ZFS file caching question
Peter Schuller
peter.schuller at infidyne.com
Thu May 1 16:15:01 UTC 2008
> on FreeBSD. This is inefficient in a number of ways. I have also seen
> things that make me suspicious that it is not caching properly even when
> you tune it to be "large enough" (if possible given memory constraints),
> but I haven't confirmed this.
FWIW, I have had some suspiciouns here too strictly from a user-centric point
of view.
Most recently I found that I could not get ZFS to cache enough such
that "pkg_info" would not have to get down on disk (thus making all package
installation and similar dead-slow, due to the number of times /var/db/pkg is
accesses in deep dependency hierarchies).
In this case I tended up killing as much as I could on the machine (web
browsers, MUA:s, whatever else), on the theory that large files being kept
open would affect the caching strategy. Even after killing most stuff and
after multiple pkg_info:s (to accumulate frequency), it was still only
caching enough for perhaps 20% of what pkg_info needs on this machine (when
invoked without arguments, listing all packages).
I don't know if this is just a result of giving too much weight to past
history or if there is an actual bug somewhere; but the perceived end-result
for me as a user was clearly significantly worse than a plain page-wise LRU
would have given.
I ended up rebooting the machine to get package upgrades to complete within a
reasonable amount of time.
This was on my 64 bit desktop machine with:
vm.kmem_size_max="1258291200"
vfs.zfs.prefetch_disable="1"
vfs.zfs.arc_max="838860800"
--
/ Peter Schuller
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