Date: Mon, 04 Mar 2013 21:39:26 -0600 From: Karl Denninger <karl@denninger.net> Cc: freebsd-stable@freebsd.org Subject: Re: ZFS "stalls" -- and maybe we should be talking about defaults? Message-ID: <513568EE.80006@denninger.net> In-Reply-To: <8C68812328E3483BA9786EF15591124D@multiplay.co.uk> References: <513524B2.6020600@denninger.net> <1362449266.92708.8.camel@btw.pki2.com> <51355F64.4040409@denninger.net> <8C68812328E3483BA9786EF15591124D@multiplay.co.uk>
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On 3/4/2013 9:25 PM, Steven Hartland wrote: > ----- Original Message ----- From: "Karl Denninger" <karl@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. That's exactly what I can reproduce here; the stalls are anywhere from a few seconds to well north of a half-minute. It looks like the machine is hung -- but it is not. The machine in question normally runs with zero swap allocated but it always has 1.5Gb of shared memory allocated to Postgres ("shared_buffers = 1500MB" in its config file) I wonder if the ARC cache management code is misbehaving when shared segments are in use? -- -- Karl Denninger /The Market Ticker ®/ <http://market-ticker.org> Cuda Systems LLC
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