From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: Adding Reed-Solomon Personality to MD, need help/advice Date: Thu, 29 Dec 2005 10:40:33 -0800 (PST) Message-ID: References: <6.0.1.1.2.20040130235924.03f39710@mail.athenet.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Return-path: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Followup to: By author: "Mario 'BitKoenig' Holbe" In newsgroup: linux.dev.raid > > Hello, > > Nathan Lewis wrote: > > As part of my Master's thesis, I am working on adding a Reed-Solomon > > personality to the existing linux RAID structure and I would like some > > Is there any progress in implementing a generic Reed-Solomon personality > in MD since this mail from 31 Jan 2004? > Regarding the intention-question... for me, personally, it would be the > logical step inbetween raid5 resp. raid6 with survival of 1 resp. 2 > simultaneous disk failures and raid10 with survival of n/2 simultaneous > disk failures. RaidRS would give users the chance to configure > redundancy and thus survivability exactly on their demands. > This would especially make sense when I see the raid5 configurations > with 14 and more devices which some users refer to on this list. > To be honest, I was thinking about such a personality myself, too, and > then was crawling the list's archive. > It's not really in-between; generic RS RAID would be many times slower than either; however, unlike raid10 it could survive *any* m failures where m is the number of redundancy drives. The fundamental problem is that generic RS requires table lookups even in the common case, whereas RAID-6 uses shortcuts to substantially speed up the computation in the common case. RAID-6 is an important corner of the problem space, since it deals with the unfortunately fairly common problem of "disk failure discovered during recovery" with RAID-5. That doesn't mean there couldn't be a problem space where it would make sense (in fact, on the contrary), but it's still a substantial engineering effort that would have to be justified. Heck, I might even be persuaded to look for generic RS shortcuts if someone tempted me enough... -hpa