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MFV r328253: 8835 Speculative prefetch in ZFS not working for misaligned reads

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illumos/illumos-gate@5cb8d943bc

https://www.illumos.org/issues/8835:
Sequential reads not aligned to block size are not detected by ZFS
prefetcher as sequential, killing prefetch and severely hurting
performance.  It is caused by dmu_zfetch() in case of misaligned
sequential accesses being called with overlap of one block.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Alexander Motin <mav@FreeBSD.org>
This commit is contained in:
Alexander Motin 2018-01-22 05:57:14 +00:00
commit 1dbc0fc352
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=328254
1 changed files with 19 additions and 5 deletions
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24
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_zfetch.c
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@ -240,19 +240,33 @@ dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
rw_enter(&zf->zf_rwlock, RW_READER);
/*
* Find matching prefetch stream. Depending on whether the accesses
* are block-aligned, first block of the new access may either follow
* the last block of the previous access, or be equal to it.
*/
for (zs = list_head(&zf->zf_stream); zs != NULL;
zs = list_next(&zf->zf_stream, zs)) {
if (blkid == zs->zs_blkid) {
if (blkid == zs->zs_blkid || blkid + 1 == zs->zs_blkid) {
mutex_enter(&zs->zs_lock);
/*
* zs_blkid could have changed before we
* acquired zs_lock; re-check them here.
*/
if (blkid != zs->zs_blkid) {
mutex_exit(&zs->zs_lock);
continue;
if (blkid == zs->zs_blkid) {
break;
} else if (blkid + 1 == zs->zs_blkid) {
blkid++;
nblks--;
if (nblks == 0) {
/* Already prefetched this before. */
mutex_exit(&zs->zs_lock);
rw_exit(&zf->zf_rwlock);
return;
}
break;
}
break;
mutex_exit(&zs->zs_lock);
}
}