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

Steven Hartland killing at multiplay.co.uk
Tue Mar 5 03:25:19 UTC 2013

----- 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

Then it happened I thought the machine had wedged, but minutes (not
seconds) later, everything sprung into action again.

> Am I correct that a ZFS filesystem does NOT use the VM buffer cache
> at all?



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