[ILUG] xen, lvm snapshots and filesystem status

[bobb]

Niall O Broin hath declared on Friday the 02 day of February 2007  :-:
> I've been looking into doing offsite backups of running xen VMs. The  
> method being considered is making a snapshot of the VMs disk,  
> mounting this, and then rsyncing it to the backup server. This has of  
> course a number of issues with regards to consistency e.g. status of  
> database files etc. Some of these are easy to solve (in the case of  
> databases, doing some kind of dump before the snapshot is made, for  
> instance) but some are not so easy e.g. files which are being edited  
> just as the snapshot is taken.

Databases can be locked/flushed/dumped.
Most editors work in a temporary/swap file, so there should be a consistant
copy of the file since the last save, new changes will be lost, although
some might be recovered from the swap file.

> One issue I have noticed is that of filesystem inconsistency, the  
> snapshot of course being at a lower level than the filesystem. TO  
> deal with this, I have been running fsck on the snapshot, which has  
> worked so far, but I guess it's only a matter of time before I get a  
> filesystem state which fsck can't automatically handle.

xfs_freeze will suspend an (xfs) filesystem and flush buffers leaving
a consistant state on disk.  Allowing you to create a snapshot with lvm.

I haven't used it in anger, but seems to work well in testing.

- bobb

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