From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Greaves Subject: Re: Convert raid5 to raid1? Date: Fri, 11 Mar 2005 08:58:30 +0000 Message-ID: <42315DB6.5030604@dgreaves.com> References: <4230BE56.6090905@advocap.org> <4230D279.5090101@wasp.net.au> <4230E003.4040609@advocap.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit In-Reply-To: <4230E003.4040609@advocap.org> Sender: linux-raid-owner@vger.kernel.org To: John McMonagle Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids John McMonagle wrote: > Not saying its broke. good :) > Part of my reasoning to go to raid5 was that I could expand. OK that is really quite hard. Unless you run lvm2 over the top - in which case it's a doddle (though constraints apply) I'd do it anyway (run lvm2). It causes almost no performance issues and you can then grow your fs later. You could build a raid5 then linearly extend onto a raid0 > While it can be done I don't really see it as practical. > Also it's looking like I probably will not need to expand. Oh. > raid5 with 3 drives and 1 spare > or 2 - 2 drive raid1 drives have the same space. > Which is less likely to have a failure cause data loss? > I'm guessing raid1. > If I'm wrong I'd like to know now. A 4x raid5 gives 3X space and survives 1 disk failure (a spare can be added later) A 2x raid1/raid0+2 gives 2X space and if 1 disk fails then there's a 1/3 chance that a second failure will cause total failure. A 3x raid5+1s gives 2X space and the ability for 2 simultaneous* disk failures A 4x raid6 gives 2X space and the ability for 2 simultaneous** disk failures * simultaneous here = apart from the few hours whilst a resync occurs - really I'm more interested in the lack of resilience during the few weeks whilst the RMA happens. Although I ended up with a spare 'cos I waited for a failure and bought a spare on next-day delivery whilst wrapping the failed disk for RMA ** really simultaneous > > Also concerned about the resync times. It was going to take a couple > days to resync under a rather light load if it weren't for the fact > that it couldn't because of a bad drive and a kernel panic caused by > the read error. Hmm, took a few hours to resync 1Tb here. (maybe as much as overnight) > Still not certain about the cause of problem my current guess is the > sata controller. I run a 7 device SATA and it's81 stable (on 2.6.11.2 now). I had a few minor problems a few months back but I think they've been sorted. > I'm glad there is work being done on the resync issue. > Also think the ideas to attempt to fix read errors are great. Yes the "read error = kick disk" is still a significant failing :( > My only suggestion is that there should be provision to send > notification when it happens such as this one: This is an automatically generated mail message from mdadm running on cu.dgreaves.com A Fail event had been detected on md device /dev/md0. Faithfully yours, etc. man madm (or Debian sets it up automagically) HTH One other suggestion is to consider using lvm over 2 raid1 devices rather than md0/md1 - I think you'll find a lot more flexibility there. David