From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goswin von Brederlow Subject: Re: Why do I need 4 disks for a raid6? Date: Wed, 01 Apr 2009 20:07:53 +0200 Message-ID: <87prfwb62u.fsf@frosties.localdomain> References: <87ljr4hsk9.fsf@frosties.localdomain> <20090318121825.GL32416@skl-net.de> <49D39169.2050705@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: In-Reply-To: <49D39169.2050705@zytor.com> (H. Peter Anvin's message of "Wed, 01 Apr 2009 09:08:09 -0700") Sender: linux-raid-owner@vger.kernel.org To: "H. Peter Anvin" Cc: Andre Noll , Goswin von Brederlow , linux-raid@vger.kernel.org List-Id: linux-raid.ids "H. Peter Anvin" writes: > Andre Noll wrote: >> >>> So back to my original question: Why does the kernel require 4 disks >>> for a raid6 instead of allowing 3? >> >> Dunno. Maybe Dan, Neil or HPA can tell the reason for imposing this >> limitation. >> > > It's very simple: it avoids an ugly corner case in the RAID-6 > computation code. Rather than inserting special code (and verifying it, > etc.) to deal with the 3-disk RAID-6 degenerate case, it was easier to > just not permit it. > > -hpa Could you point me at the file/line where the corner case is? MfG Goswin