From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arne Jansen Subject: Re: Safe fsck / consistent backup while mounted Date: Sat, 04 Jun 2011 16:13:14 +0200 Message-ID: <4DEA3D7A.40003@gmx.net> References: <201106041225.56028.Martin@lichtvoll.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: linux-btrfs To: Martin Steigerwald Return-path: In-Reply-To: <201106041225.56028.Martin@lichtvoll.de> List-ID: On 04.06.2011 12:25, Martin Steigerwald wrote: > Hi! > > In mailing list debian-user-german we are discussing safe ways to do = a > fsck when mounted. > > I tested with Ext4 that fsck -nf works either with mount -o remount,r= o or > fsfreeze -f while writing with: > > I=3D0; while true ; let I=3DI+1 ; do touch /boot/test$I ; sleep 0.2 ;= done > > In the read only mount case the write application returns errors, in = the > fsfreeze case Linux kernel stacks the changes in memory, but the fsck > reports no errors like it should. for online fsck you can use scrub, it checks at least partially the consistency. > > > Now I thought about a way to safely backup a MySQL or other database = - > without long service interruption: > > - Tell DB to turn itself into consistent state and freeze there > - sync / btrfs filesystem sync ; fsfreeze -f /mountpoint > - btrfs subvolume snapshot > - fsfreeze -u /mountpoint > - Tell DB to continue business as usual I'd just take a snapshot and backup from there. As a snapshot is a consistent image of the filesystem at the time the snapshot is taken, and every database is required to always have an at least recoverable state on disk, the snapshot represents a state where your DB can recover from. > > My questions are: > > 1) Would this work? > > 2) Is the sync needed? And if so how to avoid the race condition betw= een > the sync and the fsfreeze invocation? Reading from the fsfreeze manpa= ge I > understand that fsfreeze allows all ongoing transactions to complete.= But > does that include everything what sync would bring to disk? > > 3) Is the fsfreeze needed at all? Does btrfs subvolume freeze the > filesystem prior to the snapshot? The manpage doesn=B4t tell it. > > Thanks, -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" = in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html