Another ZFS ARC memory question

Luke Marsden luke-lists at
Fri Feb 24 11:42:11 UTC 2012

Hi all,

Just wanted to get your opinion on best practices for ZFS.

We're running 8.2-RELEASE v15 in production on 24GB RAM amd64 machines
but have been having trouble with short spikes in application memory
usage resulting in huge amounts of swapping, bringing the whole machine
to its knees and crashing it hard.  I suspect this is because when there
is a sudden spike in memory usage the zfs arc reclaim thread is unable
to free system memory fast enough.

This most recently happened yesterday as you can see from the following
munin graphs:


Our response has been to start limiting the ZFS ARC cache to 4GB on our
production machines - trading performance for stability is fine with me
(and we have L2ARC on SSD so we still get good levels of caching).

My questions are:

      * is this a known problem?
      * what is the community's advice for production machines running
        ZFS on FreeBSD, is manually limiting the ARC cache (to ensure
        that there's enough actually free memory to handle a spike in
        application memory usage) the best solution to this
        spike-in-memory-means-crash problem?
      * has FreeBSD 9.0 / ZFS v28 solved this problem?
      * rather than setting a hard limit on the ARC cache size, is it
        possible to adjust the auto-tuning variables to leave more free
        memory for spiky memory situations?  e.g. set the auto-tuning to
        make arc eat 80% of memory instead of ~95% like it is at
      * could the arc reclaim thread be made to drop ARC pages with
        higher priority before the system starts swapping out
        application pages?

Thank you for any/all answers, and thank you for making FreeBSD
awesome :-)

Best Regards,
Luke Marsden

CTO, Hybrid Logic
+447791750420  |  +1-415-449-1165  | 

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