Dan Nelson dnelson at allantgroup.com
Tue Sep 30 04:12:20 UTC 2008

In the last episode (Sep 30), Andrew Snow said:
> Zaphod Beeblebrox wrote:
> > Also, there exists data within the ARC (I'm always tempted to say
> > the ARC Cache, but that is redundant) that is also then in paging
> > memory.
> OK, but one advantage of ZFS memory consumption is under heavy write
> loads, where much of the memory is used to store and reorder writes.
> The heavy memory consumption under reading is a shame, but ZFS has to
> cache and use more metadata than UFS, so its a price you pay for the
> extra features and benefits.
> What I think we need is a way to turn off read-caching except for
> metadata.  This allows ARC to only be used more efficiently. 
> Currently you can turn all read-ahead on or off, with the provided
> sysctl tunables, but would be easy to implement a metadata-only
> option.  I found that access speed suffers when metadata is not
> prefetched.

That'd be handy, but at least on my system the data prefetcher isn't
really called often enough to make a difference either way (assuming
the counts are accurate).  Metadata prefetch is a big win, however.

(dan at dan.15) /home/dan> uptime
11:00PM  up 5 days, 13:47, 21 users, load averages: 1.52, 1.68, 1.69
(dan at dan.15) /home/dan> sysctl kstat
kstat.zfs.misc.arcstats.hits: 211130907 (95%)
kstat.zfs.misc.arcstats.misses: 9808431
kstat.zfs.misc.arcstats.demand_data_hits: 116614377 (98%)
kstat.zfs.misc.arcstats.demand_data_misses: 2477943
kstat.zfs.misc.arcstats.demand_metadata_hits: 55805261 (96%)
kstat.zfs.misc.arcstats.demand_metadata_misses: 2310006
kstat.zfs.misc.arcstats.prefetch_data_hits: 79878 (53%)
kstat.zfs.misc.arcstats.prefetch_data_misses: 71741
kstat.zfs.misc.arcstats.prefetch_metadata_hits: 38556033 (88%)
kstat.zfs.misc.arcstats.prefetch_metadata_misses: 4947270
kstat.zfs.misc.arcstats.mru_hits: 23702582 (95%)
kstat.zfs.misc.arcstats.mru_ghost_hits: 1274189
kstat.zfs.misc.arcstats.mfu_hits: 149722171 (98%)
kstat.zfs.misc.arcstats.mfu_ghost_hits: 2944572
kstat.zfs.misc.arcstats.p: 235221504
kstat.zfs.misc.arcstats.c: 268435456
kstat.zfs.misc.arcstats.c_min: 67108864
kstat.zfs.misc.arcstats.c_max: 268435456
kstat.zfs.misc.arcstats.size: 263926784

	Dan Nelson
	dnelson at allantgroup.com

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