From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: Creating a 3-disk RAID6 array Date: Thu, 17 May 2012 15:18:33 +0200 Message-ID: <4FB4FAA9.7000100@hesbynett.no> References: <4FB4534A.5070608@volatilevoid.net> <20120517113817.7faf9c1e@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120517113817.7faf9c1e@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Oliver Martin , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 17/05/12 03:38, NeilBrown wrote: > On Thu, 17 May 2012 03:24:26 +0200 Oliver Martin > wrote: > >> Hi, >> >> is there any specific reason why md refuses to create a RAID6 array with >> 3 disks? My (probably naive) understanding suggests it should be the >> same as a 3-disk RAID1, similar to a 2-disk RAID5. > > I'll have to leave for for hpa to answer. I've occasionally thought that > maybe it should be fixed, but it never seemed worth the effort. > >> >> The reason I'm asking is that I currently have space on three disks for >> a new array, and would like to expand it when I add a fourth. I tried >> this scenario with a few loopback devices, but the only way to go from a >> 3-disk RAID1 to a 4-disk RAID6 seems to be via an intermediate 3-disk >> RAID5, requiring two reshapes. I'd like to avoid one of them, if at all >> possible. > > Yes, not possible at present. > It might be as simple and finding the places that impose the limit and delete > them... > > NeilBrown > Theoretically, a 3-disk RAID6 is like a 2-disk RAID5 or a 1-disk RAID1 - it is possible, but not much use except as a building block for expansion later. mdadm supports 1-disk RAID1 (you need a "--force" to persuade mdadm that you know what you are doing, which is nice), and I think it supports a 2-disk RAID5, though I haven't tried it. I can't think of any good reason for it /not/ to support 3-disk RAID6, as there is nothing in the algorithms to hinder it. mvh., David