From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: Growing 6 HDD RAID5 to 7 HDD RAID5 Date: Wed, 13 Apr 2011 07:15:54 +1000 Message-ID: <20110413071554.6537c52c@notabene.brown> References: <20110412231401.5bb9065c@natsu> <20110413002238.3f31bdeb@natsu> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/bOzDzmoY7xHxe0zBmzkWxEM"; protocol="application/pgp-signature" Return-path: In-Reply-To: <20110413002238.3f31bdeb@natsu> Sender: linux-raid-owner@vger.kernel.org To: Roman Mamedov Cc: Mathias =?ISO-8859-1?B?QnVy6W4=?= , Linux-RAID List-Id: linux-raid.ids --Sig_/bOzDzmoY7xHxe0zBmzkWxEM Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov wrote: > On Tue, 12 Apr 2011 18:21:13 +0100 > Mathias Bur=E9n wrote: >=20 > > If I use --layout=3Dpreserve , what impact will that have? > > If I preserve the layout, what is the final result of the array > > compared to not preserving it? >=20 > Neil wrote about this on his blog: > "It is a very similar process that can now be used to convert a RAID5 to a > RAID6. We first change the RAID5 to RAID6 with a non-standard layout that= has > the parity blocks distributed as normal, but the Q blocks all on the last > device (a new device). So this is RAID6 using the RAID6 driver, but with a > non-RAID6 layout. So we "simply" change the layout and the job is done." > http://neil.brown.name/blog/20090817000931 >=20 > Admittedly it is not completely clear to me what are the long-term downsi= des of > this layout. As I understand it does fully provide the RAID6-level redund= ancy. > Perhaps just the performance will suffer a bit? Maybe someone can explain= this > more. If you specify --layout=3Dpreserve, then all the 'Q' blocks will be on one = disk. As every write needs to update a Q block, every write will write to that di= sk. With our current RAID6 implementation that probably isn't a big cost - for any write, we need to either read from or write to each disk anyway. Anyway: the only possible problem would be a performance problem, and I really don't know what performance impact there is - if any. >=20 > If anything, I think it is safe to use this layout for a while, e.g. in c= ase > you don't want to rebuild 'right now'. You can always change the layout t= o the > traditional one later, by issuing "--grow --layout=3Dnormalise". Or perha= ps if > you plan to add another disk soon, you can normalise it on that occasion,= and > still gain the benefit of only one full reshape. Note that doing a normalise by itself later will be much slower than not doing a preserve now. Doing the normalise later when growing the the device again would be just as fast as no doing the preserve now. NeilBrown >=20 > > Will the array have redundancy during the rebuild of the new drive? >=20 > If you choose --layout=3Dpreserve, your array immediately becomes a RAID6= with > one rebuilding drive. So this is the kind of redundancy you will have dur= ing > that rebuild - tolerance of up to one more (among the "old" drives) failu= re, > in other words, identical to what you currently have with RAID5. >=20 --Sig_/bOzDzmoY7xHxe0zBmzkWxEM Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.16 (GNU/Linux) iD8DBQFNpMEKG5fc6gV+Wb0RAi6pAJwM3W7XrMeBg3PC7x4mIviX3obZdACg0fXk uHuwg43UikEvv2Cdb7IK5EU= =Txai -----END PGP SIGNATURE----- --Sig_/bOzDzmoY7xHxe0zBmzkWxEM--