Date: Wed, 19 Aug 2015 10:06:16 -0500 (CDT) From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> To: kpneal@pobox.com Cc: javocado <javocado@gmail.com>, FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: Optimizing performance with SLOG/L2ARC Message-ID: <alpine.GSO.2.01.1508190957240.15707@freddy.simplesystems.org> In-Reply-To: <20150819111243.GA44407@neutralgood.org> References: <CAP1HOmTidC3%2BG4XfhvkQxieo%2BSYMq-JWiXF9Cs4FSW2VqkktWA@mail.gmail.com> <alpine.GSO.2.01.1508182106080.4186@freddy.simplesystems.org> <20150819111243.GA44407@neutralgood.org>
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On Wed, 19 Aug 2015, kpneal@pobox.com wrote: >> Use mirrors if you can afford it. > > Mirrors are less safe than raidz* unless you have enough drives in the > mirror. Look up the failure prediction calculations that were done on one > of the zfs lists last year. (The relevant keyword might be "MTTDL".) > > Mirrors also have about the same write performance as raidz*. But reads > from mirrors scale excellently with the number of drives in the mirror. With traditional hard drives, mirrors offer more write performance than raidz because they consume fewer precious drive IOPS (e.g. 5X, 8X less) and because the pool will have more vdevs (supporting more simultaneous activities). With single large sequential writes, raidz* likely matches performance with mirrors. Performance with large sequential writes is not usually the problem when someone says that a server feels "sluggish". With today's large disk sector sizes (4k, 8k), the amount of space lost to mirroring (compared with raidz*) is not as much as it used to be. It is necessary to start at raidz2 in order to exceed the MTTDL with simple mirrors. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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