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OpenZFS 8835 - Speculative prefetch in ZFS not working for misaligned reads

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In case of misaligned I/O sequential requests are not detected as such
due to overlaps in logical block sequence:

    dmu_zfetch(fffff80198dd0ae0, 27347, 9, 1)
    dmu_zfetch(fffff80198dd0ae0, 27355, 9, 1)
    dmu_zfetch(fffff80198dd0ae0, 27363, 9, 1)
    dmu_zfetch(fffff80198dd0ae0, 27371, 9, 1)
    dmu_zfetch(fffff80198dd0ae0, 27379, 9, 1)
    dmu_zfetch(fffff80198dd0ae0, 27387, 9, 1)

This patch makes single block overlap to be counted as a stream hit,
improving performance up to several times.

Authored by: Alexander Motin <mav@FreeBSD.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Gvozden Neskovic <neskovic@gmail.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/8835
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/aab6dd482a
Closes #7062
This commit is contained in:
Alexander Motin 2017-11-20 19:56:01 +02:00 committed by Brian Behlendorf
parent 31864e3d8c
commit 916384729e
1 changed files with 19 additions and 5 deletions
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24
module/zfs/dmu_zfetch.c
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@ -226,19 +226,33 @@ 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);
}
}