ZFS "stalls" -- and maybe we should be talking about defaults?

Gary Palmer gpalmer at freebsd.org
Tue Mar 5 15:22:54 UTC 2013

On Tue, Mar 05, 2013 at 12:40:38AM -0500, Garrett Wollman wrote:
> In article <8C68812328E3483BA9786EF15591124D at multiplay.co.uk>,
> killing at multiplay.co.uk writes:
> >Now interesting you should say that I've seen a stall recently on ZFS
> >only box running on 6 x SSD RAIDZ2.
> >
> >The stall was caused by fairly large mysql import, with nothing else
> >running.
> >
> >Then it happened I thought the machine had wedged, but minutes (not
> >seconds) later, everything sprung into action again.
> I have certainly seen what you might describe as "stalls", caused, so
> far as I can tell, by kernel memory starvation.  I've seen it take as
> much as a half an hour to recover from these (which is too long for my
> users).  Right now I have the ARC limited to 64 GB (on a 96 GB file
> server) and that has made it more stable, but it's still not behaving
> quite as I would like, and I'm looking to put more memory into the
> system (to be used for non-ARC functions).  Looking at my munin
> graphs, I find that backups in particular put very heavy pressure on,
> doubling the UMA allocations over steady-state, and this takes about
> four or five hours to climb back down.  See
> <http://people.freebsd.org/~wollman/vmstat_z-day.png> for an example.
> Some of the stalls are undoubtedly caused by internal fragmentation
> rather than actual data in use.  (Solaris used to have this issue, and
> some hooks were added to allow some amount of garbage collection with
> the cooperation of the filesystem.)

Just as a note that there was a page I read in the past few months
that pointed out that having a huge ARC may not always be in the best
interests of the system.  Some operation on the filesystem (I forget
what, apologies) caused the system to churn through the ARC and discard
most of it, while regular I/O was blocked

Unfortunately I cannot remember where I found that page now and I don't
appear to have bookmarked it

>From what has been said in this thread I'm not convinced that people
are hitting this issue, however I would like to raise it for



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