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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 Tony Hutter
parent 812c36fc71
commit 516a08ebb4
6 changed files with 140 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files
module/zfs/ddt_zap.c
+13 -1
View File
@@ -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;
module/zfs/dmu.c
+16
View File
@@ -81,6 +81,13 @@ int zfs_dmu_offset_next_sync = 0;
*/
int zfs_object_remap_one_indirect_delay_ms = 0;
/*
* Limit the amount we can prefetch with one call to this amount. This
* helps to limit the amount of memory that can be used by prefetching.
* Larger objects should be prefetched a bit at a time.
*/
int dmu_prefetch_max = 8 * SPA_MAXBLOCKSIZE;
const dmu_object_type_info_t dmu_ot[DMU_OT_NUMTYPES] = {
{DMU_BSWAP_UINT8, TRUE, FALSE, FALSE, "unallocated" },
{DMU_BSWAP_ZAP, TRUE, TRUE, FALSE, "object directory" },
@@ -667,6 +674,11 @@ dmu_prefetch(objset_t *os, uint64_t object, int64_t level, uint64_t offset,
return;
}
/*
* See comment before the definition of dmu_prefetch_max.
*/
len = MIN(len, dmu_prefetch_max);
/*
* XXX - Note, if the dnode for the requested object is not
* already cached, we will do a *synchronous* read in the
@@ -2629,6 +2641,10 @@ module_param(zfs_dmu_offset_next_sync, int, 0644);
MODULE_PARM_DESC(zfs_dmu_offset_next_sync,
"Enable forcing txg sync to find holes");
module_param(dmu_prefetch_max, int, 0644);
MODULE_PARM_DESC(dmu_prefetch_max,
"Limit one prefetch call to this size");
/* END CSTYLED */
#endif
module/zfs/zap.c
+55 -1
View File
@@ -20,7 +20,7 @@
*/
/*
* Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2012, 2016 by Delphix. All rights reserved.
* Copyright (c) 2012, 2018 by Delphix. All rights reserved.
* Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
*/
@@ -49,6 +49,36 @@
#include <sys/zap_impl.h>
#include <sys/zap_leaf.h>
/*
* If zap_iterate_prefetch is set, we will prefetch the entire ZAP object
* (all leaf blocks) when we start iterating over it.
*
* For zap_cursor_init(), the callers all intend to iterate through all the
* entries. There are a few cases where an error (typically i/o error) could
* cause it to bail out early.
*
* For zap_cursor_init_serialized(), there are callers that do the iteration
* outside of ZFS. Typically they would iterate over everything, but we
* don't have control of that. E.g. zfs_ioc_snapshot_list_next(),
* zcp_snapshots_iter(), and other iterators over things in the MOS - these
* are called by /sbin/zfs and channel programs. The other example is
* zfs_readdir() which iterates over directory entries for the getdents()
* syscall. /sbin/ls iterates to the end (unless it receives a signal), but
* userland doesn't have to.
*
* Given that the ZAP entries aren't returned in a specific order, the only
* legitimate use cases for partial iteration would be:
*
* 1. Pagination: e.g. you only want to display 100 entries at a time, so you
* get the first 100 and then wait for the user to hit "next page", which
* they may never do).
*
* 2. You want to know if there are more than X entries, without relying on
* the zfs-specific implementation of the directory's st_size (which is
* the number of entries).
*/
int zap_iterate_prefetch = B_TRUE;
int fzap_default_block_shift = 14; /* 16k blocksize */
extern inline zap_phys_t *zap_f_phys(zap_t *zap);
@@ -1189,6 +1219,21 @@ fzap_cursor_retrieve(zap_t *zap, zap_cursor_t *zc, zap_attribute_t *za)
/* retrieve the next entry at or after zc_hash/zc_cd */
/* if no entry, return ENOENT */
/*
* If we are reading from the beginning, we're almost certain to
* iterate over the entire ZAP object. If there are multiple leaf
* blocks (freeblk > 2), prefetch the whole object (up to
* dmu_prefetch_max bytes), so that we read the leaf blocks
* concurrently. (Unless noprefetch was requested via
* zap_cursor_init_noprefetch()).
*/
if (zc->zc_hash == 0 && zap_iterate_prefetch &&
zc->zc_prefetch && zap_f_phys(zap)->zap_freeblk > 2) {
dmu_prefetch(zc->zc_objset, zc->zc_zapobj, 0, 0,
zap_f_phys(zap)->zap_freeblk << FZAP_BLOCK_SHIFT(zap),
ZIO_PRIORITY_ASYNC_READ);
}
if (zc->zc_leaf &&
(ZAP_HASH_IDX(zc->zc_hash,
zap_leaf_phys(zc->zc_leaf)->l_hdr.lh_prefix_len) !=
@@ -1333,3 +1378,12 @@ fzap_get_stats(zap_t *zap, zap_stats_t *zs)
}
}
}
#if defined(_KERNEL)
/* BEGIN CSTYLED */
module_param(zap_iterate_prefetch, int, 0644);
MODULE_PARM_DESC(zap_iterate_prefetch,
"When iterating ZAP object, prefetch it");
/* END CSTYLED */
#endif
module/zfs/zap_micro.c
+26 -5
View File
@@ -21,7 +21,7 @@
/*
* Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2011, 2017 by Delphix. All rights reserved.
* Copyright (c) 2011, 2018 by Delphix. All rights reserved.
* Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
* Copyright 2017 Nexenta Systems, Inc.
*/
@@ -1472,9 +1472,9 @@ zap_remove_uint64(objset_t *os, uint64_t zapobj, const uint64_t *key,
* Routines for iterating over the attributes.
*/
void
zap_cursor_init_serialized(zap_cursor_t *zc, objset_t *os, uint64_t zapobj,
uint64_t serialized)
static void
zap_cursor_init_impl(zap_cursor_t *zc, objset_t *os, uint64_t zapobj,
uint64_t serialized, boolean_t prefetch)
{
zc->zc_objset = os;
zc->zc_zap = NULL;
@@ -1483,12 +1483,33 @@ zap_cursor_init_serialized(zap_cursor_t *zc, objset_t *os, uint64_t zapobj,
zc->zc_serialized = serialized;
zc->zc_hash = 0;
zc->zc_cd = 0;
zc->zc_prefetch = prefetch;
}
void
zap_cursor_init_serialized(zap_cursor_t *zc, objset_t *os, uint64_t zapobj,
uint64_t serialized)
{
zap_cursor_init_impl(zc, os, zapobj, serialized, B_TRUE);
}
/*
* Initialize a cursor at the beginning of the ZAP object. The entire
* ZAP object will be prefetched.
*/
void
zap_cursor_init(zap_cursor_t *zc, objset_t *os, uint64_t zapobj)
{
zap_cursor_init_serialized(zc, os, zapobj, 0);
zap_cursor_init_impl(zc, os, zapobj, 0, B_TRUE);
}
/*
* Initialize a cursor at the beginning, but request that we not prefetch
* the entire ZAP object.
*/
void
zap_cursor_init_noprefetch(zap_cursor_t *zc, objset_t *os, uint64_t zapobj)
{
zap_cursor_init_impl(zc, os, zapobj, 0, B_FALSE);
}
void