Date: Thu, 12 Nov 2009 20:51:30 -0600 From: "James R. Van Artsdalen" <james-freebsd-current@jrv.org> To: Artem Belevich <fbsdlist@src.cx> Cc: freebsd-current@freebsd.org Subject: Re: Help ZFS FreeBSD 8.0 RC2 Write performance issue Message-ID: <4AFCC9B2.2050401@jrv.org> In-Reply-To: <ed91d4a80911121219sa0f3fa3s934791725028a966@mail.gmail.com> References: <11167f520911111050j36dd94far667c81e6f5c18e69@mail.gmail.com> <hdf23m$b6n$1@ger.gmane.org> <20091111204903.GI89052@dan.emsphone.com> <11167f520911111326v13bb442bt36e853afbecdf834@mail.gmail.com> <9bbcef730911111352t12188bdajbca71bcf35a5beb5@mail.gmail.com> <Pine.GSO.4.63.0911121147310.9126@muncher.cs.uoguelph.ca> <11167f520911121044l74744c30u5a4d9ca008ab863c@mail.gmail.com> <d5992baf0911121048o76c1543el2b4d9de84ffb107@mail.gmail.com> <11167f520911121101o403751ddmb544dfaf1c61bf1e@mail.gmail.com> <ed91d4a80911121219sa0f3fa3s934791725028a966@mail.gmail.com>
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Artem Belevich wrote: > Log seems to be somewhat weak point in ZFS. If you lose your log > device, you will lose your pool. In ZFS losing *any* vdev causes a pool to fault! The log vdev is no different and is not a weak point in that regard. In ZFS *every* vdev should be redundant. In the case of a log this means a MIRROR since a log cannot be a RAIDZ. That's not a big deal since you really should put the MIRROR disks on different controllers, and preferably using different drives, independent cables & power supplies, etc. (I think a cache vdev is different and need not be reliable, but I have not tested to see what happens if it is not present when the pool is imported) The ZIL may have bugs but it's a very important feature to have, necessary in a transactional filesystem. A mythical "zfsck" program could probably wipe out the ZIL allowing the pool to import from the most recent uberblock state and losing a few seconds of sync's, better than nothing. Tricking ZFS into importing a pool with the log vdev missing is probably harder. > I'd say that SSD are probably the best fit for slog role. A PCI-e SSD like the OCZ P84 with sustained writes "up to" 600 MB/s might be a possibility for a high-throughput database server, if it uses an interface we have a driver for.
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