From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gandalf Corvotempesta Subject: Re: proactive disk replacement Date: Tue, 21 Mar 2017 14:26:10 +0100 Message-ID: References: <3FA2E00F-B107-4F3C-A9D3-A10CA5F81EC0@allygray.2y.net> <11c21a22-4bbf-7b16-5e64-8932be768c68@websitemanagers.com.au> <58D1244E.3040204@hesbynett.no> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: In-Reply-To: <58D1244E.3040204@hesbynett.no> Sender: linux-raid-owner@vger.kernel.org To: David Brown Cc: Reindl Harald , Jeff Allison , Adam Goryachev , linux-raid@vger.kernel.org List-Id: linux-raid.ids 2017-03-21 14:02 GMT+01:00 David Brown : > Note that to cause failure in non-degraded RAID5 (or degraded RAID6), > your two URE's need to be on the same stripe in order to cause data > loss. The chances of getting an URE somewhere on the disk are roughly > proportional to the size of the disk - but the chance of getting an URE > on the same stripe as another URE on another disk are basically > independent of the disk size, and it is extraordinarily small. Little bit OT: is this the same even for HW RAID Controllers like LSI Megaraid or they tend to fail the rebuild in case of multiple URE even in different stripes? > No, you cannot. Your conclusion here is based on several totally > incorrect assumptions: > > 1. You think that RAID5/RAID6 recovery is more stressful, because the > parity is "all over the place". This is wrong. > > 2. You think that random IO has higher chance of getting an URE than > linear IO. This is wrong. Totally agree. > 3. You think that getting an URE on one disk, then getting an URE on a > second disk, counts as a double failure that will break an single-parity > redundancy (RAID5, RAID1, RAID6 in degraded mode). This is wrong - it > is only a problem if the two UREs are in the same stripe, which is quite > literally a one in a million chance. I'm not sure about this. The posted paper is talking about "standard" raid made with hw raid controllers and I'm not sure if they are able to finish a rebuild in case of double URE even if coming from different stripes. I think they fail the whole rebuild.