From owner-freebsd-questions@freebsd.org Tue Apr 19 19:27:52 2016 Return-Path: Delivered-To: freebsd-questions@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 2ED84B14E2A for ; Tue, 19 Apr 2016 19:27:52 +0000 (UTC) (envelope-from steve@sohara.org) Received: from uk1mail2513.mymailbank.co.uk (UK1MAIL2513-PERMANET.IE.mymailbank.co.uk [217.69.47.44]) by mx1.freebsd.org (Postfix) with ESMTP id A1ED6173D for ; Tue, 19 Apr 2016 19:27:50 +0000 (UTC) (envelope-from steve@sohara.org) Received: from smtp.lan.sohara.org (UnknownHost [88.151.27.41]) by uk1mail2513-d.mymailbank.co.uk with SMTP; Tue, 19 Apr 2016 18:07:10 +0100 Received: from [192.168.63.1] (helo=steve.lan.sohara.org) by smtp.lan.sohara.org with smtp (Exim 4.86 (FreeBSD)) (envelope-from ) id 1asZ7M-0000f6-3Q; Tue, 19 Apr 2016 17:07:16 +0000 Date: Tue, 19 Apr 2016 18:07:15 +0100 From: Steve O'Hara-Smith To: freebsd-questions@freebsd.org Cc: galtsev@kicp.uchicago.edu Subject: Re: Raid 1+0 Message-Id: <20160419180715.f788945fb67388a707c60ee7@sohara.org> In-Reply-To: <13260.128.135.52.6.1461084740.squirrel@cosmo.uchicago.edu> References: <571533F4.8040406@bananmonarki.se> <57153E6B.6090200@gmail.com> <20160418210257.GB86917@neutralgood.org> <64031.128.135.52.6.1461017122.squirrel@cosmo.uchicago.edu> <441t61efpv.fsf@be-well.ilk.org> <13260.128.135.52.6.1461084740.squirrel@cosmo.uchicago.edu> X-Mailer: Sylpheed 3.5.0 (GTK+ 2.24.29; amd64-portbld-freebsd10.1) X-Clacks-Overhead: "GNU Terry Pratchett" Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 19 Apr 2016 19:27:52 -0000 On Tue, 19 Apr 2016 11:52:20 -0500 (CDT) "Valeri Galtsev" wrote: > > On Tue, April 19, 2016 11:16 am, Lowell Gilbert wrote: > > "Valeri Galtsev" writes: > > > >> Somebody with better knowledge of probability theory will correct me if > >> I'm wrong some place. > > > > Well, you are assuming that the probabilities of two drives failing > > are entirely independent of each other. The person to whom you are > > responding asserted that this is not the case. Neither of you > > presented any evidence directly to that point. > > Correct, we didn't hear proof of one or another. I, however, can not think > of any physical mechanism that can be involved which will lead in case of > failure of one drive to failure of another. That is why I assume events > are (pretty much) independent. The increased load caused by populating the replacement drive is one such physical mechanism. Probability of failure in a period of time is affected by how busy the drive has been in that period. Next Look at the curves for failure probability against age (or total activity) and note that the classic bathtub shape and consider what happens when your whole array is on the steep bit at the end and the first drive failure happens. Yes random drive failures well within the expected life are pretty much independent but the coupling gets stronger as the drives get older due to the increasing tendency of load induced failures. -- Steve O'Hara-Smith