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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=0AHi Bob,=0A=0A--- On Wed, 7/11/12, Bob Friesenhahn <bfriesen@simple.dalla= s.tx.us> wrote:=0A=0A> The probabilty is indeed additive just as you say.= =A0 My=0A> point is that the fundamental integrity is offered at the=0A> vd= ev level.=A0 If a vdev fails, then the whole pool is=0A> gone.=A0 The MTTDL= calculations for various vdev=0A> topologies vary by orders of magnitude, = which tends to make=0A> the additive nature of more vdevs insignificant.=0A= =0A=0AThanks again for responding.=0A=0AI'm not going to beat this to death= , but just to summarize, if F is 2, then the corresponding data loss probab= ilities for RAID-Z1, -Z2, -Z3 are: 14.9%, 1.3%, and 0.086%.=0A=0ABut if com= bining multiple vdevs into a zpool (as opposed to maintaining a different z= pool for each raidz3 vdev) is additive, then raidz3 becomes .258%.=0A=0ASin= ce (I think) a lot of raidz3 adoption is due to folks desiring "some overki= ll" as they attempt to overcome the "disks got really big but didn't get an= y faster (for rebuilds)"[1] ... but they are losing some of that by combini= ng vdevs in a single pool.=0A=0ANot losing so much that they're back down t= o the failure rate of a single raidz*2* vdev, but they're not at the overki= ll level they thought they were at either.=0A=0AI think that's important, o= r at least worth noting...=0A=0A=0A[1] http://storagegaga.com/4tb-disks-the= -end-of-raid/
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