Re: [Xen-users] How was CoW implemented on the Demo LiveCD (3.0) ??

[Ernst Bachmann <e.bachmann@xxxxxxxx>]

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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