From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: Creating a 3-disk RAID6 array Date: Fri, 18 May 2012 00:30:47 -0700 Message-ID: <4FB5FAA7.4090401@zytor.com> References: <4FB4534A.5070608@volatilevoid.net> <20120517113817.7faf9c1e@notabene.brown> <4FB4FAA9.7000100@hesbynett.no> <4FB50573.6020604@anonymous.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4FB50573.6020604@anonymous.org.uk> Sender: linux-raid-owner@vger.kernel.org To: John Robinson Cc: David Brown , NeilBrown , Oliver Martin , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 05/17/2012 07:04 AM, John Robinson wrote: > On 17/05/2012 14:18, David Brown wrote: > [...] >> Theoretically, a 3-disk RAID6 is like a 2-disk RAID5 or a 1-disk RAID1 - > [...] >> 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. > > I've a vague recollection of Peter Anvin saying the implementation is > optimised in such a way that it won't work. > Yes, it would have required introducing some odd special cases. A 3-disk RAID-6 is bitwise identical to a 3-disk RAID-1, so if Neil wants to he could add "instant reshaping" support in mdadm (add a disk to a 3-disk RAID-1 turning it into a 4-disk RAID-6; and similar for 2-disk RAID-1 into 3-disk RAID-5.) -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.