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Improve speculative prefetcher for block cloning #16814

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Merged
merged 1 commit into from
Dec 4, 2024
Merged

Improve speculative prefetcher for block cloning #16814

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34 changes: 29 additions & 5 deletions module/zfs/dmu_zfetch.c
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Original file line number Diff line number Diff line change
Expand Up @@ -63,8 +63,8 @@ static unsigned int zfetch_min_distance = 4 * 1024 * 1024;
/* max bytes to prefetch per stream (default 64MB) */
unsigned int zfetch_max_distance = 64 * 1024 * 1024;
#endif
/* max bytes to prefetch indirects for per stream (default 64MB) */
unsigned int zfetch_max_idistance = 64 * 1024 * 1024;
/* max bytes to prefetch indirects for per stream (default 128MB) */
unsigned int zfetch_max_idistance = 128 * 1024 * 1024;
/* max request reorder distance within a stream (default 16MB) */
unsigned int zfetch_max_reorder = 16 * 1024 * 1024;
/* Max log2 fraction of holes in a stream */
Expand Down Expand Up @@ -472,6 +472,7 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
zstream_t *zs;
spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
zfs_prefetch_type_t os_prefetch = zf->zf_dnode->dn_objset->os_prefetch;
int64_t ipf_start, ipf_end;

if (zfs_prefetch_disable || os_prefetch == ZFS_PREFETCH_NONE)
return (NULL);
Expand Down Expand Up @@ -571,13 +572,13 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
* This access is not part of any existing stream. Create a new
* stream for it unless we are at the end of file.
*/
ASSERT0P(zs);
if (end_blkid < maxblkid)
dmu_zfetch_stream_create(zf, end_blkid);
mutex_exit(&zf->zf_lock);
if (!have_lock)
rw_exit(&zf->zf_dnode->dn_struct_rwlock);
ZFETCHSTAT_BUMP(zfetchstat_misses);
return (NULL);
ipf_start = 0;
goto prescient;

hit:
nblks = dmu_zfetch_hit(zs, nblks);
Expand Down Expand Up @@ -650,6 +651,7 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
pf_nblks = zs->zs_ipf_dist >> dbs;
if (zs->zs_ipf_start < zs->zs_pf_end)
zs->zs_ipf_start = zs->zs_pf_end;
ipf_start = zs->zs_ipf_end;
if (zs->zs_ipf_end < zs->zs_pf_end + pf_nblks)
zs->zs_ipf_end = zs->zs_pf_end + pf_nblks;

Expand All @@ -658,8 +660,30 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
zfs_refcount_add(&zs->zs_callers, NULL);
mutex_exit(&zf->zf_lock);

prescient:
/*
* Prefetch the following indirect blocks for this access to reduce
* dbuf_hold() sync read delays in dmu_buf_hold_array_by_dnode().
* This covers the gap during the first couple accesses when we can
* not predict the future yet, but know what is needed right now.
* This should be very rare for reads/writes to need more than one
* indirect, but more useful for cloning due to much bigger accesses.
*/
ipf_start = MAX(ipf_start, blkid + 1);
int epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT;
ipf_start = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs;
ipf_end = P2ROUNDUP(end_blkid, 1 << epbs) >> epbs;

int issued = 0;
for (int64_t iblk = ipf_start; iblk < ipf_end; iblk++) {
issued += dbuf_prefetch(zf->zf_dnode, 1, iblk,
ZIO_PRIORITY_SYNC_READ, ARC_FLAG_PRESCIENT_PREFETCH);
}

if (!have_lock)
rw_exit(&zf->zf_dnode->dn_struct_rwlock);
if (issued)
ZFETCHSTAT_ADD(zfetchstat_io_issued, issued);
return (zs);
}

Expand Down