Date: Sat, 07 Jul 2012 18:34:39 +0200 From: =?UTF-8?B?UmFkaW8gbcWCb2R5Y2ggYmFuZHl0w7N3?= <radiomlodychbandytow@o2.pl> To: freebsd-fs@freebsd.org Subject: Re: freebsd-fs Digest, Vol 472, Issue 5 Message-ID: <4FF8651F.8090102@o2.pl> In-Reply-To: <20120706120031.A47A41065733@hub.freebsd.org> References: <20120706120031.A47A41065733@hub.freebsd.org>
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On 2012-07-06 14:00, freebsd-fs-request@freebsd.org 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 ? Skip these calculations. They all assume that drive failures are independent and it's not the case in the real world. There's been a good study on the topic of drive failures several years ago, http://static.usenix.org/events/fast07/tech/schroeder/schroeder_html/index.html Among other findings they say: "We also present strong evidence for the existence of correlations between disk replacement interarrivals." -- Twoje radio
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