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SubjectRe: Odd data corruption problem with LVM/ReiserFS
On Sun, Feb 20, 2005 at 11:25:37PM -0600, Alex Adriaanse wrote:
> This morning was the first time my backup script took
> a snapshot since upgrading to 2.6.10-ac12 (yesterday I had taken a few
> snapshots myself for testing purposes, this seemed to work fine).

a) Activating a snapshot requires a lot of memory;

b) If a snapshot can't get the memory it needs you have to back it
out manually (using dmsetup - some combination of resume, remove &
possibly reload) to avoid locking up the volume - what you have to do
depends how far it got before it failed;

c) You should be OK once a snapshot is active and its origin has
successfully had a block written to it.

Work is underway to address the various problems with snapshot activation
- we think we understand them all - but until the fixes have worked their
way through, unless you've enough memory in the machine it's best to avoid
them.

Suggestions:
Only do one snapshot+backup at once;
Make sure logging in as root and using dmsetup does not depend on access
to anything in /var or /home (similar to the case of hard NFS mounts with
the server down) so you can still log in;

BTW Also never snapshot the root filesystem unless you've mounted it noatime
or disabled hotplug etc. - e.g. the machine can lock up attempting to
update the atime on /sbin/hotplug while writes to the filesystem are blocked

Alasdair
--
agk@redhat.com
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