ZFS performance degradation over time

Richard Todd rmtodd at ichotolot.servalan.com
Sun Jan 3 20:05:50 UTC 2010

Garrett Moore <garrettmoore at gmail.com> writes:

> I'm having problems with ZFS performance. When my system comes up,
> read/write speeds are excellent (testing with dd if=/dev/zero
> of=/tank/bigfile and dd if=/tank/bigfile of=/dev/null); I get at least
> 100MB/s on both reads and writes, and I'm happy with that.
> The longer the system is up, the worse my performance gets. Currently my
> system has been up for 4 days, and read/write performance is down to about
> 10MB/s at best.
> The system is only accessed by 3 clients: myself, my roommate, and our HTPC.
> Usually, only one client will be doing anything at a time, so it is not
> under heavy load or anything.

This could be due to the amount of memory available for ZFS caching declining
as time goes on (for reasons that are not entirely clear, though I suspect
it may be due to increasing fragmentation in the kernel memory).  You might
try doing "sysctl kstat.zfs.misc.arcstats.size" repeatedly to monitor the
amount of memory the ZFS cache is taking up as your system uptime increases.


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