Re: Odd data corruption problem with LVM/ReiserFS

[Alasdair G Kergon]

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
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