Poor ZFS ARC metadata hit/miss stats after recent ZFS updates
Yanhui Shen
shen.elf at gmail.com
Thu Jan 4 02:51:23 UTC 2018
This link might be helpful:
"Bug 222929 - ZFS ARC stats have wrong count"
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222929
Best regards,
Yanhui Shen
2016-10-17 20:34 GMT+08:00 Fabian Keil <freebsd-listen at fabiankeil.de>:
> After rebasing some of my systems from r305866 to r307312
> (plus local patches) I noticed that most of the ARC accesses
> are counted as misses now.
>
> Example:
>
> [fk at elektrobier2 ~]$ uptime
> 2:03PM up 1 day, 18:36, 7 users, load averages: 0.29, 0.36, 0.30
> [fk at elektrobier2 ~]$ zfs-stats -E
>
> ------------------------------------------------------------------------
> ZFS Subsystem Report Mon Oct 17 14:03:58 2016
> ------------------------------------------------------------------------
>
> ARC Efficiency: 3.38m
> Cache Hit Ratio: 12.87% 435.23k
> Cache Miss Ratio: 87.13% 2.95m
> Actual Hit Ratio: 9.55% 323.15k
>
> Data Demand Efficiency: 6.61% 863.01k
>
> CACHE HITS BY CACHE LIST:
> Most Recently Used: 18.97% 82.54k
> Most Frequently Used: 55.28% 240.60k
> Most Recently Used Ghost: 8.88% 38.63k
> Most Frequently Used Ghost: 24.84% 108.12k
>
> CACHE HITS BY DATA TYPE:
> Demand Data: 13.10% 57.03k
> Prefetch Data: 0.00% 0
> Demand Metadata: 32.94% 143.36k
> Prefetch Metadata: 53.96% 234.85k
>
> CACHE MISSES BY DATA TYPE:
> Demand Data: 27.35% 805.98k
> Prefetch Data: 0.00% 0
> Demand Metadata: 71.21% 2.10m
> Prefetch Metadata: 1.44% 42.48k
>
> ------------------------------------------------------------------------
>
> I suspect that this is caused by r307265 ("MFC r305323: MFV r302991:
> 6950 ARC should cache compressed data") which removed a
> ARCSTAT_CONDSTAT() call but I haven't confirmed this yet.
>
> The system performance doesn't actually seem to be negatively affected
> and repeated metadata accesses that are counted as misses are still served
> from memory. On my freshly booted laptop I get:
>
> fk at t520 /usr/ports $for i in 1 2 3; do \
> /usr/local/etc/munin/plugins/zfs-absolute-arc-hits-and-misses; \
> time git status > /dev/null; \
> done; \
> /usr/local/etc/munin/plugins/zfs-absolute-arc-hits-and-misses;
> zfs_arc_hits.value 5758
> zfs_arc_misses.value 275416
> zfs_arc_demand_metadata_hits.value 4331
> zfs_arc_demand_metadata_misses.value 270252
> zfs_arc_demand_data_hits.value 304
> zfs_arc_demand_data_misses.value 3345
> zfs_arc_prefetch_metadata_hits.value 1103
> zfs_arc_prefetch_metadata_misses.value 1489
> zfs_arc_prefetch_data_hits.value 20
> zfs_arc_prefetch_data_misses.value 334
>
> real 1m23.398s
> user 0m0.974s
> sys 0m12.273s
> zfs_arc_hits.value 11346
> zfs_arc_misses.value 389748
> zfs_arc_demand_metadata_hits.value 7723
> zfs_arc_demand_metadata_misses.value 381018
> zfs_arc_demand_data_hits.value 400
> zfs_arc_demand_data_misses.value 3412
> zfs_arc_prefetch_metadata_hits.value 3202
> zfs_arc_prefetch_metadata_misses.value 4885
> zfs_arc_prefetch_data_hits.value 21
> zfs_arc_prefetch_data_misses.value 437
>
> real 0m1.472s
> user 0m0.452s
> sys 0m1.820s
> zfs_arc_hits.value 11348
> zfs_arc_misses.value 428536
> zfs_arc_demand_metadata_hits.value 7723
> zfs_arc_demand_metadata_misses.value 419782
> zfs_arc_demand_data_hits.value 400
> zfs_arc_demand_data_misses.value 3436
> zfs_arc_prefetch_metadata_hits.value 3204
> zfs_arc_prefetch_metadata_misses.value 4885
> zfs_arc_prefetch_data_hits.value 21
> zfs_arc_prefetch_data_misses.value 437
>
> real 0m1.537s
> user 0m0.461s
> sys 0m1.860s
> zfs_arc_hits.value 11352
> zfs_arc_misses.value 467334
> zfs_arc_demand_metadata_hits.value 7723
> zfs_arc_demand_metadata_misses.value 458556
> zfs_arc_demand_data_hits.value 400
> zfs_arc_demand_data_misses.value 3460
> zfs_arc_prefetch_metadata_hits.value 3208
> zfs_arc_prefetch_metadata_misses.value 4885
> zfs_arc_prefetch_data_hits.value 21
> zfs_arc_prefetch_data_misses.value 437
>
> Disabling ARC compression through vfs.zfs.compressed_arc_enabled
> does not affect the accounting issue.
>
> Can anybody reproduce this?
>
> Fabian
>
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