From owner-freebsd-fs@FreeBSD.ORG Fri Jul 6 13:31:27 2012 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 435B51065673 for ; Fri, 6 Jul 2012 13:31:27 +0000 (UTC) (envelope-from nowakpl@platinum.linux.pl) Received: from platinum.linux.pl (platinum.edu.pl [81.161.192.4]) by mx1.freebsd.org (Postfix) with ESMTP id E7AF38FC17 for ; Fri, 6 Jul 2012 13:31:26 +0000 (UTC) Received: by platinum.linux.pl (Postfix, from userid 87) id 61A7947E14; Fri, 6 Jul 2012 15:22:58 +0200 (CEST) X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on platinum.linux.pl X-Spam-Level: X-Spam-Status: No, score=-1.5 required=3.0 tests=ALL_TRUSTED,AWL autolearn=disabled version=3.3.2 Received: from [172.19.191.4] (unknown [83.151.38.73]) by platinum.linux.pl (Postfix) with ESMTPA id C5B0B47E0F for ; Fri, 6 Jul 2012 15:22:53 +0200 (CEST) Message-ID: <4FF6E6A3.1010000@platinum.linux.pl> Date: Fri, 06 Jul 2012 15:22:43 +0200 From: Adam Nowacki User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2.28) Gecko/20120306 Thunderbird/3.1.20 MIME-Version: 1.0 To: freebsd-fs@freebsd.org References: <1341537402.58301.YahooMailClassic@web122504.mail.ne1.yahoo.com> In-Reply-To: <1341537402.58301.YahooMailClassic@web122504.mail.ne1.yahoo.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit 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: Fri, 06 Jul 2012 13:31:27 -0000 On 2012-07-06 03:16, Jason Usher wrote: > It's easy to find the failure math for raidz2 and raidz3. > > But what if you create a pool with 3 different raidz3 vdevs inside of it ? > > For instance, 3 12-drive raidz3 vdevs in one big pool. > > For each individual vdev the failure probability is now higher, since not only will it fail when 4 drives in the vdev fail, but it will also fail if four drives in any of the other two vdevs fail. > > So each raidz3 vdev now has a failure rate higher than vanilla raidz3 ... but what is that new failure rate ? Is it still higher than vanilla raidz2 ? If you're asking if 3-stripe x 12-drive raidz3 has a higher failure probability than 36-drive raidz2 then the answer is no. It also doesn't have a higher failure probability than 36-drive raidz3. But if you compare 3-stripe x 12-drive raidz3 with no stripe 12-drive raidz2 then for a disk failure probability less than ~0.16 raidz3 wins, otherwise raidz2.