Date: Thu, 4 Jan 2018 10:50:41 +0800 From: Yanhui Shen <shen.elf@gmail.com> Cc: stable@freebsd.org Subject: Re: Poor ZFS ARC metadata hit/miss stats after recent ZFS updates Message-ID: <CAEEM4vkJyzzPU1gCO=wvC4Y=5m6oVr-mf%2B%2BXfbS0UOqWTYiecQ@mail.gmail.com> In-Reply-To: <20161017143416.14024482@fabiankeil.de> References: <20161017143416.14024482@fabiankeil.de>
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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@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@elektrobier2 ~]$ uptime > 2:03PM up 1 day, 18:36, 7 users, load averages: 0.29, 0.36, 0.30 > [fk@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@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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