Date: Wed, 11 Jul 2012 08:32:34 -0700 (PDT) From: Jason Usher <jusher71@yahoo.com> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org Subject: Re: vdev/pool math with combined raidzX vdevs... Message-ID: <1342020754.79202.YahooMailClassic@web122502.mail.ne1.yahoo.com> In-Reply-To: <alpine.GSO.2.01.1207110829540.27589@freddy.simplesystems.org>
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Hi Bob, --- On Wed, 7/11/12, Bob Friesenhahn <bfriesen@simple.dallas.tx.us> wrote: > The probabilty is indeed additive just as you say. My > point is that the fundamental integrity is offered at the > vdev level. If a vdev fails, then the whole pool is > gone. The MTTDL calculations for various vdev > topologies vary by orders of magnitude, which tends to make > the additive nature of more vdevs insignificant. Thanks again for responding. I'm not going to beat this to death, but just to summarize, if F is 2, then the corresponding data loss probabilities for RAID-Z1, -Z2, -Z3 are: 14.9%, 1.3%, and 0.086%. But if combining multiple vdevs into a zpool (as opposed to maintaining a different zpool for each raidz3 vdev) is additive, then raidz3 becomes .258%. Since (I think) a lot of raidz3 adoption is due to folks desiring "some overkill" as they attempt to overcome the "disks got really big but didn't get any faster (for rebuilds)"[1] ... but they are losing some of that by combining vdevs in a single pool. Not losing so much that they're back down to the failure rate of a single raidz*2* vdev, but they're not at the overkill level they thought they were at either. I think that's important, or at least worth noting... [1] http://storagegaga.com/4tb-disks-the-end-of-raid/
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