Date: Tue, 22 Jul 2008 15:22:17 -0700 From: Matt Simerson <matt@corp.spry.com> To: freebsd-fs@freebsd.org Subject: Re: ZFS hang issue and prefetch_disable Message-ID: <BAA45F6C-FD85-4767-B6AB-F3D2C8D31645@corp.spry.com> In-Reply-To: <48865A68.1010504@quip.cz> References: <5E8D64DE-EC9B-4B11-BCB4-17BA63650BB7@corp.spry.com> <48865A68.1010504@quip.cz>
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On Jul 22, 2008, at 3:08 PM, Miroslav Lachman wrote: > Matt Simerson wrote: >> Symptoms >> Deadlocks under heavy IO load on the ZFS file system with >> prefetch_disable=0. Setting vfs.zfs.prefetch_disable=1 results in >> a stable system. > > [...] > >> With vfs.zfs.prefetch_disable=1, a hang will occur within a few >> hours (no more than a day). If I keep the i/o load (measured via >> iostat) down to a low level (< 200 iops) then I still get hangs >> but less frequently (1-6 days). The only way I have found to >> prevent the hangs is by setting vfs.zfs.prefetch_disable=1. > > "With vfs.zfs.prefetch_disable=1, a hang will occur within...", did > you realy mean prefetch_disable=1 in this sentence? Your whole e- > mail seems that prefetch_disable=1 is good workaround, so I expect > you have prefetch_disable=0 previously which causes hangs... Aye. That is exactly what I meant. With vfs.zfs.prefetch_disable=1, I get a stable system. With vfs.zfs.prefetch_disable=0 (the default) I have frequent deadlocks. Matt Rant: I really wish that variable wasn't named in the negative, creating a double negative when prefetch_disable=0. IE, it should be named vfs.zfs.prefetch_enable instead. It's much easier to express in English that prefetch_enable=1 means ON and prefetch_enable=0 means OFF. There's also the matter than in some languages, a double or triple negative still means the negative case. %-\. I'd rather not have to guess what prefetch_disable=1 means.
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