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Finally drop long disabled vdev cache.

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It was a vdev level read cache, designed to aggregate many small
reads by speculatively issuing bigger reads instead and caching
the result.  But since it has almost no idea about what is going
on with exception of ZIO_FLAG_DONT_CACHE flag set by higher layers,
it was found to make more harm than good, for which reason it was
disabled for the past 12 years.  These days we have much better
instruments to enlarge the I/Os, such as speculative and prescient
prefetches, I/O scheduler, I/O aggregation etc.

Besides just the dead code removal this removes one extra mutex
lock/unlock per write inside vdev_cache_write(), not otherwise
disabled and trying to do some work.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #14953
This commit is contained in:
Alexander Motin
2023-06-09 15:40:55 -04:00
committed by GitHub
parent 6db4ed51d6
commit 70ea484e3e
20 changed files with 13 additions and 562 deletions
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man/man4/zfs.4
-15
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@@ -2028,21 +2028,6 @@ Max vdev I/O aggregation size.
.It Sy zfs_vdev_aggregation_limit_non_rotating Ns = Ns Sy 131072 Ns B Po 128 KiB Pc Pq uint
Max vdev I/O aggregation size for non-rotating media.
.
.It Sy zfs_vdev_cache_bshift Ns = Ns Sy 16 Po 64 KiB Pc Pq uint
Shift size to inflate reads to.
.
.It Sy zfs_vdev_cache_max Ns = Ns Sy 16384 Ns B Po 16 KiB Pc Pq uint
Inflate reads smaller than this value to meet the
.Sy zfs_vdev_cache_bshift
size
.Pq default Sy 64 KiB .
.
.It Sy zfs_vdev_cache_size Ns = Ns Sy 0 Pq uint
Total size of the per-disk cache in bytes.
.Pp
Currently this feature is disabled, as it has been found to not be helpful
for performance and in some cases harmful.
.
.It Sy zfs_vdev_mirror_rotating_inc Ns = Ns Sy 0 Pq int
A number by which the balancing algorithm increments the load calculation for
the purpose of selecting the least busy mirror member when an I/O operation
man/man8/zpool-events.8
-1
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@@ -456,7 +456,6 @@ ZIO_FLAG_CANFAIL:0x00000080
ZIO_FLAG_SPECULATIVE:0x00000100
ZIO_FLAG_CONFIG_WRITER:0x00000200
ZIO_FLAG_DONT_RETRY:0x00000400
ZIO_FLAG_DONT_CACHE:0x00000800
ZIO_FLAG_NODATA:0x00001000
ZIO_FLAG_INDUCE_DAMAGE:0x00002000