From: "Eric Blake" <eblake@redhat.com>
To: "Andrew Martin" <amartin@xes-inc.com>
Cc: libvirt-users@redhat.com
Sent: Friday, August 17, 2012 9:43:23 AM
Subject: Re: [libvirt-users] Live Snapshot and Rebasing using blockpull

On 08/17/2012 08:27 AM, Andrew Martin wrote:
>
> 1. virsh snapshot-create-as to create the snapshot
> 2. cp myvm.qcow2 myvm-backup.qcow2 to copy off the now read-only backing image file
> 3. virsh blockpull to merge the base and snapshot images together into a single image again

Yep.  As long as you don't mind the single image changing names, this
works (and if you _do_ mind it changing names, you can do the process
twice to get back to the original name, although that's obviously not
very efficient).

There are future improvements coming down the pipeline for even more
efficient block manipulations, but they missed the qemu 1.2 cutoff, so
now I have to wait for qemu 1.3 (probably closer to December of this
year) before having libvirt control over those features - but I am
actively working on this area of code.

>
>
> Is this functionality only available in qemu 1.1?

snapshot-create-as was introduced in libvirt 0.9.5, and requires the
qemu 'blockdev-snapshot-sync' monitor command (since 0.14.0), but
prefers the 'transaction' monitor command (since 1.1) for safety.

blockpull was introduced in libvirt 0.9.4, and requires the qemu
'block-stream' monitor command (since 1.1).

> I am using Ubuntu 12.04 LTS on production servers which only includes qemu 1.0.x. Is there a safe alternative for accomplishing live backups like this on qemu 1.0.x? Is there a minimum version of libvirt that supports this behavior?

It sounds like you can take snapshots but not merge them back while your
domain is online, because your qemu is too old.  But you can use
'qemu-img' to do offline merging, if having your domain offline is not
an issue.

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