Date: Wed, 9 May 2012 00:48:22 +0200 From: Michael Gmelin <freebsd@grem.de> To: freebsd-fs@freebsd.org Subject: Re: ZFS resilvering strangles IO Message-ID: <1CEFF50A-4CD5-4947-8A38-2EEAE3311E67@grem.de> In-Reply-To: <alpine.GSO.2.01.1205081732090.9406@freddy.simplesystems.org> References: <73F8D020-04F3-44B2-97D4-F08E3B253C32@grem.de> <CAFHbX1K0--P-Sh0QdLszEs0V1ocWoe6Jp_SY9H%2BVJd1AQw2XKA@mail.gmail.com> <180B72CE-B285-4702-B16D-0714AA07022C@grem.de> <alpine.GSO.2.01.1205081625470.9406@freddy.simplesystems.org> <CAOjFWZ7ik_sUmUaw4im729dc-2Toq2j_z_oxiqUpzc4x_TOujQ@mail.gmail.com> <CAFqOu6hxww5a1CLwYOZZcZNkJVhwH2eUXmtJKNwm6ohNmcqP0Q@mail.gmail.com> <44759017-6FAC-4982-B382-CE17DED83262@grem.de> <alpine.GSO.2.01.1205081732090.9406@freddy.simplesystems.org>
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On May 9, 2012, at 00:42, Bob Friesenhahn wrote: > On Wed, 9 May 2012, Michael Gmelin wrote: >>>>=20 >>>> Setting vfs.zfs.vdev_max_pending=3D"4" in /boot/loader.conf (or = whatever >>>> value you want). The default is 10. >>=20 >> Do you think this will actually make a difference. As far as I >> understand my primary problem is not latency but throughput. Simple >> example is dd if=3D/dev/zero of=3Dfilename bs=3D1m, which gave me = 500kb/s. >> Latency might be an additional problem (or am I mislead and a shorter >> queue would raise the processes chance to get data through?). >=20 > The effect may be observed in real-time on a running system. Latency = and throughput go hand in hand. The 'dd' command is not threaded and is = sequential. It waits for the current I/O to return before it starts the = next one. If the wait is shorter (fewer pending requests in line), then = throughput does increase. System total throughput (which includes the = resilver operations) may not increase but the throughput observed by an = individual waiter may increase. >=20 > The default for vdev_max_pending on Solaris was/is 32. If FreeBSD = uses a default of 10 then reducing from the default may be less = dramatic. >=20 That makes sense. I will run more sophisticated I/O tests next time to get a more complete picture. --=20 Michael > Bob > --=20 > Bob Friesenhahn > bfriesen@simple.dallas.tx.us, = http://www.simplesystems.org/users/bfriesen/ > GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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