From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pavel Machek Subject: Re: RAID-6 support in kernel? Date: Tue, 4 Jun 2002 15:49:05 +0000 Sender: linux-raid-owner@vger.kernel.org Message-ID: References: <20020603113128.C13204@ucw.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: In-Reply-To: <20020603113128.C13204@ucw.cz>; from vojtech@suse.cz on Mon, Jun 03, 2002 at 11:31:28AM +0200 To: Vojtech Pavlik Cc: Derek Vadala , Roy Sigurd Karlsbakk , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, Tedd Hansen , Christian Vik , Lars Christian Nygaard List-Id: linux-raid.ids Hi! > > > It'll waste 9 drives, giving me a total capacity of 7n instead of 14n. > > > And, by definition, RAID-6 _can_ withstand _any_ two-drive failure. > > > > This is certainly not true. > > > > Combining N RAID-5 into a stripe wastes on N disks. > > > > If you combine two it wastes 2 disks, etc. > > > > That is, for each RAID-5 you waste a single disk worth of storage for > > partiy. I don't know what equation you're using where you get 9 drives > > from. > > He was thinking "mirror", not "stripe". Mirror of 2 RAID-5 arrays (would > be probably called RAID-15 (when there is a RAID-10 for mirrored stripe > arrays)), can withstand any two disks failing anytime. Even more for RAID-1 over two RAID-5s should withstand any three failures, AFAICS. You could do RAID-5 over RAID-5. That should survive any 2 failures and still be reasonably efficient. Pavel -- Philips Velo 1: 1"x4"x8", 300gram, 60, 12MB, 40bogomips, linux, mutt, details at http://atrey.karlin.mff.cuni.cz/~pavel/velo/index.html.