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Re: [libvirt] RFC: API additions for enhanced snapshot support

On 07/27/2011 04:04 AM, Stefan Hajnoczi wrote:
On Thu, Jun 16, 2011 at 6:41 AM, Eric Blake<eblake redhat com>  wrote:
Right now, libvirt has a snapshot API via virDomainSnapshotCreateXML,
but for qemu domains, it only works if all the guest disk images are
qcow2, and qemu rather than libvirt does all the work.  However, it has
a couple of drawbacks: it is inherently tied to domains (there is no way
to manage snapshots of storage volumes not tied to a domain, even though
libvirt does that for qcow2 images associated with offline qemu domains
by using the qemu-img application).  And it necessarily operates on all
of the images associated with a domain in parallel - if any disk image
is not qcow2, the snapshot fails, and there is no way to select a subset
of disks to save.  However, it works on both active (disk and memory
state) and inactive domains (just disk state).

Hi Eric,
Any updates on your proposed snapshot API enhancements?

I still need to post a v2 RFC that gives the XML changes needed to support disk snapshots on top of virDomainSnapshotCreateXML.

Meanwhile, I still want to add additional API to make it easier to manage offline storage volume snapshots (storage volumes that are not in use by a defined or running domain), although it obviously won't happen by 0.9.4.

The use case I am particularly interested in is a backup solution
using libvirt snapshot APIs to take consistent backups of guests.  The
two workflows are reading out full snapshots of disk images (full
backup) and reading only those blocks that have changed since the last
backup (incremental backup).

Full backup should be supported via virDomainSnapshotCreateXML, once I have the new XML in place, by using the new qemu snapshot_blkdev monitor commands.

Incremental backups can be done just like full backups except with an
API call to get a dirty blocks list.  The client only reads out those
dirty blocks from the snapshot.

Incremental backups will need more work - qemu does not yet have the monitor commands for exposing which blocks are dirty. Of course, as code is written for qemu, it would be nice to also be thinking about how to support that in libvirt, so once I do propose my XML changes for full snapshots, it would be nice to remember in your review to think about whether it remains extensible to the incremental case.

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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