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.
 */

#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" };

#define	SYNC_PASS_DEFERRED_FREE	1	/* defer frees after this pass */
#define	SYNC_PASS_DONT_COMPRESS	4	/* don't compress after this pass */
#define	SYNC_PASS_REWRITE	1	/* rewrite new bps after this pass */

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

#ifdef _KERNEL
extern vmem_t *zio_alloc_arena;
#endif

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

void
zio_init(void)
{
#ifdef ZIO_USE_UMA
	size_t c;
#endif
	zio_cache = kmem_cache_create("zio_cache", sizeof (zio_t), 0,
	    NULL, NULL, NULL, NULL, NULL, 0);

#ifdef ZIO_USE_UMA
	/*
	 * 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, NULL, 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];
	}
#endif

	zio_inject_init();
}

void
zio_fini(void)
{
#ifdef ZIO_USE_UMA
	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;
	}
#endif

	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)
{
#ifdef ZIO_USE_UMA
	size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

	ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

	return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
#else
	return (kmem_alloc(size, KM_SLEEP));
#endif
}

/*
 * 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)
{
#ifdef ZIO_USE_UMA
	size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

	ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

	return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
#else
	return (kmem_alloc(size, KM_SLEEP));
#endif
}

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

	ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

	kmem_cache_free(zio_buf_cache[c], buf);
#else
	kmem_free(buf, size);
#endif
}

void
zio_data_buf_free(void *buf, size_t size)
{
#ifdef ZIO_USE_UMA
	size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;

	ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);

	kmem_cache_free(zio_data_buf_cache[c], buf);
#else
	kmem_free(buf, size);
#endif
}

/*
 * ==========================================================================
 * 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_func_t *transform)
{
	zio_transform_t *zt = kmem_alloc(sizeof (zio_transform_t), KM_SLEEP);

	zt->zt_orig_data = zio->io_data;
	zt->zt_orig_size = zio->io_size;
	zt->zt_bufsize = bufsize;
	zt->zt_transform = transform;

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

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

static void
zio_pop_transforms(zio_t *zio)
{
	zio_transform_t *zt;

	while ((zt = zio->io_transform_stack) != NULL) {
		if (zt->zt_transform != NULL)
			zt->zt_transform(zio,
			    zt->zt_orig_data, zt->zt_orig_size);

		zio_buf_free(zio->io_data, zt->zt_bufsize);

		zio->io_data = zt->zt_orig_data;
		zio->io_size = zt->zt_orig_size;
		zio->io_transform_stack = zt->zt_next;

		kmem_free(zt, sizeof (zio_transform_t));
	}
}

/*
 * ==========================================================================
 * I/O transform callbacks for subblocks and decompression
 * ==========================================================================
 */
static void
zio_subblock(zio_t *zio, void *data, uint64_t size)
{
	ASSERT(zio->io_size > size);

	if (zio->io_type == ZIO_TYPE_READ)
		bcopy(zio->io_data, data, size);
}

static void
zio_decompress(zio_t *zio, void *data, uint64_t size)
{
	if (zio->io_error == 0 &&
	    zio_decompress_data(BP_GET_COMPRESS(zio->io_bp),
	    zio->io_data, zio->io_size, data, size) != 0)
		zio->io_error = EIO;
}

/*
 * ==========================================================================
 * I/O parent/child relationships and pipeline interlocks
 * ==========================================================================
 */

static void
zio_add_child(zio_t *pio, zio_t *zio)
{
	mutex_enter(&pio->io_lock);
	if (zio->io_stage < ZIO_STAGE_READY)
		pio->io_children[zio->io_child_type][ZIO_WAIT_READY]++;
	if (zio->io_stage < ZIO_STAGE_DONE)
		pio->io_children[zio->io_child_type][ZIO_WAIT_DONE]++;
	zio->io_sibling_prev = NULL;
	zio->io_sibling_next = pio->io_child;
	if (pio->io_child != NULL)
		pio->io_child->io_sibling_prev = zio;
	pio->io_child = zio;
	zio->io_parent = pio;
	mutex_exit(&pio->io_lock);
}

static void
zio_remove_child(zio_t *pio, zio_t *zio)
{
	zio_t *next, *prev;

	ASSERT(zio->io_parent == pio);

	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);
}

static boolean_t
zio_wait_for_children(zio_t *zio, enum zio_child child, enum zio_wait_type wait)
{
	uint64_t *countp = &zio->io_children[child][wait];
	boolean_t waiting = B_FALSE;

	mutex_enter(&zio->io_lock);
	ASSERT(zio->io_stall == NULL);
	if (*countp != 0) {
		zio->io_stage--;
		zio->io_stall = countp;
		waiting = B_TRUE;
	}
	mutex_exit(&zio->io_lock);

	return (waiting);
}

static void
zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait)
{
	uint64_t *countp = &pio->io_children[zio->io_child_type][wait];
	int *errorp = &pio->io_child_error[zio->io_child_type];

	mutex_enter(&pio->io_lock);
	if (zio->io_error && !(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE))
		*errorp = zio_worst_error(*errorp, zio->io_error);
	pio->io_reexecute |= zio->io_reexecute;
	ASSERT3U(*countp, >, 0);
	if (--*countp == 0 && pio->io_stall == countp) {
		pio->io_stall = NULL;
		mutex_exit(&pio->io_lock);
		zio_execute(pio);
	} else {
		mutex_exit(&pio->io_lock);
	}
}

static void
zio_inherit_child_errors(zio_t *zio, enum zio_child c)
{
	if (zio->io_child_error[c] != 0 && zio->io_error == 0)
		zio->io_error = zio->io_child_error[c];
}

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

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

	ASSERT(!vd || spa_config_held(spa, SCL_STATE_ALL, RW_READER));
	ASSERT(!bp || !(flags & ZIO_FLAG_CONFIG_WRITER));
	ASSERT(vd || stage == ZIO_STAGE_OPEN);

	zio = kmem_cache_alloc(zio_cache, KM_SLEEP);
	bzero(zio, sizeof (zio_t));

	mutex_init(&zio->io_lock, NULL, MUTEX_DEFAULT, NULL);
	cv_init(&zio->io_cv, NULL, CV_DEFAULT, NULL);

	if (vd != NULL)
		zio->io_child_type = ZIO_CHILD_VDEV;
	else if (flags & ZIO_FLAG_GANG_CHILD)
		zio->io_child_type = ZIO_CHILD_GANG;
	else
		zio->io_child_type = ZIO_CHILD_LOGICAL;

	if (bp != NULL) {
		zio->io_bp = bp;
		zio->io_bp_copy = *bp;
		zio->io_bp_orig = *bp;
		if (type != ZIO_TYPE_WRITE)
			zio->io_bp = &zio->io_bp_copy;	/* so caller can free */
		if (zio->io_child_type == ZIO_CHILD_LOGICAL) {
			if (BP_IS_GANG(bp))
				pipeline |= ZIO_GANG_STAGES;
			zio->io_logical = zio;
		}
	}

	zio->io_spa = spa;
	zio->io_txg = txg;
	zio->io_data = data;
	zio->io_size = size;
	zio->io_done = done;
	zio->io_private = private;
	zio->io_type = type;
	zio->io_priority = priority;
	zio->io_vd = vd;
	zio->io_offset = offset;
	zio->io_orig_flags = zio->io_flags = flags;
	zio->io_orig_stage = zio->io_stage = stage;
	zio->io_orig_pipeline = zio->io_pipeline = pipeline;

	if (zb != NULL)
		zio->io_bookmark = *zb;

	if (pio != NULL) {
		/*
		 * Logical I/Os can have logical, gang, or vdev children.
		 * Gang I/Os can have gang or vdev children.
		 * Vdev I/Os can only have vdev children.
		 * The following ASSERT captures all of these constraints.
		 */
		ASSERT(zio->io_child_type <= pio->io_child_type);
		if (zio->io_logical == NULL)
			zio->io_logical = pio->io_logical;
		zio_add_child(pio, zio);
	}

	return (zio);
}

static void
zio_destroy(zio_t *zio)
{
	spa_t *spa = zio->io_spa;
	uint8_t async_root = zio->io_async_root;

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

	if (async_root) {
		mutex_enter(&spa->spa_async_root_lock);
		if (--spa->spa_async_root_count == 0)
			cv_broadcast(&spa->spa_async_root_cv);
		mutex_exit(&spa->spa_async_root_lock);
	}
}

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, NULL, 0, NULL,
	    ZIO_STAGE_OPEN, ZIO_INTERLOCK_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, const blkptr_t *bp,
    void *data, uint64_t size, zio_done_func_t *done, void *private,
    int priority, int flags, const zbookmark_t *zb)
{
	zio_t *zio;

	zio = zio_create(pio, spa, bp->blk_birth, (blkptr_t *)bp,
	    data, size, done, private,
	    ZIO_TYPE_READ, priority, flags, NULL, 0, zb,
	    ZIO_STAGE_OPEN, ZIO_READ_PIPELINE);

	return (zio);
}

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

	ASSERT(zp->zp_checksum >= ZIO_CHECKSUM_OFF &&
	    zp->zp_checksum < ZIO_CHECKSUM_FUNCTIONS &&
	    zp->zp_compress >= ZIO_COMPRESS_OFF &&
	    zp->zp_compress < ZIO_COMPRESS_FUNCTIONS &&
	    zp->zp_type < DMU_OT_NUMTYPES &&
	    zp->zp_level < 32 &&
	    zp->zp_ndvas > 0 &&
	    zp->zp_ndvas <= spa_max_replication(spa));
	ASSERT(ready != NULL);

	zio = zio_create(pio, spa, txg, bp, data, size, done, private,
	    ZIO_TYPE_WRITE, priority, flags, NULL, 0, zb,
	    ZIO_STAGE_OPEN, ZIO_WRITE_PIPELINE);

	zio->io_ready = ready;
	zio->io_prop = *zp;

	return (zio);
}

zio_t *
zio_rewrite(zio_t *pio, spa_t *spa, 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, NULL, 0, zb,
	    ZIO_STAGE_OPEN, ZIO_REWRITE_PIPELINE);

	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, int flags)
{
	zio_t *zio;

	ASSERT(!BP_IS_HOLE(bp));

	if (bp->blk_fill == BLK_FILL_ALREADY_FREED)
		return (zio_null(pio, spa, NULL, NULL, flags));

	if (txg == spa->spa_syncing_txg &&
	    spa_sync_pass(spa) > SYNC_PASS_DEFERRED_FREE) {
		bplist_enqueue_deferred(&spa->spa_sync_bplist, bp);
		return (zio_null(pio, spa, NULL, NULL, flags));
	}

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

	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, int flags)
{
	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, BP_GET_PSIZE(bp),
	    done, private, ZIO_TYPE_CLAIM, ZIO_PRIORITY_NOW, flags,
	    NULL, 0, NULL, ZIO_STAGE_OPEN, ZIO_CLAIM_PIPELINE);

	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, vd, 0, NULL,
		    ZIO_STAGE_OPEN, ZIO_IOCTL_PIPELINE);

		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);
}

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;

	ASSERT(vd->vdev_children == 0);
	ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
	    offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
	ASSERT3U(offset + size, <=, vd->vdev_psize);

	zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, done, private,
	    ZIO_TYPE_READ, priority, flags, vd, offset, NULL,
	    ZIO_STAGE_OPEN, ZIO_READ_PHYS_PIPELINE);

	zio->io_prop.zp_checksum = checksum;

	return (zio);
}

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

	ASSERT(vd->vdev_children == 0);
	ASSERT(!labels || offset + size <= VDEV_LABEL_START_SIZE ||
	    offset >= vd->vdev_psize - VDEV_LABEL_END_SIZE);
	ASSERT3U(offset + size, <=, vd->vdev_psize);

	zio = zio_create(pio, vd->vdev_spa, 0, NULL, data, size, done, private,
	    ZIO_TYPE_WRITE, priority, flags, vd, offset, NULL,
	    ZIO_STAGE_OPEN, ZIO_WRITE_PHYS_PIPELINE);

	zio->io_prop.zp_checksum = checksum;

	if (zio_checksum_table[checksum].ci_zbt) {
		/*
		 * zbt checksums are necessarily destructive -- they modify
		 * the end of the write buffer to hold the verifier/checksum.
		 * Therefore, we must make a local copy in case the data is
		 * being written to multiple places in parallel.
		 */
		void *wbuf = zio_buf_alloc(size);
		bcopy(data, wbuf, size);
		zio_push_transform(zio, wbuf, size, size, NULL);
	}

	return (zio);
}

/*
 * Create a child I/O to do some work for us.
 */
zio_t *
zio_vdev_child_io(zio_t *pio, blkptr_t *bp, vdev_t *vd, uint64_t offset,
	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 *zio;

	ASSERT(vd->vdev_parent ==
	    (pio->io_vd ? pio->io_vd : pio->io_spa->spa_root_vdev));

	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;
		pio->io_pipeline &= ~(1U << ZIO_STAGE_CHECKSUM_VERIFY);
	}

	if (vd->vdev_children == 0)
		offset += VDEV_LABEL_START_SIZE;

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

	return (zio);
}

zio_t *
zio_vdev_delegated_io(vdev_t *vd, uint64_t offset, void *data, uint64_t size,
	int type, int priority, int flags, zio_done_func_t *done, void *private)
{
	zio_t *zio;

	ASSERT(vd->vdev_ops->vdev_op_leaf);

	zio = zio_create(NULL, vd->vdev_spa, 0, NULL,
	    data, size, done, private, type, priority,
	    flags | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_RETRY,
	    vd, offset, NULL,
	    ZIO_STAGE_VDEV_IO_START - 1, ZIO_VDEV_CHILD_PIPELINE);

	return (zio);
}

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

/*
 * ==========================================================================
 * Prepare to read and write logical blocks
 * ==========================================================================
 */

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

	if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF && zio->io_logical == zio) {
		uint64_t csize = BP_GET_PSIZE(bp);
		void *cbuf = zio_buf_alloc(csize);

		zio_push_transform(zio, cbuf, csize, csize, zio_decompress);
	}

	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_write_bp_init(zio_t *zio)
{
	zio_prop_t *zp = &zio->io_prop;
	int compress = zp->zp_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 = 1;

	/*
	 * If our children haven't all reached the ready stage,
	 * wait for them and then repeat this pipeline stage.
	 */
	if (zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_READY) ||
	    zio_wait_for_children(zio, ZIO_CHILD_LOGICAL, ZIO_WAIT_READY))
		return (ZIO_PIPELINE_STOP);

	if (!IO_IS_ALLOCATING(zio))
		return (ZIO_PIPELINE_CONTINUE);

	ASSERT(compress != ZIO_COMPRESS_INHERIT);

	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);
		ASSERT(pass > 1);

		if (pass > SYNC_PASS_DONT_COMPRESS)
			compress = ZIO_COMPRESS_OFF;

		/*
		 * Only MOS (objset 0) data should need to be rewritten.
		 */
		ASSERT(zio->io_logical->io_bookmark.zb_objset == 0);

		/* Make sure someone doesn't change their mind on overwrites */
		ASSERT(MIN(zp->zp_ndvas + BP_IS_GANG(bp),
		    spa_max_replication(zio->io_spa)) == BP_GET_NDVAS(bp));
	}

	if (compress != ZIO_COMPRESS_OFF) {
		if (!zio_compress_data(compress, zio->io_data, zio->io_size,
		    &cbuf, &csize, &cbufsize)) {
			compress = ZIO_COMPRESS_OFF;
		} else if (csize != 0) {
			zio_push_transform(zio, cbuf, csize, cbufsize, NULL);
		}
	}

	/*
	 * The final pass of spa_sync() must be all rewrites, but the first
	 * few passes offer a trade-off: allocating blocks defers convergence,
	 * but newly allocated blocks are sequential, so they can be written
	 * to disk faster.  Therefore, we allow the first few passes of
	 * spa_sync() to allocate new blocks, but force rewrites after that.
	 * There should only be a handful of blocks after pass 1 in any case.
	 */
	if (bp->blk_birth == zio->io_txg && BP_GET_PSIZE(bp) == csize &&
	    pass > SYNC_PASS_REWRITE) {
		ASSERT(csize != 0);
		uint32_t gang_stages = zio->io_pipeline & ZIO_GANG_STAGES;
		zio->io_pipeline = ZIO_REWRITE_PIPELINE | gang_stages;
		zio->io_flags |= ZIO_FLAG_IO_REWRITE;
	} else {
		BP_ZERO(bp);
		zio->io_pipeline = ZIO_WRITE_PIPELINE;
	}

	if (csize == 0) {
		zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;
	} else {
		ASSERT(zp->zp_checksum != ZIO_CHECKSUM_GANG_HEADER);
		BP_SET_LSIZE(bp, lsize);
		BP_SET_PSIZE(bp, csize);
		BP_SET_COMPRESS(bp, compress);
		BP_SET_CHECKSUM(bp, zp->zp_checksum);
		BP_SET_TYPE(bp, zp->zp_type);
		BP_SET_LEVEL(bp, zp->zp_level);
		BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
	}

	return (ZIO_PIPELINE_CONTINUE);
}

/*
 * ==========================================================================
 * Execute the I/O pipeline
 * ==========================================================================
 */

static void
zio_taskq_dispatch(zio_t *zio, enum zio_taskq_type q)
{
	zio_type_t t = zio->io_type;

	/*
	 * If we're a config writer, the normal issue and interrupt threads
	 * may all be blocked waiting for the config lock.  In this case,
	 * select the otherwise-unused taskq for ZIO_TYPE_NULL.
	 */
	if (zio->io_flags & ZIO_FLAG_CONFIG_WRITER)
		t = ZIO_TYPE_NULL;

	/*
	 * A similar issue exists for the L2ARC write thread until L2ARC 2.0.
	 */
	if (t == ZIO_TYPE_WRITE && zio->io_vd && zio->io_vd->vdev_aux)
		t = ZIO_TYPE_NULL;

	(void) taskq_dispatch(zio->io_spa->spa_zio_taskq[t][q],
	    (task_func_t *)zio_execute, zio, TQ_SLEEP);
}

static boolean_t
zio_taskq_member(zio_t *zio, enum zio_taskq_type q)
{
	kthread_t *executor = zio->io_executor;
	spa_t *spa = zio->io_spa;

	for (zio_type_t t = 0; t < ZIO_TYPES; t++)
		if (taskq_member(spa->spa_zio_taskq[t][q], executor))
			return (B_TRUE);

	return (B_FALSE);
}

static int
zio_issue_async(zio_t *zio)
{
	zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);

	return (ZIO_PIPELINE_STOP);
}

void
zio_interrupt(zio_t *zio)
{
	zio_taskq_dispatch(zio, ZIO_TASKQ_INTERRUPT);
}

/*
 * 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.
 */
static zio_pipe_stage_t *zio_pipeline[ZIO_STAGES];

void
zio_execute(zio_t *zio)
{
	zio->io_executor = curthread;

	while (zio->io_stage < ZIO_STAGE_DONE) {
		uint32_t pipeline = zio->io_pipeline;
		zio_stage_t stage = zio->io_stage;
		int rv;

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

		while (((1U << ++stage) & pipeline) == 0)
			continue;

		ASSERT(stage <= ZIO_STAGE_DONE);
		ASSERT(zio->io_stall == NULL);

		/*
		 * If we are in interrupt context and this pipeline stage
		 * will grab a config lock that is held across I/O,
		 * issue async to avoid deadlock.
		 */
		if (((1U << stage) & ZIO_CONFIG_LOCK_BLOCKING_STAGES) &&
		    zio->io_vd == NULL &&
		    zio_taskq_member(zio, ZIO_TASKQ_INTERRUPT)) {
			zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);
			return;
		}

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

		if (rv == ZIO_PIPELINE_STOP)
			return;

		ASSERT(rv == ZIO_PIPELINE_CONTINUE);
	}
}

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

	ASSERT(zio->io_stage == ZIO_STAGE_OPEN);
	ASSERT(zio->io_executor == NULL);

	zio->io_waiter = curthread;

	zio_execute(zio);

	mutex_enter(&zio->io_lock);
	while (zio->io_executor != NULL)
		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)
{
	ASSERT(zio->io_executor == NULL);

	if (zio->io_parent == NULL && zio->io_child_type == ZIO_CHILD_LOGICAL) {
		/*
		 * This is a logical async I/O with no parent to wait for it.
		 * Attach it to the pool's global async root zio so that
		 * spa_unload() has a way of waiting for async I/O to finish.
		 */
		spa_t *spa = zio->io_spa;
		zio->io_async_root = B_TRUE;
		mutex_enter(&spa->spa_async_root_lock);
		spa->spa_async_root_count++;
		mutex_exit(&spa->spa_async_root_lock);
	}

	zio_execute(zio);
}

/*
 * ==========================================================================
 * Reexecute or suspend/resume failed I/O
 * ==========================================================================
 */

static void
zio_reexecute(zio_t *pio)
{
	zio_t *zio, *zio_next;

	pio->io_flags = pio->io_orig_flags;
	pio->io_stage = pio->io_orig_stage;
	pio->io_pipeline = pio->io_orig_pipeline;
	pio->io_reexecute = 0;
	pio->io_error = 0;
	for (int c = 0; c < ZIO_CHILD_TYPES; c++)
		pio->io_child_error[c] = 0;

	if (IO_IS_ALLOCATING(pio)) {
		/*
		 * Remember the failed bp so that the io_ready() callback
		 * can update its accounting upon reexecution.  The block
		 * was already freed in zio_done(); we indicate this with
		 * a fill count of -1 so that zio_free() knows to skip it.
		 */
		blkptr_t *bp = pio->io_bp;
		ASSERT(bp->blk_birth == 0 || bp->blk_birth == pio->io_txg);
		bp->blk_fill = BLK_FILL_ALREADY_FREED;
		pio->io_bp_orig = *bp;
		BP_ZERO(bp);
	}

	/*
	 * As we reexecute pio's children, new children could be created.
	 * New children go to the head of the io_child list, however,
	 * so we will (correctly) not reexecute them.  The key is that
	 * the remainder of the io_child list, from 'zio_next' onward,
	 * cannot be affected by any side effects of reexecuting 'zio'.
	 */
	for (zio = pio->io_child; zio != NULL; zio = zio_next) {
		zio_next = zio->io_sibling_next;
		mutex_enter(&pio->io_lock);
		pio->io_children[zio->io_child_type][ZIO_WAIT_READY]++;
		pio->io_children[zio->io_child_type][ZIO_WAIT_DONE]++;
		mutex_exit(&pio->io_lock);
		zio_reexecute(zio);
	}

	/*
	 * Now that all children have been reexecuted, execute the parent.
	 */
	zio_execute(pio);
}

void
zio_suspend(spa_t *spa, zio_t *zio)
{
	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);

	mutex_enter(&spa->spa_suspend_lock);

	if (spa->spa_suspend_zio_root == NULL)
		spa->spa_suspend_zio_root = zio_root(spa, NULL, NULL, 0);

	spa->spa_suspended = B_TRUE;

	if (zio != NULL) {
		ASSERT(zio != spa->spa_suspend_zio_root);
		ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
		ASSERT(zio->io_parent == NULL);
		ASSERT(zio->io_stage == ZIO_STAGE_DONE);
		zio_add_child(spa->spa_suspend_zio_root, zio);
	}

	mutex_exit(&spa->spa_suspend_lock);
}

void
zio_resume(spa_t *spa)
{
	zio_t *pio, *zio;

	/*
	 * Reexecute all previously suspended i/o.
	 */
	mutex_enter(&spa->spa_suspend_lock);
	spa->spa_suspended = B_FALSE;
	cv_broadcast(&spa->spa_suspend_cv);
	pio = spa->spa_suspend_zio_root;
	spa->spa_suspend_zio_root = NULL;
	mutex_exit(&spa->spa_suspend_lock);

	if (pio == NULL)
		return;

	while ((zio = pio->io_child) != NULL) {
		zio_remove_child(pio, zio);
		zio->io_parent = NULL;
		zio_reexecute(zio);
	}

	ASSERT(pio->io_children[ZIO_CHILD_LOGICAL][ZIO_WAIT_DONE] == 0);

	(void) zio_wait(pio);
}

void
zio_resume_wait(spa_t *spa)
{
	mutex_enter(&spa->spa_suspend_lock);
	while (spa_suspended(spa))
		cv_wait(&spa->spa_suspend_cv, &spa->spa_suspend_lock);
	mutex_exit(&spa->spa_suspend_lock);
}

/*
 * ==========================================================================
 * Gang blocks.
 *
 * A gang block is a collection of small blocks that looks to the DMU
 * like one large block.  When zio_dva_allocate() cannot find a block
 * of the requested size, due to either severe fragmentation or the pool
 * being nearly full, it calls zio_write_gang_block() to construct the
 * block from smaller fragments.
 *
 * A gang block consists of a gang header (zio_gbh_phys_t) and up to
 * three (SPA_GBH_NBLKPTRS) gang members.  The gang header is just like
 * an indirect block: it's an array of block pointers.  It consumes
 * only one sector and hence is allocatable regardless of fragmentation.
 * The gang header's bps point to its gang members, which hold the data.
 *
 * Gang blocks are self-checksumming, using the bp's <vdev, offset, txg>
 * as the verifier to ensure uniqueness of the SHA256 checksum.
 * Critically, the gang block bp's blk_cksum is the checksum of the data,
 * not the gang header.  This ensures that data block signatures (needed for
 * deduplication) are independent of how the block is physically stored.
 *
 * Gang blocks can be nested: a gang member may itself be a gang block.
 * Thus every gang block is a tree in which root and all interior nodes are
 * gang headers, and the leaves are normal blocks that contain user data.
 * The root of the gang tree is called the gang leader.
 *
 * To perform any operation (read, rewrite, free, claim) on a gang block,
 * zio_gang_assemble() first assembles the gang tree (minus data leaves)
 * in the io_gang_tree field of the original logical i/o by recursively
 * reading the gang leader and all gang headers below it.  This yields
 * an in-core tree containing the contents of every gang header and the
 * bps for every constituent of the gang block.
 *
 * With the gang tree now assembled, zio_gang_issue() just walks the gang tree
 * and invokes a callback on each bp.  To free a gang block, zio_gang_issue()
 * calls zio_free_gang() -- a trivial wrapper around zio_free() -- for each bp.
 * zio_claim_gang() provides a similarly trivial wrapper for zio_claim().
 * zio_read_gang() is a wrapper around zio_read() that omits reading gang
 * headers, since we already have those in io_gang_tree.  zio_rewrite_gang()
 * performs a zio_rewrite() of the data or, for gang headers, a zio_rewrite()
 * of the gang header plus zio_checksum_compute() of the data to update the
 * gang header's blk_cksum as described above.
 *
 * The two-phase assemble/issue model solves the problem of partial failure --
 * what if you'd freed part of a gang block but then couldn't read the
 * gang header for another part?  Assembling the entire gang tree first
 * ensures that all the necessary gang header I/O has succeeded before
 * starting the actual work of free, claim, or write.  Once the gang tree
 * is assembled, free and claim are in-memory operations that cannot fail.
 *
 * In the event that a gang write fails, zio_dva_unallocate() walks the
 * gang tree to immediately free (i.e. insert back into the space map)
 * everything we've allocated.  This ensures that we don't get ENOSPC
 * errors during repeated suspend/resume cycles due to a flaky device.
 *
 * Gang rewrites only happen during sync-to-convergence.  If we can't assemble
 * the gang tree, we won't modify the block, so we can safely defer the free
 * (knowing that the block is still intact).  If we *can* assemble the gang
 * tree, then even if some of the rewrites fail, zio_dva_unallocate() will free
 * each constituent bp and we can allocate a new block on the next sync pass.
 *
 * In all cases, the gang tree allows complete recovery from partial failure.
 * ==========================================================================
 */

static zio_t *
zio_read_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
	if (gn != NULL)
		return (pio);

	return (zio_read(pio, pio->io_spa, bp, data, BP_GET_PSIZE(bp),
	    NULL, NULL, pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
	    &pio->io_bookmark));
}

zio_t *
zio_rewrite_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
	zio_t *zio;

	if (gn != NULL) {
		zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
		    gn->gn_gbh, SPA_GANGBLOCKSIZE, NULL, NULL, pio->io_priority,
		    ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
		/*
		 * As we rewrite each gang header, the pipeline will compute
		 * a new gang block header checksum for it; but no one will
		 * compute a new data checksum, so we do that here.  The one
		 * exception is the gang leader: the pipeline already computed
		 * its data checksum because that stage precedes gang assembly.
		 * (Presently, nothing actually uses interior data checksums;
		 * this is just good hygiene.)
		 */
		if (gn != pio->io_logical->io_gang_tree) {
			zio_checksum_compute(zio, BP_GET_CHECKSUM(bp),
			    data, BP_GET_PSIZE(bp));
		}
	} else {
		zio = zio_rewrite(pio, pio->io_spa, pio->io_txg, bp,
		    data, BP_GET_PSIZE(bp), NULL, NULL, pio->io_priority,
		    ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);
	}

	return (zio);
}

/* ARGSUSED */
zio_t *
zio_free_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
	return (zio_free(pio, pio->io_spa, pio->io_txg, bp,
	    NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio)));
}

/* ARGSUSED */
zio_t *
zio_claim_gang(zio_t *pio, blkptr_t *bp, zio_gang_node_t *gn, void *data)
{
	return (zio_claim(pio, pio->io_spa, pio->io_txg, bp,
	    NULL, NULL, ZIO_GANG_CHILD_FLAGS(pio)));
}

static zio_gang_issue_func_t *zio_gang_issue_func[ZIO_TYPES] = {
	NULL,
	zio_read_gang,
	zio_rewrite_gang,
	zio_free_gang,
	zio_claim_gang,
	NULL
};

static void zio_gang_tree_assemble_done(zio_t *zio);

static zio_gang_node_t *
zio_gang_node_alloc(zio_gang_node_t **gnpp)
{
	zio_gang_node_t *gn;

	ASSERT(*gnpp == NULL);

	gn = kmem_zalloc(sizeof (*gn), KM_SLEEP);
	gn->gn_gbh = zio_buf_alloc(SPA_GANGBLOCKSIZE);
	*gnpp = gn;

	return (gn);
}

static void
zio_gang_node_free(zio_gang_node_t **gnpp)
{
	zio_gang_node_t *gn = *gnpp;

	for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
		ASSERT(gn->gn_child[g] == NULL);

	zio_buf_free(gn->gn_gbh, SPA_GANGBLOCKSIZE);
	kmem_free(gn, sizeof (*gn));
	*gnpp = NULL;
}

static void
zio_gang_tree_free(zio_gang_node_t **gnpp)
{
	zio_gang_node_t *gn = *gnpp;

	if (gn == NULL)
		return;

	for (int g = 0; g < SPA_GBH_NBLKPTRS; g++)
		zio_gang_tree_free(&gn->gn_child[g]);

	zio_gang_node_free(gnpp);
}

static void
zio_gang_tree_assemble(zio_t *lio, blkptr_t *bp, zio_gang_node_t **gnpp)
{
	zio_gang_node_t *gn = zio_gang_node_alloc(gnpp);

	ASSERT(lio->io_logical == lio);
	ASSERT(BP_IS_GANG(bp));

	zio_nowait(zio_read(lio, lio->io_spa, bp, gn->gn_gbh,
	    SPA_GANGBLOCKSIZE, zio_gang_tree_assemble_done, gn,
	    lio->io_priority, ZIO_GANG_CHILD_FLAGS(lio), &lio->io_bookmark));
}

static void
zio_gang_tree_assemble_done(zio_t *zio)
{
	zio_t *lio = zio->io_logical;
	zio_gang_node_t *gn = zio->io_private;
	blkptr_t *bp = zio->io_bp;

	ASSERT(zio->io_parent == lio);
	ASSERT(zio->io_child == NULL);

	if (zio->io_error)
		return;

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

	ASSERT(zio->io_data == gn->gn_gbh);
	ASSERT(zio->io_size == SPA_GANGBLOCKSIZE);
	ASSERT(gn->gn_gbh->zg_tail.zbt_magic == ZBT_MAGIC);

	for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
		blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
		if (!BP_IS_GANG(gbp))
			continue;
		zio_gang_tree_assemble(lio, gbp, &gn->gn_child[g]);
	}
}

static void
zio_gang_tree_issue(zio_t *pio, zio_gang_node_t *gn, blkptr_t *bp, void *data)
{
	zio_t *lio = pio->io_logical;
	zio_t *zio;

	ASSERT(BP_IS_GANG(bp) == !!gn);
	ASSERT(BP_GET_CHECKSUM(bp) == BP_GET_CHECKSUM(lio->io_bp));
	ASSERT(BP_GET_LSIZE(bp) == BP_GET_PSIZE(bp) || gn == lio->io_gang_tree);

	/*
	 * If you're a gang header, your data is in gn->gn_gbh.
	 * If you're a gang member, your data is in 'data' and gn == NULL.
	 */
	zio = zio_gang_issue_func[lio->io_type](pio, bp, gn, data);

	if (gn != NULL) {
		ASSERT(gn->gn_gbh->zg_tail.zbt_magic == ZBT_MAGIC);

		for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
			blkptr_t *gbp = &gn->gn_gbh->zg_blkptr[g];
			if (BP_IS_HOLE(gbp))
				continue;
			zio_gang_tree_issue(zio, gn->gn_child[g], gbp, data);
			data = (char *)data + BP_GET_PSIZE(gbp);
		}
	}

	if (gn == lio->io_gang_tree)
		ASSERT3P((char *)lio->io_data + lio->io_size, ==, data);

	if (zio != pio)
		zio_nowait(zio);
}

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

	ASSERT(BP_IS_GANG(bp) && zio == zio->io_logical);

	zio_gang_tree_assemble(zio, bp, &zio->io_gang_tree);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_gang_issue(zio_t *zio)
{
	zio_t *lio = zio->io_logical;
	blkptr_t *bp = zio->io_bp;

	if (zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_DONE))
		return (ZIO_PIPELINE_STOP);

	ASSERT(BP_IS_GANG(bp) && zio == lio);

	if (zio->io_child_error[ZIO_CHILD_GANG] == 0)
		zio_gang_tree_issue(lio, lio->io_gang_tree, bp, lio->io_data);
	else
		zio_gang_tree_free(&lio->io_gang_tree);

	zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

	return (ZIO_PIPELINE_CONTINUE);
}

static void
zio_write_gang_member_ready(zio_t *zio)
{
	zio_t *pio = zio->io_parent;
	zio_t *lio = zio->io_logical;
	dva_t *cdva = zio->io_bp->blk_dva;
	dva_t *pdva = pio->io_bp->blk_dva;
	uint64_t asize;

	if (BP_IS_HOLE(zio->io_bp))
		return;

	ASSERT(BP_IS_HOLE(&zio->io_bp_orig));

	ASSERT(zio->io_child_type == ZIO_CHILD_GANG);
	ASSERT3U(zio->io_prop.zp_ndvas, ==, lio->io_prop.zp_ndvas);
	ASSERT3U(zio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(zio->io_bp));
	ASSERT3U(pio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(pio->io_bp));
	ASSERT3U(BP_GET_NDVAS(zio->io_bp), <=, BP_GET_NDVAS(pio->io_bp));

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

static int
zio_write_gang_block(zio_t *pio)
{
	spa_t *spa = pio->io_spa;
	blkptr_t *bp = pio->io_bp;
	zio_t *lio = pio->io_logical;
	zio_t *zio;
	zio_gang_node_t *gn, **gnpp;
	zio_gbh_phys_t *gbh;
	uint64_t txg = pio->io_txg;
	uint64_t resid = pio->io_size;
	uint64_t lsize;
	int ndvas = lio->io_prop.zp_ndvas;
	int gbh_ndvas = MIN(ndvas + 1, spa_max_replication(spa));
	zio_prop_t zp;
	int error;

	error = metaslab_alloc(spa, spa->spa_normal_class, SPA_GANGBLOCKSIZE,
	    bp, gbh_ndvas, txg, pio == lio ? NULL : lio->io_bp,
	    METASLAB_HINTBP_FAVOR | METASLAB_GANG_HEADER);
	if (error) {
		pio->io_error = error;
		return (ZIO_PIPELINE_CONTINUE);
	}

	if (pio == lio) {
		gnpp = &lio->io_gang_tree;
	} else {
		gnpp = pio->io_private;
		ASSERT(pio->io_ready == zio_write_gang_member_ready);
	}

	gn = zio_gang_node_alloc(gnpp);
	gbh = gn->gn_gbh;
	bzero(gbh, SPA_GANGBLOCKSIZE);

	/*
	 * Create the gang header.
	 */
	zio = zio_rewrite(pio, spa, txg, bp, gbh, SPA_GANGBLOCKSIZE, NULL, NULL,
	    pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio), &pio->io_bookmark);

	/*
	 * Create and nowait the gang children.
	 */
	for (int g = 0; resid != 0; resid -= lsize, g++) {
		lsize = P2ROUNDUP(resid / (SPA_GBH_NBLKPTRS - g),
		    SPA_MINBLOCKSIZE);
		ASSERT(lsize >= SPA_MINBLOCKSIZE && lsize <= resid);

		zp.zp_checksum = lio->io_prop.zp_checksum;
		zp.zp_compress = ZIO_COMPRESS_OFF;
		zp.zp_type = DMU_OT_NONE;
		zp.zp_level = 0;
		zp.zp_ndvas = lio->io_prop.zp_ndvas;

		zio_nowait(zio_write(zio, spa, txg, &gbh->zg_blkptr[g],
		    (char *)pio->io_data + (pio->io_size - resid), lsize, &zp,
		    zio_write_gang_member_ready, NULL, &gn->gn_child[g],
		    pio->io_priority, ZIO_GANG_CHILD_FLAGS(pio),
		    &pio->io_bookmark));
	}

	/*
	 * Set pio's pipeline to just wait for zio to finish.
	 */
	pio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

	zio_nowait(zio);

	return (ZIO_PIPELINE_CONTINUE);
}

/*
 * ==========================================================================
 * 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_prop.zp_ndvas, >, 0);
	ASSERT3U(zio->io_prop.zp_ndvas, <=, spa_max_replication(spa));
	ASSERT3U(zio->io_size, ==, BP_GET_PSIZE(bp));

	error = metaslab_alloc(spa, mc, zio->io_size, bp,
	    zio->io_prop.zp_ndvas, zio->io_txg, NULL, 0);

	if (error) {
		if (error == ENOSPC && zio->io_size > SPA_MINBLOCKSIZE)
			return (zio_write_gang_block(zio));
		zio->io_error = error;
	}

	return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_dva_free(zio_t *zio)
{
	metaslab_free(zio->io_spa, zio->io_bp, zio->io_txg, B_FALSE);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_dva_claim(zio_t *zio)
{
	int error;

	error = metaslab_claim(zio->io_spa, zio->io_bp, zio->io_txg);
	if (error)
		zio->io_error = error;

	return (ZIO_PIPELINE_CONTINUE);
}

/*
 * Undo an allocation.  This is used by zio_done() when an I/O fails
 * and we want to give back the block we just allocated.
 * This handles both normal blocks and gang blocks.
 */
static void
zio_dva_unallocate(zio_t *zio, zio_gang_node_t *gn, blkptr_t *bp)
{
	spa_t *spa = zio->io_spa;
	boolean_t now = !(zio->io_flags & ZIO_FLAG_IO_REWRITE);

	ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp));

	if (zio->io_bp == bp && !now) {
		/*
		 * This is a rewrite for sync-to-convergence.
		 * We can't do a metaslab_free(NOW) because bp wasn't allocated
		 * during this sync pass, which means that metaslab_sync()
		 * already committed the allocation.
		 */
		ASSERT(DVA_EQUAL(BP_IDENTITY(bp),
		    BP_IDENTITY(&zio->io_bp_orig)));
		ASSERT(spa_sync_pass(spa) > 1);

		if (BP_IS_GANG(bp) && gn == NULL) {
			/*
			 * This is a gang leader whose gang header(s) we
			 * couldn't read now, so defer the free until later.
			 * The block should still be intact because without
			 * the headers, we'd never even start the rewrite.
			 */
			bplist_enqueue_deferred(&spa->spa_sync_bplist, bp);
			return;
		}
	}

	if (!BP_IS_HOLE(bp))
		metaslab_free(spa, bp, bp->blk_birth, now);

	if (gn != NULL) {
		for (int g = 0; g < SPA_GBH_NBLKPTRS; g++) {
			zio_dva_unallocate(zio, gn->gn_child[g],
			    &gn->gn_gbh->zg_blkptr[g]);
		}
	}
}

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

	error = metaslab_alloc(spa, spa->spa_log_class, size,
	    new_bp, 1, txg, old_bp, METASLAB_HINTBP_AVOID);

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

	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);
	}

	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));

	metaslab_free(spa, bp, txg, B_FALSE);
}

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

static void
zio_vdev_io_probe_done(zio_t *zio)
{
	zio_t *dio;
	vdev_t *vd = zio->io_private;

	mutex_enter(&vd->vdev_probe_lock);
	ASSERT(vd->vdev_probe_zio == zio);
	vd->vdev_probe_zio = NULL;
	mutex_exit(&vd->vdev_probe_lock);

	while ((dio = zio->io_delegate_list) != NULL) {
		zio->io_delegate_list = dio->io_delegate_next;
		dio->io_delegate_next = NULL;
		if (!vdev_accessible(vd, dio))
			dio->io_error = ENXIO;
		zio_execute(dio);
	}
}

/*
 * Probe the device to determine whether I/O failure is specific to this
 * zio (e.g. a bad sector) or affects the entire vdev (e.g. unplugged).
 */
static int
zio_vdev_io_probe(zio_t *zio)
{
	vdev_t *vd = zio->io_vd;
	zio_t *pio = NULL;
	boolean_t created_pio = B_FALSE;

	/*
	 * Don't probe the probe.
	 */
	if (zio->io_flags & ZIO_FLAG_PROBE)
		return (ZIO_PIPELINE_CONTINUE);

	/*
	 * To prevent 'probe storms' when a device fails, we create
	 * just one probe i/o at a time.  All zios that want to probe
	 * this vdev will join the probe zio's io_delegate_list.
	 */
	mutex_enter(&vd->vdev_probe_lock);

	if ((pio = vd->vdev_probe_zio) == NULL) {
		vd->vdev_probe_zio = pio = zio_root(zio->io_spa,
		    zio_vdev_io_probe_done, vd, ZIO_FLAG_CANFAIL);
		created_pio = B_TRUE;
		vd->vdev_probe_wanted = B_TRUE;
		spa_async_request(zio->io_spa, SPA_ASYNC_PROBE);
	}

	zio->io_delegate_next = pio->io_delegate_list;
	pio->io_delegate_list = zio;

	mutex_exit(&vd->vdev_probe_lock);

	if (created_pio) {
		zio_nowait(vdev_probe(vd, pio));
		zio_nowait(pio);
	}

	return (ZIO_PIPELINE_STOP);
}

static int
zio_vdev_io_start(zio_t *zio)
{
	vdev_t *vd = zio->io_vd;
	uint64_t align;
	spa_t *spa = zio->io_spa;

	ASSERT(zio->io_error == 0);
	ASSERT(zio->io_child_error[ZIO_CHILD_VDEV] == 0);

	if (vd == NULL) {
		if (!(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
			spa_config_enter(spa, SCL_ZIO, zio, RW_READER);

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

	align = 1ULL << vd->vdev_top->vdev_ashift;

	if (P2PHASE(zio->io_size, align) != 0) {
		uint64_t asize = P2ROUNDUP(zio->io_size, align);
		char *abuf = zio_buf_alloc(asize);
		ASSERT(vd == vd->vdev_top);
		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, zio_subblock);
	}

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

	if (vd->vdev_ops->vdev_op_leaf &&
	    (zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE)) {

		if (zio->io_type == ZIO_TYPE_READ && vdev_cache_read(zio) == 0)
			return (ZIO_PIPELINE_STOP);

		if ((zio = vdev_queue_io(zio)) == NULL)
			return (ZIO_PIPELINE_STOP);

		if (!vdev_accessible(vd, zio)) {
			zio->io_error = ENXIO;
			zio_interrupt(zio);
			return (ZIO_PIPELINE_STOP);
		}

	}

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

static int
zio_vdev_io_done(zio_t *zio)
{
	vdev_t *vd = zio->io_vd;
	vdev_ops_t *ops = vd ? vd->vdev_ops : &vdev_mirror_ops;
	boolean_t unexpected_error = B_FALSE;

	if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE))
		return (ZIO_PIPELINE_STOP);

	ASSERT(zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE);

	if (vd != NULL && vd->vdev_ops->vdev_op_leaf) {

		vdev_queue_io_done(zio);

		if (zio->io_type == ZIO_TYPE_WRITE)
			vdev_cache_write(zio);

		if (zio_injection_enabled && zio->io_error == 0)
			zio->io_error = zio_handle_device_injection(vd, EIO);

		if (zio_injection_enabled && zio->io_error == 0)
			zio->io_error = zio_handle_label_injection(zio, EIO);

		if (zio->io_error) {
			if (!vdev_accessible(vd, zio)) {
				zio->io_error = ENXIO;
			} else {
				unexpected_error = B_TRUE;
			}
		}
	}

	ops->vdev_op_io_done(zio);

	if (unexpected_error)
		return (zio_vdev_io_probe(zio));

	return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_vdev_io_assess(zio_t *zio)
{
	vdev_t *vd = zio->io_vd;

	if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE))
		return (ZIO_PIPELINE_STOP);

	if (vd == NULL && !(zio->io_flags & ZIO_FLAG_CONFIG_WRITER))
		spa_config_exit(zio->io_spa, SCL_ZIO, zio);

	if (zio->io_vsd != NULL) {
		zio->io_vsd_free(zio);
		zio->io_vsd = NULL;
	}

	if (zio_injection_enabled && zio->io_error == 0)
		zio->io_error = zio_handle_fault_injection(zio, EIO);

	/*
	 * If the I/O failed, determine whether we should attempt to retry it.
	 */
	if (zio->io_error && vd == NULL &&
	    !(zio->io_flags & (ZIO_FLAG_DONT_RETRY | ZIO_FLAG_IO_RETRY))) {
		ASSERT(!(zio->io_flags & ZIO_FLAG_DONT_QUEUE));	/* not a leaf */
		ASSERT(!(zio->io_flags & ZIO_FLAG_IO_BYPASS));	/* not a leaf */
		zio->io_error = 0;
		zio->io_flags |= ZIO_FLAG_IO_RETRY |
		    ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_AGGREGATE;
		zio->io_stage = ZIO_STAGE_VDEV_IO_START - 1;
		zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE);
		return (ZIO_PIPELINE_STOP);
	}

	/*
	 * If we got an error on a leaf device, convert it to ENXIO
	 * if the device is not accessible at all.
	 */
	if (zio->io_error && vd != NULL && vd->vdev_ops->vdev_op_leaf &&
	    !vdev_accessible(vd, zio))
		zio->io_error = ENXIO;

	/*
	 * If we can't write to an interior vdev (mirror or RAID-Z),
	 * set vdev_cant_write so that we stop trying to allocate from it.
	 */
	if (zio->io_error == ENXIO && zio->io_type == ZIO_TYPE_WRITE &&
	    vd != NULL && !vd->vdev_ops->vdev_op_leaf)
		vd->vdev_cant_write = B_TRUE;

	if (zio->io_error)
		zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

	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)
{
	blkptr_t *bp = zio->io_bp;
	enum zio_checksum checksum;

	if (bp == NULL) {
		/*
		 * This is zio_write_phys().
		 * We're either generating a label checksum, or none at all.
		 */
		checksum = zio->io_prop.zp_checksum;

		if (checksum == ZIO_CHECKSUM_OFF)
			return (ZIO_PIPELINE_CONTINUE);

		ASSERT(checksum == ZIO_CHECKSUM_LABEL);
	} else {
		if (BP_IS_GANG(bp) && zio->io_child_type == ZIO_CHILD_GANG) {
			ASSERT(!IO_IS_ALLOCATING(zio));
			checksum = ZIO_CHECKSUM_GANG_HEADER;
		} else {
			checksum = BP_GET_CHECKSUM(bp);
		}
	}

	zio_checksum_compute(zio, checksum, zio->io_data, zio->io_size);

	return (ZIO_PIPELINE_CONTINUE);
}

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

	if (bp == NULL) {
		/*
		 * This is zio_read_phys().
		 * We're either verifying a label checksum, or nothing at all.
		 */
		if (zio->io_prop.zp_checksum == ZIO_CHECKSUM_OFF)
			return (ZIO_PIPELINE_CONTINUE);

		ASSERT(zio->io_prop.zp_checksum == ZIO_CHECKSUM_LABEL);
	}

	if ((error = zio_checksum_error(zio)) != 0) {
		zio->io_error = error;
		if (!(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);
}

/*
 * ==========================================================================
 * Error rank.  Error are ranked in the order 0, ENXIO, ECKSUM, EIO, other.
 * An error of 0 indictes success.  ENXIO indicates whole-device failure,
 * which may be transient (e.g. unplugged) or permament.  ECKSUM and EIO
 * indicate errors that are specific to one I/O, and most likely permanent.
 * Any other error is presumed to be worse because we weren't expecting it.
 * ==========================================================================
 */
int
zio_worst_error(int e1, int e2)
{
	static int zio_error_rank[] = { 0, ENXIO, ECKSUM, EIO };
	int r1, r2;

	for (r1 = 0; r1 < sizeof (zio_error_rank) / sizeof (int); r1++)
		if (e1 == zio_error_rank[r1])
			break;

	for (r2 = 0; r2 < sizeof (zio_error_rank) / sizeof (int); r2++)
		if (e2 == zio_error_rank[r2])
			break;

	return (r1 > r2 ? e1 : e2);
}

/*
 * ==========================================================================
 * I/O completion
 * ==========================================================================
 */
static int
zio_ready(zio_t *zio)
{
	blkptr_t *bp = zio->io_bp;
	zio_t *pio = zio->io_parent;

	if (zio->io_ready) {
		if (BP_IS_GANG(bp) &&
		    zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_READY))
			return (ZIO_PIPELINE_STOP);

		ASSERT(IO_IS_ALLOCATING(zio));
		ASSERT(bp->blk_birth == zio->io_txg || BP_IS_HOLE(bp));
		ASSERT(zio->io_children[ZIO_CHILD_GANG][ZIO_WAIT_READY] == 0);

		zio->io_ready(zio);
	}

	if (bp != NULL && bp != &zio->io_bp_copy)
		zio->io_bp_copy = *bp;

	if (zio->io_error)
		zio->io_pipeline = ZIO_INTERLOCK_PIPELINE;

	if (pio != NULL)
		zio_notify_parent(pio, zio, ZIO_WAIT_READY);

	return (ZIO_PIPELINE_CONTINUE);
}

static int
zio_done(zio_t *zio)
{
	spa_t *spa = zio->io_spa;
	zio_t *pio = zio->io_parent;
	zio_t *lio = zio->io_logical;
	blkptr_t *bp = zio->io_bp;
	vdev_t *vd = zio->io_vd;
	uint64_t psize = zio->io_size;

	/*
	 * If our of children haven't all completed,
	 * wait for them and then repeat this pipeline stage.
	 */
	if (zio_wait_for_children(zio, ZIO_CHILD_VDEV, ZIO_WAIT_DONE) ||
	    zio_wait_for_children(zio, ZIO_CHILD_GANG, ZIO_WAIT_DONE) ||
	    zio_wait_for_children(zio, ZIO_CHILD_LOGICAL, ZIO_WAIT_DONE))
		return (ZIO_PIPELINE_STOP);

	for (int c = 0; c < ZIO_CHILD_TYPES; c++)
		for (int w = 0; w < ZIO_WAIT_TYPES; w++)
			ASSERT(zio->io_children[c][w] == 0);

	if (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 ||
		    (pio != NULL && bp == pio->io_bp));
		if (zio->io_type == ZIO_TYPE_WRITE && !BP_IS_HOLE(bp) &&
		    !(zio->io_flags & ZIO_FLAG_IO_REPAIR)) {
			ASSERT(!BP_SHOULD_BYTESWAP(bp));
			ASSERT3U(zio->io_prop.zp_ndvas, <=, BP_GET_NDVAS(bp));
			ASSERT(BP_COUNT_GANG(bp) == 0 ||
			    (BP_COUNT_GANG(bp) == BP_GET_NDVAS(bp)));
		}
	}

	/*
	 * If there were child vdev or gang errors, they apply to us now.
	 */
	zio_inherit_child_errors(zio, ZIO_CHILD_VDEV);
	zio_inherit_child_errors(zio, ZIO_CHILD_GANG);

	zio_pop_transforms(zio);	/* note: may set zio->io_error */

	vdev_stat_update(zio, psize);

	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 == lio) {
			/*
			 * For logical I/O requests, tell the SPA to log the
			 * error and generate a logical data ereport.
			 */
			spa_log_error(spa, zio);
			zfs_ereport_post(FM_EREPORT_ZFS_DATA, spa, NULL, zio,
			    0, 0);
		}
	}

	if (zio->io_error && zio == lio) {
		/*
		 * Determine whether zio should be reexecuted.  This will
		 * propagate all the way to the root via zio_notify_parent().
		 */
		ASSERT(vd == NULL && bp != NULL);

		if (IO_IS_ALLOCATING(zio))
			if (zio->io_error != ENOSPC)
				zio->io_reexecute |= ZIO_REEXECUTE_NOW;
			else
				zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;

		if ((zio->io_type == ZIO_TYPE_READ ||
		    zio->io_type == ZIO_TYPE_FREE) &&
		    zio->io_error == ENXIO &&
		    spa_get_failmode(spa) != ZIO_FAILURE_MODE_CONTINUE)
			zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;

		if (!(zio->io_flags & ZIO_FLAG_CANFAIL) && !zio->io_reexecute)
			zio->io_reexecute |= ZIO_REEXECUTE_SUSPEND;
	}

	/*
	 * If there were logical child errors, they apply to us now.
	 * We defer this until now to avoid conflating logical child
	 * errors with errors that happened to the zio itself when
	 * updating vdev stats and reporting FMA events above.
	 */
	zio_inherit_child_errors(zio, ZIO_CHILD_LOGICAL);

	if (zio->io_reexecute) {
		/*
		 * This is a logical I/O that wants to reexecute.
		 *
		 * Reexecute is top-down.  When an i/o fails, if it's not
		 * the root, it simply notifies its parent and sticks around.
		 * The parent, seeing that it still has children in zio_done(),
		 * does the same.  This percolates all the way up to the root.
		 * The root i/o will reexecute or suspend the entire tree.
		 *
		 * This approach ensures that zio_reexecute() honors
		 * all the original i/o dependency relationships, e.g.
		 * parents not executing until children are ready.
		 */
		ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);

		if (IO_IS_ALLOCATING(zio))
			zio_dva_unallocate(zio, zio->io_gang_tree, bp);

		zio_gang_tree_free(&zio->io_gang_tree);

		if (pio != NULL) {
			/*
			 * We're not a root i/o, so there's nothing to do
			 * but notify our parent.  Don't propagate errors
			 * upward since we haven't permanently failed yet.
			 */
			zio->io_flags |= ZIO_FLAG_DONT_PROPAGATE;
			zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
		} else if (zio->io_reexecute & ZIO_REEXECUTE_SUSPEND) {
			/*
			 * We'd fail again if we reexecuted now, so suspend
			 * until conditions improve (e.g. device comes online).
			 */
			zio_suspend(spa, zio);
		} else {
			/*
			 * Reexecution is potentially a huge amount of work.
			 * Hand it off to the otherwise-unused claim taskq.
			 */
			(void) taskq_dispatch(
			    spa->spa_zio_taskq[ZIO_TYPE_CLAIM][ZIO_TASKQ_ISSUE],
			    (task_func_t *)zio_reexecute, zio, TQ_SLEEP);
		}
		return (ZIO_PIPELINE_STOP);
	}

	ASSERT(zio->io_child == NULL);
	ASSERT(zio->io_reexecute == 0);
	ASSERT(zio->io_error == 0 || (zio->io_flags & ZIO_FLAG_CANFAIL));

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

	zio_gang_tree_free(&zio->io_gang_tree);

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

	if (pio != NULL) {
		zio_remove_child(pio, zio);
		zio_notify_parent(pio, zio, ZIO_WAIT_DONE);
	}

	if (zio->io_waiter != NULL) {
		mutex_enter(&zio->io_lock);
		zio->io_executor = NULL;
		cv_broadcast(&zio->io_cv);
		mutex_exit(&zio->io_lock);
	} else {
		zio_destroy(zio);
	}

	return (ZIO_PIPELINE_STOP);
}

/*
 * ==========================================================================
 * I/O pipeline definition
 * ==========================================================================
 */
static zio_pipe_stage_t *zio_pipeline[ZIO_STAGES] = {
	NULL,
	zio_issue_async,
	zio_read_bp_init,
	zio_write_bp_init,
	zio_checksum_generate,
	zio_gang_assemble,
	zio_gang_issue,
	zio_dva_allocate,
	zio_dva_free,
	zio_dva_claim,
	zio_ready,
	zio_vdev_io_start,
	zio_vdev_io_done,
	zio_vdev_io_assess,
	zio_checksum_verify,
	zio_done
};