From mboxrd@z Thu Jan 1 00:00:00 1970 From: Norman Schmidt Subject: Re: split RAID1 during backups? Date: Tue, 25 Oct 2005 21:20:41 +0200 Message-ID: <435E8589.9000303@naa.net> References: <435DEE69.8030605@naa.net> <17246.28805.267226.810872@smtp.charter.net> Reply-To: schmidt@naa.net Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <17246.28805.267226.810872@smtp.charter.net> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids John Stoffel schrieb: > Norman> What you should be able to do with software raid1 is the > Norman> following: Stop the raid, mount both underlying devices > Norman> instead of the raid device, but of course READ ONLY. Both > Norman> contain the complete data and filesystem, and in addition to > Norman> that the md superblock at the end. Both should be identical > Norman> copies of that. Thus, you do not have to resync > Norman> afterwards. You then can backup the one disk while serving the > Norman> web server from the other. When you are done, unmount, > Norman> assemble the raid, mount it and go on. > > Umm... so what you're proposing would mean that he would have to stop > his application, make sure nothing is accessing that volume, and then > restart the application? > > Somehow I don't see that happening. > > Why do you feel that you have to stop and umount the volume for the > splitting off of a read-only mirror pair for backups? > > John All this "pulling a disk" sounded to me like a resync, and with that much data and slow access this resync would take ages. And as far as I remember, there was a suggestion with stopping or interrupting applications anyway, was there not? Unfortunately, I have thrown most of the posts away. If you keep the volume running, ok - it should still work from the data integrity point of view. But if you keep using the data from a functional raid1 (/dev/md1 on /data), you cannot disentangle the access, since the raid1 will read from both mirrors. But what was desired was a faster backup and being able to continue serving the data. If one disk (without the read-balancing of raid1) would provide enough treoughput for the webserver, one could even do the following: mount /dev/md1 /writeaccess mount -r /dev/hda /data mount -r /dev/hdc /backup So write from /weiteacces (mirrored to both drives) and read from one for the webserver (statically) and from the other one for the backup. Could that work? Norman. -- Norman Schmidt Institut fuer Physikal. u. Theoret. Chemie Dipl.-Chem. Univ. Friedrich-Alexander-Universitaet schmidt@naa.net Erlangen-Nuernberg +49 9131 852 7321 IT-Systembetreuer Physikalische Chemie