From mboxrd@z Thu Jan  1 00:00:00 1970
From: Martin Steigerwald <Martin@lichtvoll.de>
Subject: Safe fsck / consistent backup while mounted
Date: Sat, 4 Jun 2011 12:25:55 +0200
Message-ID: <201106041225.56028.Martin@lichtvoll.de>
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To: "linux-btrfs" <linux-btrfs@vger.kernel.org>
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Hi!

In mailing list debian-user-german we are discussing safe ways to do a=20
fsck when mounted.

I tested with Ext4 that fsck -nf works either with mount -o remount,ro or=20
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=20
fsfreeze case Linux kernel stacks the changes in memory, but the fsck=20
reports no errors like it should.


Now I thought about a way to safely backup a MySQL or other database -=20
without long service interruption:

=2D Tell DB to turn itself into consistent state and freeze there
=2D sync / btrfs filesystem sync ; fsfreeze -f /mountpoint
=2D btrfs subvolume snapshot
=2D fsfreeze -u /mountpoint
=2D Tell DB to continue business as usual

My questions are:

1) Would this work?

2) Is the sync needed? And if so how to avoid the race condition between=20
the sync and the fsfreeze invocation? Reading from the fsfreeze manpage I=20
understand that fsfreeze allows all ongoing transactions to complete. But=20
does that include everything what sync would bring to disk?

3) Is the fsfreeze needed at all? Does btrfs subvolume freeze the=20
filesystem prior to the snapshot? The manpage doesn=B4t tell it.

Thanks,
=2D-=20
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7

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