[sheepdog-users] [libvirt] How to start a snapshot of Sheepdog volume in Libvirt

Eric Blake eblake at redhat.com
Mon Feb 4 21:49:56 CET 2013

On 02/02/2013 08:51 PM, harryxiyou wrote:
> Hi all,
> I wanna test start a snapshot of Sheepdog volume in Libvirt. However, i
> cannot find a test description from
> https://github.com/collie/sheepdog/wiki/Libvirt.
>>From http://libvirt.org/formatsnapshot.html, i can find some snapshot operations
> in Libvirt, which may be different from Sheepdog volume snapshot, right? Now,
> i have some questions like following.
> 1, What are the differences between Libvirt snapshot operations and Sheepdog
> Volume snapshot operations?  The object of Libvirt snapshot is a file
> and Sheepdog
> volume is a block device, right?

Libvirt snapshots are designed to be an abstraction around whatever the
hypervisor can provide.  It is flexible enough to cover both internal
snapshots (something that qcow2 definitely supports, but it also sounds
like sheepdog supports internal snapshots, based solely on what I've
read from your comments; but I have no actual experience with sheepdog
snapshots myself).  Libvirt snapshots are designed to cover as much as
the entire state of a guest: all disk state, and also all RAM state
needed to restore a running guest back to the same point in time.  But,
while the design aims to be flexible, the implementation has to catch up
to the design.

Sheepdog, on the other hand, is a storage protocol.  So at most, it can
only take snapshots of a single disk state (it probably cannot store the
RAM state of a guest that is using the disk, and if you have a guest
with more than one sheepdog disk image, it takes metadata outside of
sheepdog to coordinate if you are taking a snapshot of a guest across
all of its disks at once).

> 2, Can we use Libvirt XML layout from http://libvirt.org/formatsnapshot.html
> to create a Sheepdog volume snapshot and start from it? If not, which
> Libvirt XML
> form should we use to create&start from this snapshot?

Again, I haven't done it myself, so I'm not sure what has been wired
into qemu yet.  For a running guest, the question boils down to whether
the HMP command 'savevm' is wired up to support sheepdog images (if so,
qemu would automatically create a sheepdog internal snapshot as part of
saving vm state). I also know that people are working on qemu patches to
expose more functionality and control over portions of the overall
domain snapshot process, so that it would be possible to create a
sheepdog snapshot of a disk in isolation without having to bundle it as
part of the 'savevm' command.

For an offline guest, libvirt does not yet drive 'collie' to create any
sheepdog snapshots; I don't know if 'qemu-img' has that capability
either.  In other words, you are treading on frontier territory, where
you may find that a lot of things have not been coded up yet.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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