On Thu, 28 Jun 2012 17:56:39 +0200 "Stefan G. Weichinger" <lists@xxxxxxxx> wrote: > Am 28.06.2012 13:22, schrieb NeilBrown: > > >> Do I have to fear read-errors as with RAID5 now? > > > > If you get a read error, then that block in the new devices cannot > > be recovered, so the recovery will abort. But you have nothing to > > fear except fear itself :-) > > Ah, yes. Not exactly raid-specific, but I agree ;-) (we have a poem by > Mischa Kaleko in german reflecting this, btw ...) > > So if there is one non-readable block on the 2 disks I started with > (the degraded array) the recovery will fail? > > As sd[ab]3 were part of the array earlier, would that mean that maybe > they bring the missing bit, just in case? > > > >> I still don't fully understand if there are also 2 bits of > >> parity-informations available in a degraded RAID6 array on 2 > >> disks only. > > > > In a 4-drive RAID6 like yours, each stripe contains 2 data blocks > > and 2 parity blocks (Called 'P' and 'Q'). When two devices are > > failed/missing, some stripes will have 2 data blocks and no parity, > > some will have both parity blocks and no data (but can of course > > compute the data blocks from the parity blocks). Some will have one > > of each. > > > > Does that answer the question? > > Yes, it does. > > But ... I still don't fully understand it :-P > > What I want to understand and know: > > There is this issue with RAID5: resyncing the array after swapping a > failed disk for a new one stresses the old drives, and if there is one > read-problem on them the whole array blows up. > > As far as I read RAID6 protects me against this because of the 2 > parity blocks (instead of one) because it is much more unlikely that I > can't read both of them, right? Right. > > Does this apply to only a N-1 degraded RAID6 or also an N-2 degraded > array? As far as I understand, it is correct for both cases. Only an N-1 degraded array. An N-2 degraded RAID6 is much like an N-1 degraded RAID5 and would suffer the same fate on a read error during recovery. > > - > > I faced this RAID5-related problem 2 times already (breaking the array > ...) and therefore started to use RAID6 for the servers I deploy, > mostly using 4 disks, sometimes 6 or 8. > > If this doesn't really protect things better, I should rethink that, > maybe. Your current array had lost 2 drives. If it had been a RAID5 you would be wishing you had better backups right now. so I think RAID6 really does provide better protection :-) However it isn't perfect - it cannot protect against concurrent failures on 3 drives... NeilBrown > > - > > Right now my recovery still needs around 80mins to go: > > md0 : active raid6 sdb3[4](S) sda3[5] sdc3[2] sdd3[3] > 3903891200 blocks level 6, 64k chunk, algorithm 2 [4/2] [__UU] > [================>....] recovery = 83.0% > (1621636224/1951945600) finish=81.5min speed=67477K/sec > > I assume it is OK in this state of things that sdb3 is marked as > (S)pare ... > > Thanks, greetings, Stefan
Attachment:
signature.asc
Description: PGP signature
