From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Greaves Subject: Re: Date: Tue, 17 Jan 2006 12:12:22 +0000 Message-ID: <43CCDF26.1000704@dgreaves.com> References: <20060111144943.93BAA396A20@mxo3.broadbandsupport.net> <43C63A94.2020702@dgreaves.com> <20060112172038.GA12426@lug.udel.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20060112172038.GA12426@lug.udel.edu> Sender: linux-raid-owner@vger.kernel.org To: Ross Vandegrift Cc: bhess@patmedia.net, linux-raid@vger.kernel.org List-Id: linux-raid.ids Ross Vandegrift wrote: >On Thu, Jan 12, 2006 at 11:16:36AM +0000, David Greaves wrote: > > >>ok, first off: a 14 device raid1 is 14 times more likely to lose *all* >>your data than a single device. >> >> > >No, this is completely incorrect. Let A denote the event that a single >disk has failed, A_i denote the event that i disks have failed. >Suppose P(A) = x. Then by Bayes's Law the probability that an n disk RAID >will lose all of your data is: > >n_1 = P(A) = x >n_2 = P(A_2) = P(A) * P(A_1 | A) = x^2 >n_3 = P(A_3) = P(A) * P(A_2 | A) = x^3 >... >n_i = P(A_i) = P(A) * P(A_{i-1} | A) = x^i > >ie, RAID1 is expoentially more reliable as you add extra disks! > >This assumes that disk failures are independant - ie, that you >correctly configure disks (don't use master and slave on an IDE >channel!), and replace failed disks as soon as they fail. > >This is why adding more disks to a RAID1 is rare - x^2 is going to be >a really low probability! It will be far, far more common for >operator error to break a RAID than for both devices to honestly fail. > > > sorry, read it all as 'linear', not mirrored which is why I was writing drivel ;) David --