fs/zfs/zio.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or 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 2008 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" };

/* 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 & (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);

		}
	}

	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_PUSHPAGE));
}

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

	ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

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

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

	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;
	}
	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_timestamp = lbolt64;
	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(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(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 (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, 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, bp->blk_birth, bp, data, size, done, private,
	    ZIO_TYPE_WRITE, priority, flags | ZIO_FLAG_USER,
	    ZIO_STAGE_OPEN, ZIO_REWRITE_PIPELINE(bp));

	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(bp));

	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(bp));

	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
 * ==========================================================================
 */
static void
zio_destroy(zio_t *zio)
{
	mutex_destroy(&zio->io_lock);
	cv_destroy(&zio->io_cv);
	if (zio->io_failed_vds != NULL) {
		kmem_free(zio->io_failed_vds,
		    zio->io_failed_vds_count * sizeof (vdev_t *));
		zio->io_failed_vds = NULL;
		zio->io_failed_vds_count = 0;
	}
	kmem_cache_free(zio_cache, zio);
}

int
zio_wait(zio_t *zio)
{
	int error;

	ASSERT(zio->io_stage == ZIO_STAGE_OPEN);

	zio->io_waiter = curthread;

	zio_execute(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;
	zio_destroy(zio);

	return (error);
}

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

void
zio_interrupt(zio_t *zio)
{
	(void) taskq_dispatch(zio->io_spa->spa_zio_intr_taskq[zio->io_type],
	    (task_func_t *)zio_execute, zio, TQ_SLEEP);
}

static int
zio_issue_async(zio_t *zio)
{
	(void) taskq_dispatch(zio->io_spa->spa_zio_issue_taskq[zio->io_type],
	    (task_func_t *)zio_execute, zio, TQ_SLEEP);

	return (ZIO_PIPELINE_STOP);
}

/*
 * ==========================================================================
 * I/O pipeline interlocks: parent/child dependency scoreboarding
 * ==========================================================================
 */
static int
zio_wait_for_children(zio_t *zio, uint32_t stage, uint64_t *countp)
{
	int rv = ZIO_PIPELINE_CONTINUE;

	mutex_enter(&zio->io_lock);
	ASSERT(zio->io_stalled == 0);
	if (*countp != 0) {
		zio->io_stalled = stage;
		rv = ZIO_PIPELINE_STOP;
	}
	mutex_exit(&zio->io_lock);

	return (rv);
}

static void
zio_add_failed_vdev(zio_t *pio, zio_t *zio)
{
	uint64_t oldcount = pio->io_failed_vds_count;
	vdev_t **new_vds;
	int i;

	ASSERT(MUTEX_HELD(&pio->io_lock));

	if (zio->io_vd == NULL)
		return;

	for (i = 0; i < oldcount; i++) {
		if (pio->io_failed_vds[i] == zio->io_vd)
			return;
	}

	new_vds = kmem_zalloc((oldcount + 1) * sizeof (vdev_t *), KM_SLEEP);
	if (pio->io_failed_vds != NULL) {
		bcopy(pio->io_failed_vds, new_vds,
		    oldcount * sizeof (vdev_t *));
		kmem_free(pio->io_failed_vds, oldcount * sizeof (vdev_t *));
	}
	pio->io_failed_vds = new_vds;
	pio->io_failed_vds[oldcount] = zio->io_vd;
	pio->io_failed_vds_count++;
}

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;
		if (zio->io_error && zio->io_error != ENOTSUP)
			zio_add_failed_vdev(pio, zio);
	}
	ASSERT3U(*countp, >, 0);
	if (--*countp == 0 && pio->io_stalled == stage) {
		pio->io_stalled = 0;
		mutex_exit(&pio->io_lock);
		zio_execute(pio);
	} else {
		mutex_exit(&pio->io_lock);
	}
}

int
zio_wait_for_children_ready(zio_t *zio)
{
	return (zio_wait_for_children(zio, ZIO_STAGE_WAIT_FOR_CHILDREN_READY,
	    &zio->io_children_notready));
}

int
zio_wait_for_children_done(zio_t *zio)
{
	return (zio_wait_for_children(zio, ZIO_STAGE_WAIT_FOR_CHILDREN_DONE,
	    &zio->io_children_notdone));
}

static int
zio_read_init(zio_t *zio)
{
	blkptr_t *bp = zio->io_bp;

	if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF) {
		uint64_t csize = BP_GET_PSIZE(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(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;
	}

	if (!dmu_ot[BP_GET_TYPE(bp)].ot_metadata && BP_GET_LEVEL(bp) == 0)
		zio->io_flags |= ZIO_FLAG_DONT_CACHE;

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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_FOR_CHILDREN_READY,
		    &pio->io_children_notready);

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

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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_FOR_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;

	return (ZIO_PIPELINE_CONTINUE);
}

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, (task_func_t *)zio_execute,
		    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 int
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);

	return (ZIO_PIPELINE_STOP);
}

static void
zio_handle_io_failure(zio_t *zio, vdev_t *vd)
{
	spa_t *spa = zio->io_spa;
	blkptr_t *bp = zio->io_bp;
	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);
	if (blkbuf)
		kmem_free(blkbuf, BP_SPRINTF_LEN);
#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));
	}
	zfs_ereport_post(FM_EREPORT_ZFS_IO_FAILURE, spa, NULL, NULL, 0, 0);
	vdev_set_state(vd, vd == spa->spa_root_vdev ? B_TRUE : B_FALSE,
	    VDEV_STATE_FAULTED, VDEV_AUX_IO_FAILURE);
}

static int
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))
			return (zio_vdev_retry_io(zio));

		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)) {
			int i;

			for (i = 0; i < zio->io_failed_vds_count; i++) {
				zio_handle_io_failure(zio,
				    zio->io_failed_vds[i]);
			}
			if (zio->io_failed_vds_count == 0) {
				zio_handle_io_failure(zio,
				    vd ? vd : spa->spa_root_vdev);
			}
			if (zio->io_failed_vds != NULL) {
				kmem_free(zio->io_failed_vds,
				    zio->io_failed_vds_count *
				    sizeof (vdev_t *));
				zio->io_failed_vds = NULL;
				zio->io_failed_vds_count = 0;
			}
			return (zio_vdev_suspend_io(zio));
		}
	}
	ASSERT(!(zio->io_flags & ZIO_FLAG_WRITE_RETRY));
	ASSERT(zio->io_children_notready == 0);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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_FOR_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 {
		zio_destroy(zio);
	}

	return (ZIO_PIPELINE_STOP);
}

/*
 * ==========================================================================
 * Compression support
 * ==========================================================================
 */
static int
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(bp);
	} 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);
		}
	}

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

/*
 * ==========================================================================
 * Gang block support
 * ==========================================================================
 */
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 int
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));

	return (zio_wait_for_children_done(zio));
}

static int
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);

	return (zio_wait_for_children_done(zio));
}

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

	zio_push_transform(zio, gbh, gsize, gbufsize);

	return (zio_wait_for_children_ready(zio));
}

static int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

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) {
		zio->io_error = error;
		return (ZIO_PIPELINE_CONTINUE);
	}

	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);

	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) {
				zio->io_error = error;
				return (ZIO_PIPELINE_CONTINUE);
			}
			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, 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 'ready' instead of 'done',
	 * 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.
	 */
	return (zio_wait_for_children_done(zio));
}

/*
 * ==========================================================================
 * Allocate and free blocks
 * ==========================================================================
 */
static int
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 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) {
		return (zio_write_allocate_gang_members(zio, mc));
	} else {
		zio->io_error = error;
	}

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

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

	return (ZIO_PIPELINE_CONTINUE);
}

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

static int
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)
		return (zio_vdev_suspend_io(zio));

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

	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));

	return (vd->vdev_ops->vdev_op_io_start(zio));
}

static int
zio_vdev_io_done(zio_t *zio)
{
	if (zio->io_vd == NULL)
		return (vdev_mirror_ops.vdev_op_io_done(zio));

	return (zio->io_vd->vdev_ops->vdev_op_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 != NULL) {
		if (vd != vd->vdev_top)
			return (B_FALSE);
		if (vd->vdev_is_failing)
			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 int
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_RETRY_INHERIT;
		/* XXPOLICY */
		zio->io_flags &= ~ZIO_FLAG_FAILFAST;
		zio->io_flags |= ZIO_FLAG_DONT_CACHE;
		zio->io_stage = ZIO_STAGE_VDEV_IO_START - 1;

		return (ZIO_PIPELINE_CONTINUE);
	}

	return (ZIO_PIPELINE_CONTINUE);
}

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 int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
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);
	}

	return (ZIO_PIPELINE_CONTINUE);
}

/*
 * 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 int zio_pipe_stage_t(zio_t *zio);

zio_pipe_stage_t *zio_pipeline[ZIO_STAGE_DONE + 2] = {
	NULL,
	zio_wait_for_children_ready,
	zio_read_init,
	zio_issue_async,
	zio_write_compress,
	zio_checksum_generate,
	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_vdev_io_start,
	zio_vdev_io_done,
	zio_vdev_io_assess,
	zio_wait_for_children_done,
	zio_checksum_verify,
	zio_read_gang_members,
	zio_read_decompress,
	zio_assess,
	zio_done,
	NULL
};

/*
 * Execute the I/O pipeline until one of the following occurs:
 * (1) the I/O completes; (2) the pipeline stalls waiting for
 * dependent child I/Os; (3) the I/O issues, so we're waiting
 * for an I/O completion interrupt; (4) the I/O is delegated by
 * vdev-level caching or aggregation; (5) the I/O is deferred
 * due to vdev-level queueing; (6) the I/O is handed off to
 * another thread.  In all cases, the pipeline stops whenever
 * there's no CPU work; it never burns a thread in cv_wait().
 *
 * 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_execute(zio_t *zio)
{
	while (zio->io_stage < ZIO_STAGE_DONE) {
		uint32_t pipeline = zio->io_pipeline;
		int rv;

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

		/*
		 * If an error occurred outside the vdev stack,
		 * just execute the interlock stages to clean up.
		 */
		if (zio->io_error &&
		    ((1U << zio->io_stage) & ZIO_VDEV_IO_STAGES) == 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);

		rv = zio_pipeline[zio->io_stage](zio);

		if (rv == ZIO_PIPELINE_STOP)
			return;

		ASSERT(rv == ZIO_PIPELINE_CONTINUE);
	}
}

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(zio_t *zio, vdev_t *vd)
{
	zio_nowait(zio_ioctl(zio, zio->io_spa, vd, DKIOCFLUSHWRITECACHE,
	    NULL, NULL, ZIO_PRIORITY_NOW,
	    ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_RETRY));
}