ZFS arc sizing (maybe related to kern/145229)
Adam Nowacki
nowak at xpam.de
Thu Apr 8 00:00:04 UTC 2010
check kstat.zfs.misc.arcstats.memory_throttle_count
This counter is increased every time zfs thinks system is running low on
memory and will force a write flush and reduce arc size to minimum.
Biggest problem is that the code is counting only free memory and
completely ignoring other memory that can be immediately freed like
cached files from ufs. This is very easy to trigger on mixed ufs and zfs
system by just reading enough data from ufs to fill its cache, zfs will
begin throttling and will continue doing so even with no further ufs
reads or writes.
Rich wrote:
> A datapoint for you:
> Now running 8-STABLE (plus the mbuf leak fix which went in recently),
> here's my ARC stats and ARC sysctl settings after the server was up
> for about a week (5 days) after that:
> ARC Size:
> Current Size: 587.49M (arcsize)
> Target Size: (Adaptive) 587.63M (c)
> Min Size (Hard Limit): 512.00M (arc_min)
> Max Size (Hard Limit): 3072.00M (arc_max)
>
> ARC Size Breakdown:
> Recently Used Cache Size: 98.28% 577.50M (p)
> Frequently Used Cache Size: 1.72% 10.12M (c-p)
>
> ARC Efficiency:
> Cache Access Total: 2602789964
> Cache Hit Ratio: 96.11% 2501461882
> Cache Miss Ratio: 3.89% 101328082
> Actual Hit Ratio: 87.65% 2281380527
>
> and
>
> vfs.zfs.arc_meta_limit=1073741824
> vfs.zfs.arc_meta_used=548265792
> vfs.zfs.arc_min=536870912
> vfs.zfs.arc_max=3221225472
>
> So it very clearly limits to near the minimum size, but whether this
> is design or accidental behavior, I'm unsure.
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