Date: Fri, 06 Jul 2012 15:22:43 +0200 From: Adam Nowacki <nowakpl@platinum.linux.pl> To: freebsd-fs@freebsd.org Subject: Re: vdev/pool math with combined raidzX vdevs... Message-ID: <4FF6E6A3.1010000@platinum.linux.pl> In-Reply-To: <1341537402.58301.YahooMailClassic@web122504.mail.ne1.yahoo.com> References: <1341537402.58301.YahooMailClassic@web122504.mail.ne1.yahoo.com>
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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.
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