Date: Tue, 8 May 2012 15:58:33 +0100 From: Tom Evans <tevans.uk@googlemail.com> To: Michael Gmelin <freebsd@grem.de> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS resilvering strangles IO Message-ID: <CAFHbX1K0--P-Sh0QdLszEs0V1ocWoe6Jp_SY9H%2BVJd1AQw2XKA@mail.gmail.com> In-Reply-To: <73F8D020-04F3-44B2-97D4-F08E3B253C32@grem.de> References: <73F8D020-04F3-44B2-97D4-F08E3B253C32@grem.de>
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On Tue, May 8, 2012 at 3:33 PM, Michael Gmelin <freebsd@grem.de> wrote: > So the question is, is there anything I can do to improve the situation? > Is this because of memory constraints? Are there any other knobs to > adjust? As far as I know zfs_resilver_delay can't be changed in FreeBSD yet. > > I have more drives around, so I could replace another one in the server, > just to replicate the exact situation. > In general, raidz is pretty fast, but when it's resilvering it is just too busy. The first thing I would do to speed up writes is to add a log device, preferably a SSD. Having a log device will allow the pool to buffer writes to the pool much more effectively than normally during a resilver. Having lots of small writes will kill read speed during the resilver, which is the critical thing. If your workload would benefit, you could split the SSD down the middle, use half for a log device, and half for a cache device to accelerate reads. I've never tried using a regular disk as a log device, I wonder if that would speed up resilvering? Cheers Tom
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