[libvirt-users] some problem with snapshot by libvirt

Kashyap Chamarthy kchamart at redhat.com
Fri Nov 21 21:02:09 UTC 2014

On Fri, Nov 21, 2014 at 07:51:33PM +0000, Payes wrote:
> Eric Blake <eblake at ...> writes:

[. . .]

> > 
> > Yep.  You've run into a case where I haven't yet been able to code
> > things.  The problem is that reverting to a disk image has a question -
> > you are starting with two files (the snapshot backing file and the new
> > qcow2 wrapper), but the moment you reload the backing file for
> > modification, you invalidate the qcow2 wrapper.  There are two sane
> > options - create yet another qcow2 wrapper around the same backing file,
> > or delete the qcow2 wrapper as part of reverting to the snapshot, but we
> > would probably need two new flags to the virDomainRevertToSnapshot API
> > to select one of these two behaviors.  In the meantime, the only way to
> > revert to a disk-only snapshot is to manually edit the domain XML to
> > match what you want to do, possibly with manual qemu-img operations as well.
> > 
> > Help in writing the patches to improve the functionality would be welcome.
> > 
> Has the issue ( revert to external disk snapshot ) been solved in later
> versions of libvirt?

It's not merely an "issue". As Eric stated above, it's tricky and care
needs to be taken when dealing with external snapshots, where you have
multiple files in play. 

> I am using:
> Compiled against library: libvirt 1.2.2
> Using library: libvirt 1.2.2

If it's possible in your environment, you might want to move to
the latest release libvirt-1.2.10

> Using API: QEMU 1.2.2
> Running hypervisor: QEMU 2.1.92
> On trying to revert to an  external snapshot,
> Getting the error: revert to external disk snapshot not supported yet
> If not is there any alternate.

There is more information in the below wikis on methods to deal with
external snapshots, ways to shorten disk image chains etc with some
possible scenarios



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