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

Dennis Glatting dg at pki2.com
Tue Mar 5 03:54:43 UTC 2013

On Tue, 2013-03-05 at 03:25 +0000, Steven Hartland wrote:
> ----- Original Message ----- 
> From: "Karl Denninger" <karl at denninger.net>
> > Stick this in /boot/loader.conf and see if your lockups goes away:
> >
> > vfs.zfs.write_limit_override=1024000000
> ...
> > If it turns out that the write_limit_override tunable is the one
> > responsible for stopping the hangs I can drop the ARC limit tunable
> > although I'm not sure I want to; I don't see much if any performance
> > penalty from leaving it where it is and if the larger cache isn't
> > helping anything then why use it?  I'm inclined to stick an SSD in the
> > cabinet as a cache drive instead of dedicating RAM to this -- even
> > though it's not AS fast as RAM it's still MASSIVELY quicker than getting
> > data off a rotating plate of rust.
> 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've seen this too.

> > Am I correct that a ZFS filesystem does NOT use the VM buffer cache
> > at all?
> Correct
>     Regards
>     Steve
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Dennis Glatting <dg at pki2.com>

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