From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?UTF-8?Q?Mathias_Bur=C3=A9n?= Subject: Re: Growing 6 HDD RAID5 to 7 HDD RAID5 Date: Tue, 12 Apr 2011 22:53:44 +0100 Message-ID: References: <20110412231401.5bb9065c@natsu> <20110413002238.3f31bdeb@natsu> <20110413071554.6537c52c@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110413071554.6537c52c@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Roman Mamedov , Linux-RAID List-Id: linux-raid.ids On 12 April 2011 22:15, NeilBrown wrote: > On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov wrot= e: > >> On Tue, 12 Apr 2011 18:21:13 +0100 >> Mathias Bur=C3=A9n wrote: >> >> > 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? >> >> Neil wrote about this on his blog: >> "It is a very similar process that can now be used to convert a RAID= 5 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 do= ne." >> http://neil.brown.name/blog/20090817000931 >> >> Admittedly it is not completely clear to me what are the long-term d= ownsides of >> this layout. As I understand it does fully provide the RAID6-level r= edundancy. >> Perhaps just the performance will suffer a bit? Maybe someone can ex= plain this >> more. > > If you specify --layout=3Dpreserve, then all the 'Q' blocks will be o= n one disk. > As every write needs to update a Q block, every write will write to t= hat disk. > > 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: =C2=A0the only possible problem would be a performance proble= m, and I > really don't know what performance impact there is - if any. > >> >> If anything, I think it is safe to use this layout for a while, e.g.= in case >> you don't want to rebuild 'right now'. You can always change the lay= out to the >> traditional one later, by issuing "--grow --layout=3Dnormalise". Or = perhaps if >> you plan to add another disk soon, you can normalise it on that occa= sion, 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 > > >> >> > =C2=A0Will the array have redundancy during the rebuild of the new= drive? >> >> If you choose --layout=3Dpreserve, your array immediately becomes a = RAID6 with >> one rebuilding drive. So this is the kind of redundancy you will hav= e during >> that rebuild - tolerance of up to one more (among the "old" drives) = failure, >> in other words, identical to what you currently have with RAID5. >> > > Right, so using --preserve seems like a sane and good option. Thanks for the info, I'll let you know what happens, HDD should arrive the next few days. // Mathias -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html