On 2006-07-08 12:44, Josef Wolf wrote:
On Fri, Jul 07, 2006 at 11:13:44PM +0200, Paul Bijnens wrote:
There are two approaches (maybe even many more -- I'm not the
specialist here).
One approach is to let the snapshot mechanism understand the filesystem
and work on that level. That is how xfs and solaris ufs snapshots seem
to work.
The other approach is a layer deeper: on the logical volume manager.
This implementation is not interested in where the inodes are located
or how directories and datablocks are layed out on disk. The LVM just
manages large blocks. The LVM2 snapshots in linux work on this layer.
If the snapshot is on the driver layer, there is no way to ensure that
the snapshot is consistent? Thus the snapshot would look like the power
switch was turned off at that moment. Without journalling, data loss can
be expected. I would rather not base my backups on such a mechanism...
Using method 1, you can probably get more optimized (quicker, using less
diskspace). Using method 2 is more general, and is is independent of
the filesystems on it. In my test setup I even managed to make a
snapshots of a vfat filesystem.
I don't think vfat have journalling.
Yes, indeed! You better have journalling to be really useful.
But after sync and with a "quiet" filesystem (if really necessary
unmount it) you *can* make a snapshot.
It's not that snapshotting vfat is elegant, but it shows the power of
doing snapshots a on the blocklevel instead of on the filesystem level.
--
Paul Bijnens, xplanation Technology Services Tel +32 16 397.511
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM Fax +32 16 397.512
http://www.xplanation.com/ email: Paul.Bijnens AT xplanation DOT com
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