From owner-freebsd-fs@FreeBSD.ORG Tue Nov 13 17:41:17 2012 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 5DFA8C43 for ; Tue, 13 Nov 2012 17:41:17 +0000 (UTC) (envelope-from bfriesen@simple.dallas.tx.us) Received: from blade.simplesystems.org (blade.simplesystems.org [65.66.246.74]) by mx1.freebsd.org (Postfix) with ESMTP id 145818FC13 for ; Tue, 13 Nov 2012 17:41:16 +0000 (UTC) Received: from freddy.simplesystems.org (freddy.simplesystems.org [65.66.246.65]) by blade.simplesystems.org (8.14.4+Sun/8.14.4) with ESMTP id qADHf8GE019718; Tue, 13 Nov 2012 11:41:09 -0600 (CST) Date: Tue, 13 Nov 2012 11:41:08 -0600 (CST) From: Bob Friesenhahn X-X-Sender: bfriesen@freddy.simplesystems.org To: kpneal@pobox.com Subject: Re: RHEL to FreeBSD file server In-Reply-To: <20121113043409.GA70601@neutralgood.org> Message-ID: References: <50A130B7.4080604@cse.yorku.ca> <20121113043409.GA70601@neutralgood.org> User-Agent: Alpine 2.01 (GSO 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; format=flowed; charset=US-ASCII X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.2 (blade.simplesystems.org [65.66.246.90]); Tue, 13 Nov 2012 11:41:09 -0600 (CST) Cc: freebsd-fs@freebsd.org X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 13 Nov 2012 17:41:17 -0000 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/