From 916384729e5d828e578c8f5c3003742360fb89ea Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Mon, 20 Nov 2017 19:56:01 +0200 Subject: OpenZFS 8835 - Speculative prefetch in ZFS not working for misaligned reads 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 Approved by: Gordon Ross Reviewed by: Matthew Ahrens Reviewed by: Allan Jude Reviewed by: Gvozden Neskovic Reviewed by: George Melikov Ported-by: Brian Behlendorf OpenZFS-issue: https://www.illumos.org/issues/8835 OpenZFS-commit: https://github.com/openzfs/openzfs/commit/aab6dd482a Closes #7062 --- module/zfs/dmu_zfetch.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-) (limited to 'module') diff --git a/module/zfs/dmu_zfetch.c b/module/zfs/dmu_zfetch.c index ea9afa282..a2bf80993 100644 --- a/module/zfs/dmu_zfetch.c +++ b/module/zfs/dmu_zfetch.c @@ -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); } } -- cgit v1.2.3