From owner-freebsd-fs@FreeBSD.ORG Wed Jul 11 13:49:08 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 7ADA71065674 for ; Wed, 11 Jul 2012 13:49:08 +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 C7F5E8FC12 for ; Wed, 11 Jul 2012 13:49:07 +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 q6BDn4or029874; Wed, 11 Jul 2012 08:49:04 -0500 (CDT) Date: Wed, 11 Jul 2012 08:49:04 -0500 (CDT) From: Bob Friesenhahn X-X-Sender: bfriesen@freddy.simplesystems.org To: Jason Usher In-Reply-To: <1341992974.53118.YahooMailClassic@web122503.mail.ne1.yahoo.com> Message-ID: References: <1341992974.53118.YahooMailClassic@web122503.mail.ne1.yahoo.com> User-Agent: Alpine 2.01 (GSO 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.2 (blade.simplesystems.org [65.66.246.90]); Wed, 11 Jul 2012 08:49:04 -0500 (CDT) Cc: freebsd-fs@freebsd.org Subject: Re: vdev/pool math with combined raidzX vdevs... X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 11 Jul 2012 13:49:08 -0000 On Wed, 11 Jul 2012, Jason Usher wrote: > > Thanks for responding. So I must be mistaken, and the failure > probability of each vdev is not additive ? As I mentioned earlier > in the thread, I am not a probability person, nor would I trust my > own calculations if I tried. 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. Here are some useful blog articles about MTTDL: https://blogs.oracle.com/relling/entry/raid_recommendations_space_vs_mttdl http://blog.richardelling.com/2010/02/zfs-data-protection-comparison.html http://www.servethehome.com/raid-reliability-failure-anthology-part-1-primer/ Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/