Finally drop long disabled vdev cache.

It was a vdev level read cache, designed to aggregate many small
reads by speculatively issuing bigger reads instead and caching
the result.  But since it has almost no idea about what is going
on with exception of ZIO_FLAG_DONT_CACHE flag set by higher layers,
it was found to make more harm than good, for which reason it was
disabled for the past 12 years.  These days we have much better
instruments to enlarge the I/Os, such as speculative and prescient
prefetches, I/O scheduler, I/O aggregation etc.

Besides just the dead code removal this removes one extra mutex
lock/unlock per write inside vdev_cache_write(), not otherwise
disabled and trying to do some work.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #14953
This commit is contained in:
Alexander Motin
2023-06-09 15:40:55 -04:00
committed by GitHub
parent 6db4ed51d6
commit 70ea484e3e
20 changed files with 13 additions and 562 deletions
-1
View File
@@ -382,7 +382,6 @@ ZFS_OBJS := \
uberblock.o \
unique.o \
vdev.o \
vdev_cache.o \
vdev_draid.o \
vdev_draid_rand.o \
vdev_indirect.o \
-1
View File
@@ -308,7 +308,6 @@ SRCS+= abd.c \
uberblock.c \
unique.c \
vdev.c \
vdev_cache.c \
vdev_draid.c \
vdev_draid_rand.c \
vdev_indirect.c \
-2
View File
@@ -872,8 +872,6 @@ SYSCTL_INT(_vfs_zfs, OID_AUTO, validate_skip,
"Enable to bypass vdev_validate().");
/* END CSTYLED */
/* vdev_cache.c */
/* vdev_mirror.c */
/* vdev_queue.c */
+4 -7
View File
@@ -6106,8 +6106,7 @@ top:
asize, abd,
ZIO_CHECKSUM_OFF,
l2arc_read_done, cb, priority,
zio_flags | ZIO_FLAG_DONT_CACHE |
ZIO_FLAG_CANFAIL |
zio_flags | ZIO_FLAG_CANFAIL |
ZIO_FLAG_DONT_PROPAGATE |
ZIO_FLAG_DONT_RETRY, B_FALSE);
acb->acb_zio_head = rzio;
@@ -10177,8 +10176,7 @@ l2arc_dev_hdr_read(l2arc_dev_t *dev)
err = zio_wait(zio_read_phys(NULL, dev->l2ad_vdev,
VDEV_LABEL_START_SIZE, l2dhdr_asize, abd,
ZIO_CHECKSUM_LABEL, NULL, NULL, ZIO_PRIORITY_SYNC_READ,
ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY |
ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY |
ZIO_FLAG_SPECULATIVE, B_FALSE));
abd_free(abd);
@@ -10498,11 +10496,10 @@ l2arc_log_blk_fetch(vdev_t *vd, const l2arc_log_blkptr_t *lbp,
cb = kmem_zalloc(sizeof (l2arc_read_callback_t), KM_SLEEP);
cb->l2rcb_abd = abd_get_from_buf(lb, asize);
pio = zio_root(vd->vdev_spa, l2arc_blk_fetch_done, cb,
ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE |
ZIO_FLAG_DONT_RETRY);
ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY);
(void) zio_nowait(zio_read_phys(pio, vd, lbp->lbp_daddr, asize,
cb->l2rcb_abd, ZIO_CHECKSUM_OFF, NULL, NULL,
ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL |
ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY, B_FALSE));
return (pio);
+2 -2
View File
@@ -1371,8 +1371,8 @@ do_corrective_recv(struct receive_writer_arg *rwa, struct drr_write *drrw,
dnode_t *dn;
abd_t *abd = rrd->abd;
zio_cksum_t bp_cksum = bp->blk_cksum;
zio_flag_t flags = ZIO_FLAG_SPECULATIVE |
ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_RETRY | ZIO_FLAG_CANFAIL;
zio_flag_t flags = ZIO_FLAG_SPECULATIVE | ZIO_FLAG_DONT_RETRY |
ZIO_FLAG_CANFAIL;
if (rwa->raw)
flags |= ZIO_FLAG_RAW;
-2
View File
@@ -2438,7 +2438,6 @@ spa_init(spa_mode_t mode)
zio_init();
dmu_init();
zil_init();
vdev_cache_stat_init();
vdev_mirror_stat_init();
vdev_raidz_math_init();
vdev_file_init();
@@ -2462,7 +2461,6 @@ spa_fini(void)
spa_evict_all();
vdev_file_fini();
vdev_cache_stat_fini();
vdev_mirror_stat_fini();
vdev_raidz_math_fini();
chksum_fini();
+1 -6
View File
@@ -715,7 +715,6 @@ vdev_alloc_common(spa_t *spa, uint_t id, uint64_t guid, vdev_ops_t *ops)
offsetof(struct vdev, vdev_dtl_node));
vd->vdev_stat.vs_timestamp = gethrtime();
vdev_queue_init(vd);
vdev_cache_init(vd);
return (vd);
}
@@ -1096,7 +1095,6 @@ vdev_free(vdev_t *vd)
* Clean up vdev structure.
*/
vdev_queue_fini(vd);
vdev_cache_fini(vd);
if (vd->vdev_path)
spa_strfree(vd->vdev_path);
@@ -1720,8 +1718,7 @@ vdev_probe(vdev_t *vd, zio_t *zio)
vps = kmem_zalloc(sizeof (*vps), KM_SLEEP);
vps->vps_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_PROBE |
ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_AGGREGATE |
ZIO_FLAG_TRYHARD;
ZIO_FLAG_DONT_AGGREGATE | ZIO_FLAG_TRYHARD;
if (spa_config_held(spa, SCL_ZIO, RW_WRITER)) {
/*
@@ -2612,8 +2609,6 @@ vdev_close(vdev_t *vd)
vd->vdev_ops->vdev_op_close(vd);
vdev_cache_purge(vd);
/*
* We record the previous state before we close it, so that if we are
* doing a reopen(), we don't generate FMA ereports if we notice that
-436
View File
@@ -1,436 +0,0 @@
/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or https://opensource.org/licenses/CDDL-1.0.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2009 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/*
* Copyright (c) 2013, 2016 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/vdev_impl.h>
#include <sys/zio.h>
#include <sys/kstat.h>
#include <sys/abd.h>
/*
* Virtual device read-ahead caching.
*
* This file implements a simple LRU read-ahead cache. When the DMU reads
* a given block, it will often want other, nearby blocks soon thereafter.
* We take advantage of this by reading a larger disk region and caching
* the result. In the best case, this can turn 128 back-to-back 512-byte
* reads into a single 64k read followed by 127 cache hits; this reduces
* latency dramatically. In the worst case, it can turn an isolated 512-byte
* read into a 64k read, which doesn't affect latency all that much but is
* terribly wasteful of bandwidth. A more intelligent version of the cache
* could keep track of access patterns and not do read-ahead unless it sees
* at least two temporally close I/Os to the same region. Currently, only
* metadata I/O is inflated. A further enhancement could take advantage of
* more semantic information about the I/O. And it could use something
* faster than an AVL tree; that was chosen solely for convenience.
*
* There are five cache operations: allocate, fill, read, write, evict.
*
* (1) Allocate. This reserves a cache entry for the specified region.
* We separate the allocate and fill operations so that multiple threads
* don't generate I/O for the same cache miss.
*
* (2) Fill. When the I/O for a cache miss completes, the fill routine
* places the data in the previously allocated cache entry.
*
* (3) Read. Read data from the cache.
*
* (4) Write. Update cache contents after write completion.
*
* (5) Evict. When allocating a new entry, we evict the oldest (LRU) entry
* if the total cache size exceeds zfs_vdev_cache_size.
*/
/*
* These tunables are for performance analysis.
*/
/*
* All i/os smaller than zfs_vdev_cache_max will be turned into
* 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software
* track buffer). At most zfs_vdev_cache_size bytes will be kept in each
* vdev's vdev_cache.
*
* TODO: Note that with the current ZFS code, it turns out that the
* vdev cache is not helpful, and in some cases actually harmful. It
* is better if we disable this. Once some time has passed, we should
* actually remove this to simplify the code. For now we just disable
* it by setting the zfs_vdev_cache_size to zero. Note that Solaris 11
* has made these same changes.
*/
static uint_t zfs_vdev_cache_max = 1 << 14; /* 16KB */
static uint_t zfs_vdev_cache_size = 0;
static uint_t zfs_vdev_cache_bshift = 16;
#define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
static kstat_t *vdc_ksp = NULL;
typedef struct vdc_stats {
kstat_named_t vdc_stat_delegations;
kstat_named_t vdc_stat_hits;
kstat_named_t vdc_stat_misses;
} vdc_stats_t;
static vdc_stats_t vdc_stats = {
{ "delegations", KSTAT_DATA_UINT64 },
{ "hits", KSTAT_DATA_UINT64 },
{ "misses", KSTAT_DATA_UINT64 }
};
#define VDCSTAT_BUMP(stat) atomic_inc_64(&vdc_stats.stat.value.ui64);
static inline int
vdev_cache_offset_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
return (TREE_CMP(ve1->ve_offset, ve2->ve_offset));
}
static int
vdev_cache_lastused_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
int cmp = TREE_CMP(ve1->ve_lastused, ve2->ve_lastused);
if (likely(cmp))
return (cmp);
/*
* Among equally old entries, sort by offset to ensure uniqueness.
*/
return (vdev_cache_offset_compare(a1, a2));
}
/*
* Evict the specified entry from the cache.
*/
static void
vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
{
ASSERT(MUTEX_HELD(&vc->vc_lock));
ASSERT3P(ve->ve_fill_io, ==, NULL);
ASSERT3P(ve->ve_abd, !=, NULL);
avl_remove(&vc->vc_lastused_tree, ve);
avl_remove(&vc->vc_offset_tree, ve);
abd_free(ve->ve_abd);
kmem_free(ve, sizeof (vdev_cache_entry_t));
}
/*
* Allocate an entry in the cache. At the point we don't have the data,
* we're just creating a placeholder so that multiple threads don't all
* go off and read the same blocks.
*/
static vdev_cache_entry_t *
vdev_cache_allocate(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
uint64_t offset = P2ALIGN(zio->io_offset, VCBS);
vdev_cache_entry_t *ve;
ASSERT(MUTEX_HELD(&vc->vc_lock));
if (zfs_vdev_cache_size == 0)
return (NULL);
/*
* If adding a new entry would exceed the cache size,
* evict the oldest entry (LRU).
*/
if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) >
zfs_vdev_cache_size) {
ve = avl_first(&vc->vc_lastused_tree);
if (ve->ve_fill_io != NULL)
return (NULL);
ASSERT3U(ve->ve_hits, !=, 0);
vdev_cache_evict(vc, ve);
}
ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
ve->ve_offset = offset;
ve->ve_lastused = ddi_get_lbolt();
ve->ve_abd = abd_alloc_for_io(VCBS, B_TRUE);
avl_add(&vc->vc_offset_tree, ve);
avl_add(&vc->vc_lastused_tree, ve);
return (ve);
}
static void
vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
{
uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
ASSERT(MUTEX_HELD(&vc->vc_lock));
ASSERT3P(ve->ve_fill_io, ==, NULL);
if (ve->ve_lastused != ddi_get_lbolt()) {
avl_remove(&vc->vc_lastused_tree, ve);
ve->ve_lastused = ddi_get_lbolt();
avl_add(&vc->vc_lastused_tree, ve);
}
ve->ve_hits++;
abd_copy_off(zio->io_abd, ve->ve_abd, 0, cache_phase, zio->io_size);
}
/*
* Fill a previously allocated cache entry with data.
*/
static void
vdev_cache_fill(zio_t *fio)
{
vdev_t *vd = fio->io_vd;
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_entry_t *ve = fio->io_private;
zio_t *pio;
ASSERT3U(fio->io_size, ==, VCBS);
/*
* Add data to the cache.
*/
mutex_enter(&vc->vc_lock);
ASSERT3P(ve->ve_fill_io, ==, fio);
ASSERT3U(ve->ve_offset, ==, fio->io_offset);
ASSERT3P(ve->ve_abd, ==, fio->io_abd);
ve->ve_fill_io = NULL;
/*
* Even if this cache line was invalidated by a missed write update,
* any reads that were queued up before the missed update are still
* valid, so we can satisfy them from this line before we evict it.
*/
zio_link_t *zl = NULL;
while ((pio = zio_walk_parents(fio, &zl)) != NULL)
vdev_cache_hit(vc, ve, pio);
if (fio->io_error || ve->ve_missed_update)
vdev_cache_evict(vc, ve);
mutex_exit(&vc->vc_lock);
}
/*
* Read data from the cache. Returns B_TRUE cache hit, B_FALSE on miss.
*/
boolean_t
vdev_cache_read(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
vdev_cache_entry_t *ve, ve_search;
uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS);
zio_t *fio;
uint64_t cache_phase __maybe_unused = P2PHASE(zio->io_offset, VCBS);
ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
if (zfs_vdev_cache_size == 0)
return (B_FALSE);
if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
return (B_FALSE);
if (zio->io_size > zfs_vdev_cache_max)
return (B_FALSE);
/*
* If the I/O straddles two or more cache blocks, don't cache it.
*/
if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
return (B_FALSE);
ASSERT3U(cache_phase + zio->io_size, <=, VCBS);
mutex_enter(&vc->vc_lock);
ve_search.ve_offset = cache_offset;
ve = avl_find(&vc->vc_offset_tree, &ve_search, NULL);
if (ve != NULL) {
if (ve->ve_missed_update) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
if ((fio = ve->ve_fill_io) != NULL) {
zio_vdev_io_bypass(zio);
zio_add_child(zio, fio);
mutex_exit(&vc->vc_lock);
VDCSTAT_BUMP(vdc_stat_delegations);
return (B_TRUE);
}
vdev_cache_hit(vc, ve, zio);
zio_vdev_io_bypass(zio);
mutex_exit(&vc->vc_lock);
VDCSTAT_BUMP(vdc_stat_hits);
return (B_TRUE);
}
ve = vdev_cache_allocate(zio);
if (ve == NULL) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
ve->ve_abd, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
ve->ve_fill_io = fio;
zio_vdev_io_bypass(zio);
zio_add_child(zio, fio);
mutex_exit(&vc->vc_lock);
zio_nowait(fio);
VDCSTAT_BUMP(vdc_stat_misses);
return (B_TRUE);
}
/*
* Update cache contents upon write completion.
*/
void
vdev_cache_write(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
vdev_cache_entry_t *ve, ve_search;
uint64_t io_start = zio->io_offset;
uint64_t io_end = io_start + zio->io_size;
uint64_t min_offset = P2ALIGN(io_start, VCBS);
uint64_t max_offset = P2ROUNDUP(io_end, VCBS);
avl_index_t where;
ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
mutex_enter(&vc->vc_lock);
ve_search.ve_offset = min_offset;
ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
if (ve == NULL)
ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
while (ve != NULL && ve->ve_offset < max_offset) {
uint64_t start = MAX(ve->ve_offset, io_start);
uint64_t end = MIN(ve->ve_offset + VCBS, io_end);
if (ve->ve_fill_io != NULL) {
ve->ve_missed_update = 1;
} else {
abd_copy_off(ve->ve_abd, zio->io_abd,
start - ve->ve_offset, start - io_start,
end - start);
}
ve = AVL_NEXT(&vc->vc_offset_tree, ve);
}
mutex_exit(&vc->vc_lock);
}
void
vdev_cache_purge(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_entry_t *ve;
mutex_enter(&vc->vc_lock);
while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
vdev_cache_evict(vc, ve);
mutex_exit(&vc->vc_lock);
}
void
vdev_cache_init(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
sizeof (vdev_cache_entry_t),
offsetof(struct vdev_cache_entry, ve_offset_node));
avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
sizeof (vdev_cache_entry_t),
offsetof(struct vdev_cache_entry, ve_lastused_node));
}
void
vdev_cache_fini(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_purge(vd);
avl_destroy(&vc->vc_offset_tree);
avl_destroy(&vc->vc_lastused_tree);
mutex_destroy(&vc->vc_lock);
}
void
vdev_cache_stat_init(void)
{
vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
KSTAT_FLAG_VIRTUAL);
if (vdc_ksp != NULL) {
vdc_ksp->ks_data = &vdc_stats;
kstat_install(vdc_ksp);
}
}
void
vdev_cache_stat_fini(void)
{
if (vdc_ksp != NULL) {
kstat_delete(vdc_ksp);
vdc_ksp = NULL;
}
}
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_max, UINT, ZMOD_RW,
"Inflate reads small than max");
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_size, UINT, ZMOD_RD,
"Total size of the per-disk cache");
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_bshift, UINT, ZMOD_RW,
"Shift size to inflate reads too");
+2 -3
View File
@@ -748,8 +748,7 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
aio = zio_vdev_delegated_io(first->io_vd, first->io_offset,
abd, size, first->io_type, zio->io_priority,
flags | ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE,
vdev_queue_agg_io_done, NULL);
flags | ZIO_FLAG_DONT_QUEUE, vdev_queue_agg_io_done, NULL);
aio->io_timestamp = first->io_timestamp;
nio = first;
@@ -907,7 +906,7 @@ vdev_queue_io(zio_t *zio)
ASSERT(zio->io_priority == ZIO_PRIORITY_TRIM);
}
zio->io_flags |= ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE;
zio->io_flags |= ZIO_FLAG_DONT_QUEUE;
zio->io_timestamp = gethrtime();
mutex_enter(&vq->vq_lock);
+1 -14
View File
@@ -1617,12 +1617,6 @@ zio_read_bp_init(zio_t *zio)
ASSERT3P(zio->io_bp, ==, &zio->io_bp_copy);
}
if (!DMU_OT_IS_METADATA(BP_GET_TYPE(bp)) && BP_GET_LEVEL(bp) == 0)
zio->io_flags |= ZIO_FLAG_DONT_CACHE;
if (BP_GET_TYPE(bp) == DMU_OT_DDT_ZAP)
zio->io_flags |= ZIO_FLAG_DONT_CACHE;
if (BP_GET_DEDUP(bp) && zio->io_child_type == ZIO_CHILD_LOGICAL)
zio->io_pipeline = ZIO_DDT_READ_PIPELINE;
@@ -3955,9 +3949,6 @@ zio_vdev_io_start(zio_t *zio)
zio->io_type == ZIO_TYPE_WRITE ||
zio->io_type == ZIO_TYPE_TRIM)) {
if (zio->io_type == ZIO_TYPE_READ && vdev_cache_read(zio))
return (zio);
if ((zio = vdev_queue_io(zio)) == NULL)
return (NULL);
@@ -3994,9 +3985,6 @@ zio_vdev_io_done(zio_t *zio)
vd->vdev_ops != &vdev_draid_spare_ops) {
vdev_queue_io_done(zio);
if (zio->io_type == ZIO_TYPE_WRITE)
vdev_cache_write(zio);
if (zio_injection_enabled && zio->io_error == 0)
zio->io_error = zio_handle_device_injections(vd, zio,
EIO, EILSEQ);
@@ -4106,8 +4094,7 @@ zio_vdev_io_assess(zio_t *zio)
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_flags |= ZIO_FLAG_IO_RETRY | ZIO_FLAG_DONT_AGGREGATE;
zio->io_stage = ZIO_STAGE_VDEV_IO_START >> 1;
zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE,
zio_requeue_io_start_cut_in_line);