Date: Tue, 13 Nov 2012 11:41:08 -0600 (CST) From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> To: kpneal@pobox.com Cc: freebsd-fs@freebsd.org Subject: Re: RHEL to FreeBSD file server Message-ID: <alpine.GSO.2.01.1211131132110.14586@freddy.simplesystems.org> In-Reply-To: <20121113043409.GA70601@neutralgood.org> References: <50A130B7.4080604@cse.yorku.ca> <20121113043409.GA70601@neutralgood.org>
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On Mon, 12 Nov 2012, kpneal@pobox.com wrote: > > With your setup of 11 mirrors you have a good mixture of read and write > performance, but you've compromised on the safety. The reason that RAID 6 > (and thus raidz2) and up were invented was because drives that get used > together tend to fail together. If you lose a drive in a mirror there is > an elevated probability that the replacement drive will not be in place > before the remaining leg of the mirror fails. If that happens then you've > lost the pool. (Drive failures are _not_ independent.) Do you have a reference to independent data which supports this claim that drive failures are not independent? The whole function of RAID assumes that drive failures are independent. If drives share a chassis, care should be taken to make sure that redundant drives are not in physical proximity to each other and that they are supported via a different controller, I/O path, and power supply. If the drives are in a different chassis then their failures should be completely independent outside of a shared event like power surge, fire, EMP, flood, or sun-spot activity. The idea of raidz2 vdevs of four drives each sounds nice but will suffer from decreased performance and increased time to replace a failed disk. There are always tradeoffs. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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