Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 5 Jun 2002 21:20:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 5 Jun 2002 21:20:06 -0400 Received: from jaded.cynicism.com ([206.129.95.68]:51208 "HELO jaded.cynicism.com") by vger.kernel.org with SMTP id ; Wed, 5 Jun 2002 21:19:59 -0400 Date: Wed, 5 Jun 2002 18:19:53 -0700 (PDT) From: Derek Vadala To: Bill Davidsen Cc: Linux Kernel Mailing List , linux-raid@vger.kernel.org Subject: Re: RAID-6 support in kernel? In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 4 Jun 2002, Bill Davidsen wrote: > On Sun, 2 Jun 2002, Derek Vadala wrote: > > > You can always fake this effect by combining two 8-disk RAID-5s into a > > RAID-0. It's not technically RAID-6, but can withstand a 2-disk failure, > > although not _any_ 2-disk failure. > > I think (hope) you meant 1+5, which will stand any three disk failure, and > up to 1+N/2 if just the right drives fail. They never do, of course. I did mean RAID-0 combined with RAID-5. You can search for RAID-50 for more info. The configuration you describe (RAID-5s combined into a mirror) would have a disk overhead that is worse than RAID-10/RAID-0+1. For two 5-disk RAID-5s combined into a RAID-1 you end up using six of your disks for parity and disk mirroring: RAID-1 --------> RAID-5 (D0,D1,D2,D3,P0) |--> RAID-5 (D0,D1,D2,D3,P0) (four disks used for data, only one from each RAID-5 can fail) With RAID-10: RAID-0 --------> RAID-1 (D0,D0) |--> RAID-1 (D1,D1) |--> RAID-1 (D2,D2) |--> RAID-1 (D3,D3) |--> RAID-1 (D4,D4) (five disks used for data, one from each mirror can fail) With RAID-50: RAID-0 --------> RAID-5 (D0,D2,D4,D6,P0) |--> RAID-5 (D1,D3,D5,D7,P0) (two disks wasted only one from each RAID-5 can fail) I believe that I/O performance would be similar for each configuration. I'll try to run some tests in the next few days. > I doubt it. Unless you run a system with heavy CPU demand there are lots > of cycles for this stuff. I run 0+1 several places and I don't see serious > CPU load. I would be very interested in RAID-6 in the kernel, but I have Mirroing doesnt hit the CPU nearly as much as RAID-5 does. I suspect RAID-6 would incur greater overhead because of its double parity blocks. But, there's no point in arguing about kernel RAID-6 without data to back it up. --- Derek Vadala, derek@cynicism.com, http://www.cynicism.com/~derek - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/