From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: Re-adding disks to RAID6 in a Fujitsu NAS: old mdadm? Date: Fri, 29 Jun 2012 07:39:38 +1000 Message-ID: <20120629073938.4270fd76@notabene.brown> References: <4FE9BFC1.5070607@xunil.at> <20120628163205.2c6a1122@notabene.brown> <4FEC1CE7.5010709@xunil.at> <4FEC2071.8010504@xunil.at> <20120628212257.5ca8fb05@notabene.brown> <4FEC7EB7.1000401@xunil.at> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/F8eYrz+jbJkD7XBfA12bS3a"; protocol="application/pgp-signature" Return-path: In-Reply-To: <4FEC7EB7.1000401@xunil.at> Sender: linux-raid-owner@vger.kernel.org To: lists@xunil.at Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids --Sig_/F8eYrz+jbJkD7XBfA12bS3a Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Thu, 28 Jun 2012 17:56:39 +0200 "Stefan G. Weichinger" wrote: > Am 28.06.2012 13:22, schrieb NeilBrown: >=20 > >> Do I have to fear read-errors as with RAID5 now? > >=20 > > 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 :-) >=20 > Ah, yes. Not exactly raid-specific, but I agree ;-) (we have a poem by > Mischa Kaleko in german reflecting this, btw ...) >=20 > So if there is one non-readable block on the 2 disks I started with > (the degraded array) the recovery will fail? >=20 > As sd[ab]3 were part of the array earlier, would that mean that maybe > they bring the missing bit, just in case? >=20 >=20 > >> I still don't fully understand if there are also 2 bits of=20 > >> parity-informations available in a degraded RAID6 array on 2 > >> disks only. > >=20 > > 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=20 > > compute the data blocks from the parity blocks). Some will have one > > of each. > >=20 > > Does that answer the question? >=20 > Yes, it does. >=20 > But ... I still don't fully understand it :-P >=20 > What I want to understand and know: >=20 > 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. >=20 > 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. >=20 > 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 t= he same fate on a read error during recovery. >=20 > - >=20 > 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. >=20 > 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 >=20 > - >=20 > Right now my recovery still needs around 80mins to go: >=20 > md0 : active raid6 sdb3[4](S) sda3[5] sdc3[2] sdd3[3] > 3903891200 blocks level 6, 64k chunk, algorithm 2 [4/2] [__UU] > [=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>....] recovery = =3D 83.0% > (1621636224/1951945600) finish=3D81.5min speed=3D67477K/sec >=20 > I assume it is OK in this state of things that sdb3 is marked as > (S)pare ... >=20 > Thanks, greetings, Stefan --Sig_/F8eYrz+jbJkD7XBfA12bS3a Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (GNU/Linux) iQIVAwUBT+zPGjnsnt1WYoG5AQL+oA/+P7X/ij+0fTJBsx2T1b62zRu56fHFYXw/ 29rQMKDOVUpv4ktcYizgsbIaPD5MmhpvPKktqYxKevhdWyZg9S/BNB1UDNUpU+hd FVbguRwSdudnF6BnpsjcnUetythCxZXPgweG/E7jBUm6O1bYlIoSrBxLSdGuqCjK 0muBkYkurqS0p9BZGmbbOiLBfHHkT6U3D5ruotPXRb6k9DZpwvzbcmA2PP90nKzy 7y6Q0lELEDl/ozDMDsGHrolpPhn7//rM9MK5xu0ZYa0tNU9RaU/DrQ0WEvU8FQVZ KXkyDgKJORJDoFs8zpDvCsd4vLTcl4Ayxq6DOzhHOT6FRfhmSQ4foqrwwssxcZ8i XxGkJl1xk2JlpZv8EtGhD7w58KkExOvJGTcir79Ym/NrNeFUiL+gLzEwxVnoEy/Y 1qplluU0QI0OS8SopmqahzG076R6eZdjPS8PKtrfWCPaB/ermzzDssuoxT3Z7DXq mT4ZkVgV90fjxy7qd0/cQhHOFyoZ7MvsJBOO6dx4FqZU+28//DT7lf5Q0EqIxI3e S5EpMm2LPA2bCYna5UZQ2+tN5canvB9YG/ZjTgUV/lfaL1jWipPpof7aYMSGf2i1 uY02cQKY9cIliK4B4BjdHa2hAa4OfNUr3sey3Cb9jW/IYi4FvVrsd4O/AXJTLUFn zNSIrwfk7Mo= =JVUl -----END PGP SIGNATURE----- --Sig_/F8eYrz+jbJkD7XBfA12bS3a--