On Wednesday 25 January 2006 11:14, Tim Deegan wrote:
> On Wed, Jan 25, 2006 at 12:55:16AM +0530, Anand wrote:
> > Can the same method of cow used in the LiveCD be implemented with LVM ?
> > instead of the flat images ?
>
> I've never tried it, but it should be possible to use the dmsetup runes
> on any block device. In fact Andrew Peace's create_cow script from the
> Xen3 demo CD does try to do the right thing if you give it a block
> device instead of a file (i.e. not pass it through a loop device).
>
> Don't the LVM tools provide a copy-on-write snapshot facility already?
LVM is just a "wrapper" arround the device mapper, it provides a more
"user-friendly" interface for creating dm rules, and stores them persistently
in its superblock.
But the LVM Cow features seem to be more geared towards backups, i.e. create a
RO snapshot, backup it, release the snapshot again.
And they seem to be quite unstable, I'd not try to use it on a LVM VG
containing valuable data yet.
And for backing a domU root partition, block based CoW implementations are
generally not very usefull, as you don't have a way to change the "master
copy" (like installing a security update etc) and have those changes appear
in all clones.
File-based implementations like unionfs are IMHO better suited for that task.
/Ernst
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