Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030221AbWAXJcJ (ORCPT ); Tue, 24 Jan 2006 04:32:09 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030424AbWAXJcJ (ORCPT ); Tue, 24 Jan 2006 04:32:09 -0500 Received: from cantor.suse.de ([195.135.220.2]:48830 "EHLO mx1.suse.de") by vger.kernel.org with ESMTP id S1030221AbWAXJcI (ORCPT ); Tue, 24 Jan 2006 04:32:08 -0500 From: Neil Brown To: Lars Marowsky-Bree Date: Tue, 24 Jan 2006 20:32:02 +1100 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <17365.62482.462589.875480@cse.unsw.edu.au> Cc: linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 000 of 7] md: Introduction - raid5 reshape mark-2 In-Reply-To: message from Lars Marowsky-Bree on Tuesday January 24 References: <20060124112626.4447.patches@notabene> <20060124092303.GD22870@marowsky-bree.de> X-Mailer: VM 7.19 under Emacs 21.4.1 X-face: v[Gw_3E*Gng}4rRrKRYotwlE?.2|**#s9D On 2006-01-24T11:40:47, NeilBrown wrote: > > > I am expecting that I will ultimately support online conversion of > > raid5 to raid6 with only one extra device. This process is not > > (efficiently) checkpointable and so will be at-your-risk. > > So the best way to go about that, if one wants to keep that option open > w/o that risk, would be to not create a raid5 in the first place, but a > raid6 with one disk missing? > > Maybe even have mdadm default to that - as long as just one parity disk > is missing, no slowdown should happen, right? Not exactly.... raid6 has rotating parity drives, for both P and Q (the two different 'parity' blocks). With one missing device, some Ps, some Qs, and some data would be missing, and you would definitely get a slowdown trying to generate some of it. We could define a raid6 layout that didn't rotate Q. Then you would be able to do what you suggest. However it would then be no different from creating a normal raid5 and supporting online conversion from raid5 to raid6-with-non-rotating-Q. This conversion doesn't need an reshaping pass, just a recovery of the now-missing device. raid6-with-non-rotating-Q would have similar issues to raid4 - one drive becomes a hot-spot for writes. I don't know how much of an issue this really is though. NeilBrown - 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/