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fat zap should prefetch when iterating

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When iterating over a ZAP object, we're almost always certain to iterate
over the entire object. If there are multiple leaf blocks, we can
realize a performance win by issuing reads for all the leaf blocks in
parallel when the iteration begins.

For example, if we have 10,000 snapshots, "zfs destroy -nv
pool/fs@1%9999" can take 30 minutes when the cache is cold. This change
provides a >3x performance improvement, by issuing the reads for all ~64
blocks of each ZAP object in parallel.

Reviewed-by: Andreas Dilger <andreas.dilger@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-58347
Closes #8862
This commit is contained in:
Matthew Ahrens
2019-06-12 13:13:09 -07:00
committed by Brian Behlendorf
parent d9cd66e45f
commit d9b4bf0665
6 changed files with 140 additions and 9 deletions
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module/zfs/ddt_zap.c
+13 -1
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@@ -21,6 +21,7 @@
/*
* Copyright (c) 2009, 2010, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2018 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
@@ -117,7 +118,18 @@ ddt_zap_walk(objset_t *os, uint64_t object, ddt_entry_t *dde, uint64_t *walk)
zap_attribute_t za;
int error;
zap_cursor_init_serialized(&zc, os, object, *walk);
if (*walk == 0) {
/*
* We don't want to prefetch the entire ZAP object, because
* it can be enormous. Also the primary use of DDT iteration
* is for scrubbing, in which case we will be issuing many
* scrub I/Os for each ZAP block that we read in, so
* reading the ZAP is unlikely to be the bottleneck.
*/
zap_cursor_init_noprefetch(&zc, os, object);
} else {
zap_cursor_init_serialized(&zc, os, object, *walk);
}
if ((error = zap_cursor_retrieve(&zc, &za)) == 0) {
uchar_t cbuf[sizeof (dde->dde_phys) + 1];
uint64_t csize = za.za_num_integers;