (type != ZIO_TYPE_NULL &&
            !(flags & ZIO_FLAG_CONFIG_HELD)) {
            spa_config_enter(zio->io_spa, RW_READER, zio);
            zio->io_flags |= ZIO_FLAG_CONFIG_GRABBED;
        }
        zio->io_root = zio;
    } else {
        zio->io_root = pio->io_root;
        if (!(flags & ZIO_FLAG_NOBOOKMARK))
            zio->io_logical = pio->io_logical;
        mutex_enter(&pio->io_lock);
        if (pio->io_parent == NULL &&
            pio->io_type == ZIO_TYPE_NULL &&
            !(pio->io_flags & ZIO_FLAG_CONFIG_GRABBED) &&
            !(pio->io_flags & ZIO_FLAG_CONFIG_HELD)) {
            pio->io_flags |= ZIO_FLAG_CONFIG_GRABBED;
            spa_config_enter(zio->io_spa, RW_READER, pio);
        }
        if (stage < ZIO_STAGE_READY)
            pio->io_children_notready++;
        pio->io_children_notdone++;
        zio->io_sibling_next = pio->io_child;
        zio->io_sibling_prev = NULL;
        if (pio->io_child != NULL)
            pio->io_child->io_sibling_prev = zio;
        pio->io_child = zio;
        zio->io_ndvas = pio->io_ndvas;
        mutex_exit(&pio->io_lock);
    }

    /*
     * Save off the original state incase we need to retry later.
     */
    zio->io_orig_stage = zio->io_stage;
    zio->io_orig_pipeline = zio->io_pipeline;
    zio->io_orig_flags = zio->io_flags;

    return (zio);
}

static void
zio_reset(zio_t *zio)
{
    zio_clear_transform_stack(zio);

    zio->io_flags = zio->io_orig_flags;
    zio->io_stage = zio->io_orig_stage;
    zio->io_pipeline = zio->io_orig_pipeline;
    zio_push_transform(zio, zio->io_data, zio->io_size, zio->io_size);
}

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

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

    return (zio);
}

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

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

    ASSERT3U(size, ==, BP_GET_LSIZE(bp));

    /*
     * If the user has specified that we allow I/Os to continue
     * then attempt to satisfy the read.
     */
    if (spa_get_failmode(spa) != ZIO_FAILURE_MODE_CONTINUE)
        ZIO_ENTER(spa);

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

    zio->io_logical = zio;

    /*
     * Work off our copy of the bp so the caller can free it.
     */
    zio->io_bp = &zio->io_bp_copy;

    return (zio);
}

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

    ASSERT(checksum >= ZIO_CHECKSUM_OFF &&
        checksum < ZIO_CHECKSUM_FUNCTIONS);

    ASSERT(compress >= ZIO_COMPRESS_OFF &&
        compress < ZIO_COMPRESS_FUNCTIONS);

    ZIO_ENTER(spa);

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

    zio->io_ready = ready;

    zio->io_bookmark = *zb;

    zio->io_logical = zio;

    zio->io_checksum = checksum;
    zio->io_compress = compress;
    zio->io_ndvas = ncopies;

    if (compress != ZIO_COMPRESS_OFF)
        zio->io_async_stages |= 1U << ZIO_STAGE_WRITE_COMPRESS;

    if (bp->blk_birth != txg) {
        /* XXX the bp usually (always?) gets re-zeroed later */
        BP_ZERO(bp);
        BP_SET_LSIZE(bp, size);
        BP_SET_PSIZE(bp, size);
    } else {
        /* Make sure someone doesn't change their mind on overwrites */
        ASSERT(MIN(zio->io_ndvas + BP_IS_GANG(bp),
            spa_max_replication(spa)) == BP_GET_NDVAS(bp));
    }

    return (zio);
}

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

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

    zio->io_bookmark = *zb;
    zio->io_checksum = checksum;
    zio->io_compress = ZIO_COMPRESS_OFF;

    if (pio != NULL)
        ASSERT3U(zio->io_ndvas, <=, BP_GET_NDVAS(bp));

    return (zio);
}

static void
zio_write_allocate_ready(zio_t *zio)
{
    /* Free up the previous block */
    if (!BP_IS_HOLE(&zio->io_bp_orig)) {
        zio_nowait(zio_free(zio, zio->io_spa, zio->io_txg,
            &zio->io_bp_orig, NULL, NULL));
    }
}

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

    BP_ZERO(bp);
    BP_SET_LSIZE(bp, size);
    BP_SET_PSIZE(bp, size);
    BP_SET_COMPRESS(bp, ZIO_COMPRESS_OFF);

    zio = zio_create(pio, spa, txg, bp, data, size, done, private,
        ZIO_TYPE_WRITE, priority, flags,
        ZIO_STAGE_OPEN, ZIO_WRITE_ALLOCATE_PIPELINE);

    zio->io_checksum = checksum;
    zio->io_compress = ZIO_COMPRESS_OFF;
    zio->io_ready = zio_write_allocate_ready;

    return (zio);
}

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

    ASSERT(!BP_IS_HOLE(bp));

    if (txg == spa->spa_syncing_txg &&
        spa->spa_sync_pass > zio_sync_pass.zp_defer_free) {
        bplist_enqueue_deferred(&spa->spa_sync_bplist, bp);
        return (zio_null(pio, spa, NULL, NULL, 0));
    }

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

    zio->io_bp = &zio->io_bp_copy;

    return (zio);
}

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

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

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

    zio->io_bp = &zio->io_bp_copy;

    return (zio);
}

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

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

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

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

    return (zio);
}

static void
zio_phys_bp_init(vdev_t *vd, blkptr_t *bp, uint64_t offset, uint64_t size,
    int checksum, boolean_t labels)
{
    ASSERT(vd->vdev_children == 0);

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

#ifdef ZFS_DEBUG
    if (labels) {
        ASSERT(offset + size <= VDEV_LABEL_START_SIZE ||
            offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
    }
#endif
    ASSERT3U(offset + size, <=, vd->vdev_psize);

    BP_ZERO(bp);

    BP_SET_LSIZE(bp, size);
    BP_SET_PSIZE(bp, size);

    BP_SET_CHECKSUM(bp, checksum);
    BP_SET_COMPRESS(bp, ZIO_COMPRESS_OFF);
    BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);

    if (checksum != ZIO_CHECKSUM_OFF)
        ZIO_SET_CHECKSUM(&bp->blk_cksum, offset, 0, 0, 0);
}

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

    ZIO_ENTER(vd->vdev_spa);

    zio_phys_bp_init(vd, &blk, offset, size, checksum, labels);

    zio = zio_create(pio, vd->vdev_spa, 0, &blk, data, size, done, private,
        ZIO_TYPE_READ, priority, flags | ZIO_FLAG_PHYSICAL,
        ZIO_STAGE_OPEN, ZIO_READ_PHYS_PIPELINE);

    zio->io_vd = vd;
    zio->io_offset = offset;

    /*
     * Work off our copy of the bp so the caller can free it.
     */
    zio->io_bp = &zio->io_bp_copy;

    return (zio);
}

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

    ZIO_ENTER(vd->vdev_spa);

    zio_phys_bp_init(vd, &blk, offset, size, checksum, labels);

    zio = zio_create(pio, vd->vdev_spa, 0, &blk, data, size, done, private,
        ZIO_TYPE_WRITE, priority, flags | ZIO_FLAG_PHYSICAL,
        ZIO_STAGE_OPEN, ZIO_WRITE_PHYS_PIPELINE);

    zio->io_vd = vd;
    zio->io_offset = offset;

    zio->io_bp = &zio->io_bp_copy;
    zio->io_checksum = checksum;

    if (zio_checksum_table[checksum].ci_zbt) {
        /*
         * zbt checksums are necessarily destructive -- they modify
         * one word 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.
         */
        wbuf = zio_buf_alloc(size);
        bcopy(data, wbuf, size);
        zio_push_transform(zio, wbuf, size, size);

        zbt = (zio_block_tail_t *)((char *)wbuf + size) - 1;
        zbt->zbt_cksum = blk.blk_cksum;
    }

    return (zio);
}

/*
 * Create a child I/O to do some work for us.  It has no associated bp.
 */
zio_t *
zio_vdev_child_io(zio_t *zio, blkptr_t *bp, vdev_t *vd, uint64_t offset,
    void *data, uint64_t size, int type, int priority, int flags,
    zio_done_func_t *done, void *private)
{
    uint32_t pipeline = ZIO_VDEV_CHILD_PIPELINE;
    zio_t *cio;

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

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

    cio->io_vd = vd;
    cio->io_offset = offset;

    return (cio);
}

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

    ASSERT(zio->io_stage == ZIO_STAGE_OPEN);

    zio->io_waiter = curthread;

    zio_next_stage_async(zio);

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

    error = zio->io_error;
    mutex_destroy(&zio->io_lock);
    cv_destroy(&zio->io_cv);
    kmem_cache_free(zio_cache, zio);

    return (error);
}

void
zio_nowait(zio_t *zio)
{
    zio_next_stage_async(zio);
}

/*
 * ==========================================================================
 * I/O pipeline interlocks: parent/child dependency scoreboarding
 * ==========================================================================
 */
static void
zio_wait_for_children(zio_t *zio, uint32_t stage, uint64_t *countp)
{
    mutex_enter(&zio->io_lock);
    if (*countp == 0) {
        ASSERT(zio->io_stalled == 0);
        mutex_exit(&zio->io_lock);
        zio_next_stage(zio);
    } else {
        zio->io_stalled = stage;
        mutex_exit(&zio->io_lock);
    }
}

static void
zio_notify_parent(zio_t *zio, uint32_t stage, uint64_t *countp)
{
    zio_t *pio = zio->io_parent;

    mutex_enter(&pio->io_lock);
    if (pio->io_error == 0 && !(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE))
        pio->io_error = zio->io_error;
    ASSERT3U(*countp, >, 0);
    if (--*countp == 0 && pio->io_stalled == stage) {
        pio->io_stalled = 0;
        mutex_exit(&pio->io_lock);
        zio_next_stage_async(pio);
    } else {
        mutex_exit(&pio->io_lock);
    }
}

static void
zio_wait_children_ready(zio_t *zio)
{
    zio_wait_for_children(zio, ZIO_STAGE_WAIT_CHILDREN_READY,
        &zio->io_children_notready);
}

void
zio_wait_children_done(zio_t *zio)
{
    zio_wait_for_children(zio, ZIO_STAGE_WAIT_CHILDREN_DONE,
        &zio->io_children_notdone);
}

static void
zio_read_init(zio_t *zio)
{
    if (BP_GET_COMPRESS(zio->io_bp) != ZIO_COMPRESS_OFF) {
        uint64_t csize = BP_GET_PSIZE(zio->io_bp);
        void *cbuf = zio_buf_alloc(csize);

        zio_push_transform(zio, cbuf, csize, csize);
        zio->io_pipeline |= 1U << ZIO_STAGE_READ_DECOMPRESS;
    }

    if (BP_IS_GANG(zio->io_bp)) {
        uint64_t gsize = SPA_GANGBLOCKSIZE;
        void *gbuf = zio_buf_alloc(gsize);

        zio_push_transform(zio, gbuf, gsize, gsize);
        zio->io_pipeline |= 1U << ZIO_STAGE_READ_GANG_MEMBERS;
    }
    zio_next_stage(zio);
}

static void
zio_ready(zio_t *zio)
{
    zio_t *pio = zio->io_parent;

    if (zio->io_ready)
        zio->io_ready(zio);

    if (pio != NULL)
        zio_notify_parent(zio, ZIO_STAGE_WAIT_CHILDREN_READY,
            &pio->io_children_notready);

    if (zio->io_bp)
        zio->io_bp_copy = *zio->io_bp;

    zio_next_stage(zio);
}

static void
zio_vdev_retry_io(zio_t *zio)
{
    zio_t *pio = zio->io_parent;

    /*
     * Preserve the failed bp so that the io_ready() callback can
     * update the accounting accordingly. The callback will also be
     * responsible for freeing the previously allocated block, if one
     * exists.
     */
    zio->io_bp_orig = *zio->io_bp;

    /*
     * We must zero out the old DVA and blk_birth before reallocating
     * the bp.
     */
    BP_ZERO_DVAS(zio->io_bp);
    zio_reset(zio);

    if (pio) {
        /*
         * Let the parent know that we will
         * re-alloc the write (=> new bp info).
         */
        mutex_enter(&pio->io_lock);
        pio->io_children_notready++;

        /*
         * If the parent I/O is still in the open stage, then
         * don't bother telling it to retry since it hasn't
         * progressed far enough for it to care.
         */
        if (pio->io_stage > ZIO_STAGE_OPEN && IO_IS_ALLOCATING(pio))
            pio->io_flags |= ZIO_FLAG_WRITE_RETRY;

        ASSERT(pio->io_stage <= ZIO_STAGE_WAIT_CHILDREN_DONE);
        mutex_exit(&pio->io_lock);
    }

    /*
     * We are getting ready to process the retry request so clear
     * the flag and the zio's current error status.
     */
    zio->io_flags &= ~ZIO_FLAG_WRITE_RETRY;
    zio->io_error = 0;
    zio_next_stage_async(zio);
}

int
zio_vdev_resume_io(spa_t *spa)
{
    zio_t *zio;

    mutex_enter(&spa->spa_zio_lock);

    /*
     * Probe all of vdevs that have experienced an I/O error.
     * If we are still unable to verify the integrity of the vdev
     * then we prevent the resume from proceeeding.
     */
    for (zio = list_head(&spa->spa_zio_list); zio != NULL;
        zio = list_next(&spa->spa_zio_list, zio)) {
        int error = 0;

        /* We only care about I/Os that must succeed */
        if (zio->io_vd == NULL || zio->io_flags & ZIO_FLAG_CANFAIL)
            continue;
        error = vdev_probe(zio->io_vd);
        if (error) {
            mutex_exit(&spa->spa_zio_lock);
            return (error);
        }
    }

    /*
     * Clear the vdev stats so that I/O can flow.
     */
    vdev_clear(spa, NULL, B_FALSE);

    spa->spa_state = POOL_STATE_ACTIVE;
    while ((zio = list_head(&spa->spa_zio_list)) != NULL) {
        list_remove(&spa->spa_zio_list, zio);
        zio->io_error = 0;

        /*
         * If we are resuming an allocating I/O then we force it
         * to retry and let it resume operation where it left off.
         * Otherwise, go back to the ready stage and pick up from
         * there.
         */
        if (zio_write_retry && IO_IS_ALLOCATING(zio)) {
            zio->io_flags |= ZIO_FLAG_WRITE_RETRY;
            zio->io_stage--;
        } else {
            zio->io_stage = ZIO_STAGE_READY;
        }

        (void) taskq_dispatch(zio_taskq, zio_resubmit_stage_async,
            zio, TQ_SLEEP);
    }
    mutex_exit(&spa->spa_zio_lock);

    /*
     * Wait for the taskqs to finish and recheck the pool state since
     * it's possible that a resumed I/O has failed again.
     */
    taskq_wait(zio_taskq);
    if (spa_state(spa) == POOL_STATE_IO_FAILURE)
        return (EIO);

    mutex_enter(&spa->spa_zio_lock);
    cv_broadcast(&spa->spa_zio_cv);
    mutex_exit(&spa->spa_zio_lock);

    return (0);
}

static void
zio_vdev_suspend_io(zio_t *zio)
{
    spa_t *spa = zio->io_spa;

    /*
     * We've experienced an unrecoverable failure so
     * set the pool state accordingly and queue all
     * failed IOs.
     */
    spa->spa_state = POOL_STATE_IO_FAILURE;

    mutex_enter(&spa->spa_zio_lock);
    list_insert_tail(&spa->spa_zio_list, zio);

#ifndef _KERNEL
    /* Used to notify ztest that the pool has suspended */
    cv_broadcast(&spa->spa_zio_cv);
#endif
    mutex_exit(&spa->spa_zio_lock);
}

static void
zio_assess(zio_t *zio)
{
    spa_t *spa = zio->io_spa;
    blkptr_t *bp = zio->io_bp;
    vdev_t *vd = zio->io_vd;

    ASSERT(zio->io_children_notready == 0);
    ASSERT(zio->io_children_notdone == 0);

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

    /*
     * Some child I/O has indicated that a retry is necessary, so
     * we set an error on the I/O and let the logic below do the
     * rest.
     */
    if (zio->io_flags & ZIO_FLAG_WRITE_RETRY)
        zio->io_error = ERESTART;

    if (vd != NULL)
        vdev_stat_update(zio);

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

        if ((zio->io_error == EIO ||
            !(zio->io_flags & ZIO_FLAG_SPECULATIVE)) &&
            zio->io_logical == zio) {
            /*
             * For root I/O requests, tell the SPA to log the error
             * appropriately.  Also, generate a logical data
             * ereport.
             */
            spa_log_error(spa, zio);

            zfs_ereport_post(FM_EREPORT_ZFS_DATA, spa, NULL, zio,
                0, 0);
        }

        /*
         * If we are an allocating I/O then we attempt to reissue
         * the I/O on another vdev unless the pool is out of space.
         * We handle this condition based on the spa's failmode
         * property.
         */
        if (zio_write_retry && zio->io_error != ENOSPC &&
            IO_IS_ALLOCATING(zio)) {
            zio_vdev_retry_io(zio);
            return;
        }
        ASSERT(!(zio->io_flags & ZIO_FLAG_WRITE_RETRY));

        /*
         * For I/O requests that cannot fail, we carry out
         * the requested behavior based on the failmode pool
         * property.
         *
         * XXX - Need to differentiate between an ENOSPC as
         * a result of vdev failures vs. a full pool.
         */
        if (!(zio->io_flags & ZIO_FLAG_CANFAIL)) {
            char *blkbuf;

#ifdef ZFS_DEBUG
            blkbuf = kmem_alloc(BP_SPRINTF_LEN, KM_NOSLEEP);
            if (blkbuf) {
                sprintf_blkptr(blkbuf, BP_SPRINTF_LEN,
                    bp ? bp : &zio->io_bp_copy);
            }
            cmn_err(CE_WARN, "ZFS: %s (%s on %s off %llx: zio %p "
                "%s): error %d", zio->io_error == ECKSUM ?
                "bad checksum" : "I/O failure",
                zio_type_name[zio->io_type],
                vdev_description(vd),
                (u_longlong_t)zio->io_offset,
                (void *)zio, blkbuf ? blkbuf : "", zio->io_error);
#endif

            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));
            } else {
                cmn_err(CE_WARN, "Pool '%s' has encountered "
                    "an uncorrectable I/O error. Manual "
                    "intervention is required.",
                    spa_name(spa));
                zio_vdev_suspend_io(zio);
            }
            return;
        }
    }
    ASSERT(!(zio->io_flags & ZIO_FLAG_WRITE_RETRY));
    ASSERT(zio->io_children_notready == 0);
    zio_next_stage(zio);
}

static void
zio_done(zio_t *zio)
{
    zio_t *pio = zio->io_parent;
    spa_t *spa = zio->io_spa;

    ASSERT(zio->io_children_notready == 0);
    ASSERT(zio->io_children_notdone == 0);

    zio_clear_transform_stack(zio);

    if (zio->io_done)
        zio->io_done(zio);

    ASSERT(zio->io_delegate_list == NULL);
    ASSERT(zio->io_delegate_next == NULL);

    if (pio != NULL) {
        zio_t *next, *prev;

        mutex_enter(&pio->io_lock);
        next = zio->io_sibling_next;
        prev = zio->io_sibling_prev;
        if (next != NULL)
            next->io_sibling_prev = prev;
        if (prev != NULL)
            prev->io_sibling_next = next;
        if (pio->io_child == zio)
            pio->io_child = next;
        mutex_exit(&pio->io_lock);

        zio_notify_parent(zio, ZIO_STAGE_WAIT_CHILDREN_DONE,
            &pio->io_children_notdone);
    }

    /*
     * Note: this I/O is now done, and will shortly be freed, so there is no
     * need to clear this (or any other) flag.
     */
    if (zio->io_flags & ZIO_FLAG_CONFIG_GRABBED)
        spa_config_exit(spa, zio);

    if (zio->io_waiter != NULL) {
        mutex_enter(&zio->io_lock);
        ASSERT(zio->io_stage == ZIO_STAGE_DONE);
        zio->io_stalled = zio->io_stage;
        cv_broadcast(&zio->io_cv);
        mutex_exit(&zio->io_lock);
    } else {
        mutex_destroy(&zio->io_lock);
        cv_destroy(&zio->io_cv);
        kmem_cache_free(zio_cache, zio);
    }
}

/*
 * ==========================================================================
 * Compression support
 * ==========================================================================
 */
static void
zio_write_compress(zio_t *zio)
{
    int compress = zio->io_compress;
    blkptr_t *bp = zio->io_bp;
    void *cbuf;
    uint64_t lsize = zio->io_size;
    uint64_t csize = lsize;
    uint64_t cbufsize = 0;
    int pass;

    if (bp->blk_birth == zio->io_txg) {
        /*
         * We're rewriting an existing block, which means we're
         * working on behalf of spa_sync().  For spa_sync() to
         * converge, it must eventually be the case that we don't
         * have to allocate new blocks.  But compression changes
         * the blocksize, which forces a reallocate, and makes
         * convergence take longer.  Therefore, after the first
         * few passes, stop compressing to ensure convergence.
         */
        pass = spa_sync_pass(zio->io_spa);
        if (pass > zio_sync_pass.zp_dontcompress)
            compress = ZIO_COMPRESS_OFF;
    } else {
        ASSERT(BP_IS_HOLE(bp));
        pass = 1;
    }

    if (compress != ZIO_COMPRESS_OFF)
        if (!zio_compress_data(compress, zio->io_data, zio->io_size,
            &cbuf, &csize, &cbufsize))
            compress = ZIO_COMPRESS_OFF;

    if (compress != ZIO_COMPRESS_OFF && csize != 0)
        zio_push_transform(zio, cbuf, csize, cbufsize);

    /*
     * 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 reallocate new blocks, but force rewrites after that.
     * There should only be a handful of blocks after pass 1 in any case.
     */
    if (bp->blk_birth == zio->io_txg && BP_GET_PSIZE(bp) == csize &&
        pass > zio_sync_pass.zp_rewrite) {
        ASSERT(csize != 0);
        BP_SET_LSIZE(bp, lsize);
        BP_SET_COMPRESS(bp, compress);
        zio->io_pipeline = ZIO_REWRITE_PIPELINE;
    } else {
        if (bp->blk_birth == zio->io_txg)
            BP_ZERO(bp);
        if (csize == 0) {
            BP_ZERO(bp);
            zio->io_pipeline = ZIO_WAIT_FOR_CHILDREN_PIPELINE;
        } else {
            ASSERT3U(BP_GET_NDVAS(bp), ==, 0);
            BP_SET_LSIZE(bp, lsize);
            BP_SET_PSIZE(bp, csize);
            BP_SET_COMPRESS(bp, compress);
            zio->io_pipeline = ZIO_WRITE_ALLOCATE_PIPELINE;
        }
    }

    zio_next_stage(zio);
}

static void
zio_read_decompress(zio_t *zio)
{
    blkptr_t *bp = zio->io_bp;
    void *data;
    uint64_t size;
    uint64_t bufsize;
    int compress = BP_GET_COMPRESS(bp);

    ASSERT(compress != ZIO_COMPRESS_OFF);

    zio_pop_transform(zio, &data, &size, &bufsize);

    if (zio_decompress_data(compress, data, size,
        zio->io_data, zio->io_size))
        zio->io_error = EIO;

    zio_buf_free(data, bufsize);

    zio_next_stage(zio);
}

/*
 * ==========================================================================
 * Gang block support
 * ==========================================================================
 */
static void
zio_gang_pipeline(zio_t *zio)
{
    /*
     * By default, the pipeline assumes that we're dealing with a gang
     * block.  If we're not, strip out any gang-specific stages.
     */
    if (!BP_IS_GANG(zio->io_bp))
        zio->io_pipeline &= ~ZIO_GANG_STAGES;

    zio_next_stage(zio);
}

static void
zio_gang_byteswap(zio_t *zio)
{
    ASSERT(zio->io_size == SPA_GANGBLOCKSIZE);

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

static void
zio_get_gang_header(zio_t *zio)
{
    blkptr_t *bp = zio->io_bp;
    uint64_t gsize = SPA_GANGBLOCKSIZE;
    void *gbuf = zio_buf_alloc(gsize);

    ASSERT(BP_IS_GANG(bp));

    zio_push_transform(zio, gbuf, gsize, gsize);

    zio_nowait(zio_create(zio, zio->io_spa, bp->blk_birth, bp, gbuf, gsize,
        NULL, NULL, ZIO_TYPE_READ, zio->io_priority,
        zio->io_flags & ZIO_FLAG_GANG_INHERIT,
        ZIO_STAGE_OPEN, ZIO_READ_GANG_PIPELINE));

    zio_wait_children_done(zio);
}

static void
zio_read_gang_members(zio_t *zio)
{
    zio_gbh_phys_t *gbh;
    uint64_t gsize, gbufsize, loff, lsize;
    int i;

    ASSERT(BP_IS_GANG(zio->io_bp));

    zio_gang_byteswap(zio);
    zio_pop_transform(zio, (void **)&gbh, &gsize, &gbufsize);

    for (loff = 0, i = 0; loff != zio->io_size; loff += lsize, i++) {
        blkptr_t *gbp = &gbh->zg_blkptr[i];
        lsize = BP_GET_PSIZE(gbp);

        ASSERT(BP_GET_COMPRESS(gbp) == ZIO_COMPRESS_OFF);
        ASSERT3U(lsize, ==, BP_GET_LSIZE(gbp));
        ASSERT3U(loff + lsize, <=, zio->io_size);
        ASSERT(i < SPA_GBH_NBLKPTRS);
        ASSERT(!BP_IS_HOLE(gbp));

        zio_nowait(zio_read(zio, zio->io_spa, gbp,
            (char *)zio->io_data + loff, lsize, NULL, NULL,
            zio->io_priority, zio->io_flags & ZIO_FLAG_GANG_INHERIT,
            &zio->io_bookmark));
    }

    zio_buf_free(gbh, gbufsize);
    zio_wait_children_done(zio);
}

static void
zio_rewrite_gang_members(zio_t *zio)
{
    zio_gbh_phys_t *gbh;
    uint64_t gsize, gbufsize, loff, lsize;
    int i;

    ASSERT(BP_IS_GANG(zio->io_bp));
    ASSERT3U(zio->io_size, ==, SPA_GANGBLOCKSIZE);

    zio_gang_byteswap(zio);
    zio_pop_transform(zio, (void **)&gbh, &gsize, &gbufsize);

    ASSERT(gsize == gbufsize);

    for (loff = 0, i = 0; loff != zio->io_size; loff += lsize, i++) {
        blkptr_t *gbp = &gbh->zg_blkptr[i];
        lsize = BP_GET_PSIZE(gbp);

        ASSERT(BP_GET_COMPRESS(gbp) == ZIO_COMPRESS_OFF);
        ASSERT3U(lsize, ==, BP_GET_LSIZE(gbp));
        ASSERT3U(loff + lsize, <=, zio->io_size);
        ASSERT(i < SPA_GBH_NBLKPTRS);
        ASSERT(!BP_IS_HOLE(gbp));

        zio_nowait(zio_rewrite(zio, zio->io_spa, zio->io_checksum,
            zio->io_txg, gbp, (char *)zio->io_data + loff, lsize,
            NULL, NULL, zio->io_priority, zio->io_flags,
            &zio->io_bookmark));
    }

    zio_push_transform(zio, gbh, gsize, gbufsize);
    zio_wait_children_ready(zio);
}

static void
zio_free_gang_members(zio_t *zio)
{
    zio_gbh_phys_t *gbh;
    uint64_t gsize, gbufsize;
    int i;

    ASSERT(BP_IS_GANG(zio->io_bp));

    zio_gang_byteswap(zio);
    zio_pop_transform(zio, (void **)&gbh, &gsize, &gbufsize);

    for (i = 0; i < SPA_GBH_NBLKPTRS; i++) {
        blkptr_t *gbp = &gbh->zg_blkptr[i];

        if (BP_IS_HOLE(gbp))
            continue;
        zio_nowait(zio_free(zio, zio->io_spa, zio->io_txg,
            gbp, NULL, NULL));
    }

    zio_buf_free(gbh, gbufsize);
    zio_next_stage(zio);
}

static void
zio_claim_gang_members(zio_t *zio)
{
    zio_gbh_phys_t *gbh;
    uint64_t gsize, gbufsize;
    int i;

    ASSERT(BP_IS_GANG(zio->io_bp));

    zio_gang_byteswap(zio);
    zio_pop_transform(zio, (void **)&gbh, &gsize, &gbufsize);

    for (i = 0; i < SPA_GBH_NBLKPTRS; i++) {
        blkptr_t *gbp = &gbh->zg_blkptr[i];
        if (BP_IS_HOLE(gbp))
            continue;
        zio_nowait(zio_claim(zio, zio->io_spa, zio->io_txg,
            gbp, NULL, NULL));
    }

    zio_buf_free(gbh, gbufsize);
    zio_next_stage(zio);
}

static void
zio_write_allocate_gang_member_done(zio_t *zio)
{
    zio_t *pio = zio->io_parent;
    dva_t *cdva = zio->io_bp->blk_dva;
    dva_t *pdva = pio->io_bp->blk_dva;
    uint64_t asize;
    int d;

    ASSERT3U(pio->io_ndvas, ==, zio->io_ndvas);
    ASSERT3U(BP_GET_NDVAS(zio->io_bp), <=, BP_GET_NDVAS(pio->io_bp));
    ASSERT3U(zio->io_ndvas, <=, BP_GET_NDVAS(zio->io_bp));
    ASSERT3U(pio->io_ndvas, <=, BP_GET_NDVAS(pio->io_bp));

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

static int
zio_write_allocate_gang_members(zio_t *zio, metaslab_class_t *mc)
{
    blkptr_t *bp = zio->io_bp;
    dva_t *dva = bp->blk_dva;
    spa_t *spa = zio->io_spa;
    zio_gbh_phys_t *gbh;
    uint64_t txg = zio->io_txg;
    uint64_t resid = zio->io_size;
    uint64_t maxalloc = P2ROUNDUP(zio->io_size >> 1, SPA_MINBLOCKSIZE);
    uint64_t gsize, loff, lsize;
    uint32_t gbps_left;
    int ndvas = zio->io_ndvas;
    int gbh_ndvas = MIN(ndvas + 1, spa_max_replication(spa));
    int error;
    int i, d;

    gsize = SPA_GANGBLOCKSIZE;
    gbps_left = SPA_GBH_NBLKPTRS;

    error = metaslab_alloc(spa, mc, gsize, bp, gbh_ndvas, txg, NULL,
        B_FALSE);
    if (error)
        return (error);

    for (d = 0; d < gbh_ndvas; d++)
        DVA_SET_GANG(&dva[d], 1);

    bp->blk_birth = txg;

    gbh = zio_buf_alloc(gsize);
    bzero(gbh, gsize);

    /* We need to test multi-level gang blocks */
    if (maxalloc >= zio_gang_bang && (lbolt & 0x1) == 0)
        maxalloc = MAX(maxalloc >> 2, SPA_MINBLOCKSIZE);

    for (loff = 0, i = 0; loff != zio->io_size;
        loff += lsize, resid -= lsize, gbps_left--, i++) {
        blkptr_t *gbp = &gbh->zg_blkptr[i];
        dva = gbp->blk_dva;

        ASSERT(gbps_left != 0);
        maxalloc = MIN(maxalloc, resid);

        while (resid <= maxalloc * gbps_left) {
            error = metaslab_alloc(spa, mc, maxalloc, gbp, ndvas,
                txg, bp, B_FALSE);
            if (error == 0)
                break;
            ASSERT3U(error, ==, ENOSPC);
            /* XXX - free up previous allocations? */
            if (maxalloc == SPA_MINBLOCKSIZE)
                return (error);
            maxalloc = P2ROUNDUP(maxalloc >> 1, SPA_MINBLOCKSIZE);
        }

        if (resid <= maxalloc * gbps_left) {
            lsize = maxalloc;
            BP_SET_LSIZE(gbp, lsize);
            BP_SET_PSIZE(gbp, lsize);
            BP_SET_COMPRESS(gbp, ZIO_COMPRESS_OFF);
            gbp->blk_birth = txg;
            zio_nowait(zio_rewrite(zio, spa,
                zio->io_checksum, txg, gbp,
                (char *)zio->io_data + loff, lsize,
                zio_write_allocate_gang_member_done, NULL,
                zio->io_priority,
                zio->io_flags & ZIO_FLAG_GANG_INHERIT,
                &zio->io_bookmark));
        } else {
            lsize = P2ROUNDUP(resid / gbps_left, SPA_MINBLOCKSIZE);
            ASSERT(lsize != SPA_MINBLOCKSIZE);
            zio_nowait(zio_write_allocate(zio, spa,
                zio->io_checksum, txg, gbp,
                (char *)zio->io_data + loff, lsize,
                zio_write_allocate_gang_member_done, NULL,
                zio->io_priority,
                zio->io_flags & ZIO_FLAG_GANG_INHERIT));
        }
    }

    ASSERT(resid == 0 && loff == zio->io_size);

    zio->io_pipeline |= 1U << ZIO_STAGE_GANG_CHECKSUM_GENERATE;

    zio_push_transform(zio, gbh, gsize, gsize);
    /*
     * As much as we'd like this to be zio_wait_children_ready(),
     * updating our ASIZE doesn't happen until the io_done callback,
     * so we have to wait for that to finish in order for our BP
     * to be stable.
     */
    zio_wait_children_done(zio);
    return (0);
}

/*
 * ==========================================================================
 * Allocate and free blocks
 * ==========================================================================
 */
static void
zio_dva_allocate(zio_t *zio)
{
    spa_t *spa = zio->io_spa;
    metaslab_class_t *mc = spa->spa_normal_class;
    blkptr_t *bp = zio->io_bp;
    int error;

    ASSERT(BP_IS_HOLE(bp));
    ASSERT3U(BP_GET_NDVAS(bp), ==, 0);
    ASSERT3U(zio->io_ndvas, >, 0);
    ASSERT3U(zio->io_ndvas, <=, spa_max_replication(spa));

    /* For testing, make some blocks above a certain size be gang blocks */
    if (zio->io_size >= zio_gang_bang && (lbolt & 0x3) == 0) {
        error = zio_write_allocate_gang_members(zio, mc);
        if (error)
            zio->io_error = error;
        return;
    }

    /*
     * For testing purposes, we force I/Os to retry. We don't allow
     * retries beyond the first pass since those I/Os are non-allocating
     * writes.
     */
    if (zio_io_fail_shift &&
        spa_sync_pass(zio->io_spa) <= zio_sync_pass.zp_rewrite &&
        zio_io_should_fail(zio_io_fail_shift))
        zio->io_flags |= ZIO_FLAG_WRITE_RETRY;

    ASSERT3U(zio->io_size, ==, BP_GET_PSIZE(bp));

    error = metaslab_alloc(spa, mc, zio->io_size, bp, zio->io_ndvas,
        zio->io_txg, NULL, B_FALSE);

    if (error == 0) {
        bp->blk_birth = zio->io_txg;
    } else if (error == ENOSPC && zio->io_size > SPA_MINBLOCKSIZE) {
        error = zio_write_allocate_gang_members(zio, mc);
        if (error == 0)
            return;
        zio->io_error = error;
    } else {
        zio->io_error = error;
    }
    zio_next_stage(zio);
}

static void
zio_dva_free(zio_t *zio)
{
    blkptr_t *bp = zio->io_bp;

    metaslab_free(zio->io_spa, bp, zio->io_txg, B_FALSE);

    BP_ZERO(bp);

    zio_next_stage(zio);
}

static void
zio_dva_claim(zio_t *zio)
{
    zio->io_error = metaslab_claim(zio->io_spa, zio->io_bp, zio->io_txg);

    zio_next_stage(zio);
}

/*
 * ==========================================================================
 * Read and write to physical devices
 * ==========================================================================
 */

static void
zio_vdev_io_start(zio_t *zio)
{
    vdev_t *vd = zio->io_vd;
    vdev_t *tvd = vd ? vd->vdev_top : NULL;
    blkptr_t *bp = zio->io_bp;
    uint64_t align;
    spa_t *spa = zio->io_spa;

    /*
     * If the pool is already in a failure state then just suspend
     * this IO until the problem is resolved. We will reissue them
     * at that time.
     */
    if (spa_state(spa) == POOL_STATE_IO_FAILURE &&
        zio->io_type == ZIO_TYPE_WRITE) {
        zio_vdev_suspend_io(zio);
        return;
    }

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

    align = 1ULL << tvd->vdev_ashift;

    if (zio->io_retries == 0 && vd == tvd)
        zio->io_flags |= ZIO_FLAG_FAILFAST;

    if (!(zio->io_flags & ZIO_FLAG_PHYSICAL) &&
        vd->vdev_children == 0) {
        zio->io_flags |= ZIO_FLAG_PHYSICAL;
        zio->io_offset += VDEV_LABEL_START_SIZE;
    }

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

    ASSERT(P2PHASE(zio->io_offset, align) == 0);
    ASSERT(P2PHASE(zio->io_size, align) == 0);
    ASSERT(bp == NULL ||
        P2ROUNDUP(ZIO_GET_IOSIZE(zio), align) == zio->io_size);
    ASSERT(zio->io_type != ZIO_TYPE_WRITE || (spa_mode & FWRITE));

    vdev_io_start(zio);

    /* zio_next_stage_async() gets called from io completion interrupt */
}

static void
zio_vdev_io_done(zio_t *zio)
{
    if (zio->io_vd == NULL)
        /* The mirror_ops handle multiple DVAs in a single BP */
        vdev_mirror_ops.vdev_op_io_done(zio);
    else
        vdev_io_done(zio);
}

/* XXPOLICY */
boolean_t
zio_should_retry(zio_t *zio)
{
    vdev_t *vd = zio->io_vd;

    if (zio->io_error == 0)
        return (B_FALSE);
    if (zio->io_delegate_list != NULL)
        return (B_FALSE);
    if (vd && vd != vd->vdev_top)
        return (B_FALSE);
    if (zio->io_flags & ZIO_FLAG_DONT_RETRY)
        return (B_FALSE);
    if (zio->io_retries > 0)
        return (B_FALSE);

    return (B_TRUE);
}

static void
zio_vdev_io_assess(zio_t *zio)
{
    vdev_t *vd = zio->io_vd;
    vdev_t *tvd = vd ? vd->vdev_top : NULL;

    ASSERT(zio->io_vsd == NULL);

    if (zio->io_flags & ZIO_FLAG_SUBBLOCK) {
        void *abuf;
        uint64_t asize;
        ASSERT(vd == tvd);
        zio_pop_transform(zio, &abuf, &asize, &asize);
        if (zio->io_type == ZIO_TYPE_READ)
            bcopy(abuf, zio->io_data, zio->io_size);
        zio_buf_free(abuf, asize);
        zio->io_flags &= ~ZIO_FLAG_SUBBLOCK;
    }

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

    /*
     * If the I/O failed, determine whether we should attempt to retry it.
     */
    /* XXPOLICY */
    if (zio_should_retry(zio)) {
        ASSERT(tvd == vd);

        zio->io_retries++;
        zio->io_error = 0;
        zio->io_flags &= ZIO_FLAG_VDEV_INHERIT |
            ZIO_FLAG_CONFIG_GRABBED;
        /* XXPOLICY */
        zio->io_flags &= ~ZIO_FLAG_FAILFAST;
        zio->io_flags |= ZIO_FLAG_DONT_CACHE;
        zio->io_stage = ZIO_STAGE_VDEV_IO_START - 1;

        dprintf("retry #%d for %s to %s offset %llx\n",
            zio->io_retries, zio_type_name[zio->io_type],
            vdev_description(vd), zio->io_offset);

        zio_next_stage_async(zio);
        return;
    }

    zio_next_stage(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--;
}

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

    zio->io_stage--;
}

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

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

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

    ASSERT3U(zio->io_size, ==, BP_GET_PSIZE(bp));

    BP_SET_CHECKSUM(bp, checksum);
    BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);

    zio_checksum(checksum, &bp->blk_cksum, zio->io_data, zio->io_size);

    zio_next_stage(zio);
}

static void
zio_gang_checksum_generate(zio_t *zio)
{
    zio_cksum_t zc;
    zio_gbh_phys_t *gbh = zio->io_data;

    ASSERT(BP_IS_GANG(zio->io_bp));
    ASSERT3U(zio->io_size, ==, SPA_GANGBLOCKSIZE);

    zio_set_gang_verifier(zio, &gbh->zg_tail.zbt_cksum);

    zio_checksum(ZIO_CHECKSUM_GANG_HEADER, &zc, zio->io_data, zio->io_size);

    zio_next_stage(zio);
}

static void
zio_checksum_verify(zio_t *zio)
{
    if (zio->io_bp != NULL) {
        zio->io_error = zio_checksum_error(zio);
        if (zio->io_error && !(zio->io_flags & ZIO_FLAG_SPECULATIVE))
            zfs_ereport_post(FM_EREPORT_ZFS_CHECKSUM,
                zio->io_spa, zio->io_vd, zio, 0, 0);
    }

    zio_next_stage(zio);
}

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

/*
 * Set the external verifier for a gang block based on stuff in the bp
 */
void
zio_set_gang_verifier(zio_t *zio, zio_cksum_t *zcp)
{
    blkptr_t *bp = zio->io_bp;

    zcp->zc_word[0] = DVA_GET_VDEV(BP_IDENTITY(bp));
    zcp->zc_word[1] = DVA_GET_OFFSET(BP_IDENTITY(bp));
    zcp->zc_word[2] = bp->blk_birth;
    zcp->zc_word[3] = 0;
}

/*
 * ==========================================================================
 * Define the pipeline
 * ==========================================================================
 */
typedef void zio_pipe_stage_t(zio_t *zio);

static void
zio_badop(zio_t *zio)
{
    panic("Invalid I/O pipeline stage %u for zio %p", zio->io_stage, zio);
}

zio_pipe_stage_t *zio_pipeline[ZIO_STAGE_DONE + 2] = {
    zio_badop,
    zio_wait_children_ready,
    zio_write_compress,
    zio_checksum_generate,
    zio_gang_pipeline,
    zio_get_gang_header,
    zio_rewrite_gang_members,
    zio_free_gang_members,
    zio_claim_gang_members,
    zio_dva_allocate,
    zio_dva_free,
    zio_dva_claim,
    zio_gang_checksum_generate,
    zio_ready,
    zio_read_init,
    zio_vdev_io_start,
    zio_vdev_io_done,
    zio_vdev_io_assess,
    zio_wait_children_done,
    zio_checksum_verify,
    zio_read_gang_members,
    zio_read_decompress,
    zio_assess,
    zio_done,
    zio_badop
};

/*
 * Move an I/O to the next stage of the pipeline and execute that stage.
 * 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.
 */
void
zio_next_stage(zio_t *zio)
{
    uint32_t pipeline = zio->io_pipeline;

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

    if (zio->io_error) {
        dprintf("zio %p vdev %s offset %llx stage %d error %d\n",
            zio, vdev_description(zio->io_vd),
            zio->io_offset, zio->io_stage, zio->io_error);
        if (((1U << zio->io_stage) & ZIO_VDEV_IO_PIPELINE) == 0)
            pipeline &= ZIO_ERROR_PIPELINE_MASK;
    }

    while (((1U << ++zio->io_stage) & pipeline) == 0)
        continue;

    ASSERT(zio->io_stage <= ZIO_STAGE_DONE);
    ASSERT(zio->io_stalled == 0);

    /*
     * See the comment in zio_next_stage_async() about per-CPU taskqs.
     */
    if (((1U << zio->io_stage) & zio->io_async_stages) &&
        (zio->io_stage == ZIO_STAGE_WRITE_COMPRESS) &&
        !(zio->io_flags & ZIO_FLAG_METADATA)) {
        taskq_t *tq = zio->io_spa->spa_zio_issue_taskq[zio->io_type];
        (void) taskq_dispatch(tq,
            (task_func_t *)zio_pipeline[zio->io_stage], zio, TQ_SLEEP);
    } else {
        zio_pipeline[zio->io_stage](zio);
    }
}

void
zio_next_stage_async(zio_t *zio)
{
    taskq_t *tq;
    uint32_t pipeline = zio->io_pipeline;

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

    if (zio->io_error) {
        dprintf("zio %p vdev %s offset %llx stage %d error %d\n",
            zio, vdev_description(zio->io_vd),
            zio->io_offset, zio->io_stage, zio->io_error);
        if (((1U << zio->io_stage) & ZIO_VDEV_IO_PIPELINE) == 0)
            pipeline &= ZIO_ERROR_PIPELINE_MASK;
    }

    while (((1U << ++zio->io_stage) & pipeline) == 0)
        continue;

    ASSERT(zio->io_stage <= ZIO_STAGE_DONE);
    ASSERT(zio->io_stalled == 0);

    /*
     * For performance, we'll probably want two sets of task queues:
     * per-CPU issue taskqs and per-CPU completion taskqs.  The per-CPU
     * part is for read performance: since we have to make a pass over
     * the data to checksum it anyway, we want to do this on the same CPU
     * that issued the read, because (assuming CPU scheduling affinity)
     * that thread is probably still there.  Getting this optimization
     * right avoids performance-hostile cache-to-cache transfers.
     *
     * Note that having two sets of task queues is also necessary for
     * correctness: if all of the issue threads get bogged down waiting
     * for dependent reads (e.g. metaslab freelist) to complete, then
     * there won't be any threads available to service I/O completion
     * interrupts.
     */
    if ((1U << zio->io_stage) & zio->io_async_stages) {
        if (zio->io_stage < ZIO_STAGE_VDEV_IO_DONE)
            tq = zio->io_spa->spa_zio_issue_taskq[zio->io_type];
        else
            tq = zio->io_spa->spa_zio_intr_taskq[zio->io_type];
        (void) taskq_dispatch(tq,
            (task_func_t *)zio_pipeline[zio->io_stage], zio, TQ_SLEEP);
    } else {
        zio_pipeline[zio->io_stage](zio);
    }
}

void
zio_resubmit_stage_async(void *arg)
{
    zio_t *zio = (zio_t *)(uintptr_t)arg;

    zio_next_stage_async(zio);
}

static boolean_t
zio_io_should_fail(uint16_t range)
{
    static uint16_t allocs = 0;

    return (P2PHASE(allocs++, 1U<<range) == 0);
}

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

    spa_config_enter(spa, RW_READER, FTAG);

    if (zio_zil_fail_shift && zio_io_should_fail(zio_zil_fail_shift)) {
        spa_config_exit(spa, FTAG);
        return (ENOSPC);
    }

    /*
     * We were passed the previous log block's DVA in bp->blk_dva[0].
     * We use that as a hint for which vdev to allocate from next.
     */
    error = metaslab_alloc(spa, spa->spa_log_class, size,
        new_bp, 1, txg, old_bp, B_TRUE);

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

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

    spa_config_exit(spa, FTAG);

    return (error);
}

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

    spa_config_enter(spa, RW_READER, FTAG);

    metaslab_free(spa, bp, txg, B_FALSE);

    spa_config_exit(spa, FTAG);
}

/*
 * start an async flush of the write cache for this vdev
 */
void
zio_flush_vdev(spa_t *spa, uint64_t vdev, zio_t **zio)
{
    vdev_t *vd;

    /*
     * Lock out configuration changes.
     */
    spa_config_enter(spa, RW_READER, FTAG);

    if (*zio == NULL)
        *zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);

    vd = vdev_lookup_top(spa, vdev);
    ASSERT(vd);

    (void) zio_nowait(zio_ioctl(*zio, spa, vd, DKIOCFLUSHWRITECACHE,
        NULL, NULL, ZIO_PRIORITY_NOW,
        ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_RETRY));

    spa_config_exit(spa, FTAG);
}