From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: RAID 5 - One drive dropped while replacing another Date: Wed, 02 Feb 2011 17:24:35 +0100 Message-ID: References: <20110202043605.593f0c5c@natsu> <20110202144726.GB18517@cthulhu.home.robinhill.me.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20110202144726.GB18517@cthulhu.home.robinhill.me.uk> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 02/02/2011 15:47, Robin Hill wrote: > On Wed Feb 02, 2011 at 09:21:20PM +0700, hansbkk@gmail.com wrote: > >> On Wed, Feb 2, 2011 at 6:36 AM, Roman Mamedov wrote: >>> >>>> I have a RAID5 setup with 15 drives. >>> >>> Looks like you got the problem you were so desperately asking for, with this >>> crazy setup. :( >> >> Please give some more details as to what's so crazy about this. >> > Just the number of drives in a single RAID5 array I think. I'd be > looking at RAID6 well before I got to 10 drives. > >> I would think RAID6 would have made more sense, possibly with an >> additional spare if these are large drives (over a few hundred GB?) >> > With 15, RAID6 + spare would probably be what I'd go with (depending on > drive size of course, and whether you have cold spares handy). For very > large drives, multiple arrays would be safer. > >> Or is there an upper limit as to the number of drives that's advisable >> for any array? >> > I'm sure there's advice out there on this one - probably a recommended > minimum percentage of capacity used for redundancy. I've not looked > though - I tend to go with gut feeling& err on the side of caution. > >> If so, then what do people reckon a reasonable limit should be for a >> RAID6 made up of 2TB drives? >> > As the drive capacities go up, you need to be thinking more carefully > about redundancy - with a 2TB drive, your rebuild time is probably over > a day. Rebuild also tends to put more load on drives than normal, so is > more likely to cause a secondary (or even tertiary) failure. I'd be > looking at RAID6 regardless, and throwing in a hot spare if there's more > than 5 data drives. If there's more than 10 then I'd be going with > multiple arrays. > If the load due to rebuild is a problem, it can make sense to split the raid into parts. If you've got the money, you can start with a set of raid1 pairs and then build raid5 (or even raid6) on top of that. With raid 1 + 5, you can survive any 3 drive failures, and generally more than that unless you are very unlucky in the combinations. However, rebuilds are very fast - they are just a direct copy from one disk to its neighbour, and thus are less of a load on the rest of the system. Of course, there is a cost - if you have 15 2TB drives, with one being a warm spare shared amongst the raid1 pairs, you have only 6 x 2TB storage.