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Prefetch on deadlists merge

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During snapshot deletion ZFS may issue several reads for each deadlist
to merge them into next snapshot's or pool's bpobj.  Number of the dead
lists increases with number of snapshots.  On HDD pools it may take
significant time during which sync thread is blocked.

This patch introduces prescient prefetch of required blocks for up to
128 deadlists ahead.  Tests show reduction of time required to delete
dataset with 720 snapshots with randomly overwritten file on wide HDD
pool from 75-85 to 22-28 seconds.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Issue #14276 
Closes #14402
This commit is contained in:
Alexander Motin
2023-01-25 14:30:24 -05:00
committed by GitHub
parent c85ac731a0
commit dc5c8006f6
3 changed files with 131 additions and 6 deletions
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module/zfs/dsl_deadlist.c
+67 -4
View File
@@ -438,6 +438,18 @@ dle_enqueue_subobj(dsl_deadlist_t *dl, dsl_deadlist_entry_t *dle,
}
}
/*
* Prefetch metadata required for dle_enqueue_subobj().
*/
static void
dle_prefetch_subobj(dsl_deadlist_t *dl, dsl_deadlist_entry_t *dle,
uint64_t obj)
{
if (dle->dle_bpobj.bpo_object !=
dmu_objset_pool(dl->dl_os)->dp_empty_bpobj)
bpobj_prefetch_subobj(&dle->dle_bpobj, obj);
}
void
dsl_deadlist_insert(dsl_deadlist_t *dl, const blkptr_t *bp, boolean_t bp_freed,
dmu_tx_t *tx)
@@ -810,6 +822,27 @@ dsl_deadlist_insert_bpobj(dsl_deadlist_t *dl, uint64_t obj, uint64_t birth,
dle_enqueue_subobj(dl, dle, obj, tx);
}
/*
* Prefetch metadata required for dsl_deadlist_insert_bpobj().
*/
static void
dsl_deadlist_prefetch_bpobj(dsl_deadlist_t *dl, uint64_t obj, uint64_t birth)
{
dsl_deadlist_entry_t dle_tofind;
dsl_deadlist_entry_t *dle;
avl_index_t where;
ASSERT(MUTEX_HELD(&dl->dl_lock));
dsl_deadlist_load_tree(dl);
dle_tofind.dle_mintxg = birth;
dle = avl_find(&dl->dl_tree, &dle_tofind, &where);
if (dle == NULL)
dle = avl_nearest(&dl->dl_tree, where, AVL_BEFORE);
dle_prefetch_subobj(dl, dle, obj);
}
static int
dsl_deadlist_insert_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed,
dmu_tx_t *tx)
@@ -826,12 +859,12 @@ dsl_deadlist_insert_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed,
void
dsl_deadlist_merge(dsl_deadlist_t *dl, uint64_t obj, dmu_tx_t *tx)
{
zap_cursor_t zc;
zap_attribute_t za;
zap_cursor_t zc, pzc;
zap_attribute_t za, pza;
dmu_buf_t *bonus;
dsl_deadlist_phys_t *dlp;
dmu_object_info_t doi;
int error;
int error, perror, i;
VERIFY0(dmu_object_info(dl->dl_os, obj, &doi));
if (doi.doi_type == DMU_OT_BPOBJ) {
@@ -843,15 +876,32 @@ dsl_deadlist_merge(dsl_deadlist_t *dl, uint64_t obj, dmu_tx_t *tx)
}
mutex_enter(&dl->dl_lock);
/*
* Prefetch up to 128 deadlists first and then more as we progress.
* The limit is a balance between ARC use and diminishing returns.
*/
for (zap_cursor_init(&pzc, dl->dl_os, obj), i = 0;
(perror = zap_cursor_retrieve(&pzc, &pza)) == 0 && i < 128;
zap_cursor_advance(&pzc), i++) {
dsl_deadlist_prefetch_bpobj(dl, pza.za_first_integer,
zfs_strtonum(pza.za_name, NULL));
}
for (zap_cursor_init(&zc, dl->dl_os, obj);
(error = zap_cursor_retrieve(&zc, &za)) == 0;
zap_cursor_advance(&zc)) {
uint64_t mintxg = zfs_strtonum(za.za_name, NULL);
dsl_deadlist_insert_bpobj(dl, za.za_first_integer, mintxg, tx);
VERIFY0(zap_remove_int(dl->dl_os, obj, mintxg, tx));
if (perror == 0) {
dsl_deadlist_prefetch_bpobj(dl, pza.za_first_integer,
zfs_strtonum(pza.za_name, NULL));
zap_cursor_advance(&pzc);
perror = zap_cursor_retrieve(&pzc, &pza);
}
}
VERIFY3U(error, ==, ENOENT);
zap_cursor_fini(&zc);
zap_cursor_fini(&pzc);
VERIFY0(dmu_bonus_hold(dl->dl_os, obj, FTAG, &bonus));
dlp = bonus->db_data;
@@ -869,8 +919,9 @@ dsl_deadlist_move_bpobj(dsl_deadlist_t *dl, bpobj_t *bpo, uint64_t mintxg,
dmu_tx_t *tx)
{
dsl_deadlist_entry_t dle_tofind;
dsl_deadlist_entry_t *dle;
dsl_deadlist_entry_t *dle, *pdle;
avl_index_t where;
int i;
ASSERT(!dl->dl_oldfmt);
@@ -882,11 +933,23 @@ dsl_deadlist_move_bpobj(dsl_deadlist_t *dl, bpobj_t *bpo, uint64_t mintxg,
dle = avl_find(&dl->dl_tree, &dle_tofind, &where);
if (dle == NULL)
dle = avl_nearest(&dl->dl_tree, where, AVL_AFTER);
/*
* Prefetch up to 128 deadlists first and then more as we progress.
* The limit is a balance between ARC use and diminishing returns.
*/
for (pdle = dle, i = 0; pdle && i < 128; ) {
bpobj_prefetch_subobj(bpo, pdle->dle_bpobj.bpo_object);
pdle = AVL_NEXT(&dl->dl_tree, pdle);
}
while (dle) {
uint64_t used, comp, uncomp;
dsl_deadlist_entry_t *dle_next;
bpobj_enqueue_subobj(bpo, dle->dle_bpobj.bpo_object, tx);
if (pdle) {
bpobj_prefetch_subobj(bpo, pdle->dle_bpobj.bpo_object);
pdle = AVL_NEXT(&dl->dl_tree, pdle);
}
VERIFY0(bpobj_space(&dle->dle_bpobj,
&used, &comp, &uncomp));