[libvirt-users] qemu-img snapshots with backing stores?

[Eric Blake]

On 06/06/2012 10:55 AM, Sean Abbott wrote:
> So, I was attempting to use qemu snapshots with backing stores.  The
> QEMU docs (http://wiki.qemu.org/Documentation/CreateSnapshot) make it
> sound like you simply point your qemu at the snapshot after it's
> creation, and you're golden.
> 
> When attempting this with libvirt, though, it fails.

Libvirt definitely supports this, as I use it for my guests, so let's
figure out where you went wrong.  By the way, libvirt can create qcow2
files itself, rather than forcing you to hand-create it with qemu-img,
although support for this could probably be improved with more APIs and
documentation.  Patches welcome.

> 
> I created a snapshot using the above tutorial.  the resulting file is
> disk.0, and a qmeu-img info on it returns:
> 
> image: disk.0
> file format: qcow2
> virtual size: 29G (31457280000 bytes)
> disk size: 140K
> cluster_size: 65536
> backing file: /var/lib/one/public/lin_client_current.qcow2 (actual path:
> /var/lib/one/public/lin_client_current.qcow2)
> 
> So that all looks groovy, right?

Unfortunately, 'qemu-img info' output doesn't say whether you properly
populated the backing_fmt property, but I will assume that is not your
issue (do note, however, that failure to use the backing_fmt property is
a security hole - it means libvirt and/or qemu will autoprobe the format
from the backing file itself, but if the backing file is supposed to be
raw, the guest can manipulate the backing file into looking non-raw, and
cause your host to hand over control of files to the guest that should
not normally be accessible to the guest).

> 
> Then, I created (via opennebula) an xml deployment file like so: 
> http://paste.ubuntu.com/1027145/

which included:

		<disk type='file' device='disk'>
			<source file='/var/lib/one/vm/56/images/disk.0'/>
			<target dev='hda' bus='virtio'/>
			<driver name='qemu' type='qcow2' cache='none'/>

and that looked correct to me.

> 
> When I attempt to do a virsh create, I get the following errors:
> 
> virsh # create deployment.0
> error: Failed to create domain from deployment.0
> error: internal error process exited while connecting to monitor:
> file=/var/lib/one/vm/56/images/disk.0,if=none,id=drive-virtio-disk0,boot=on,format=qcow2,cache=none
> qemu-kvm: boot=on|off is deprecated and will be ignored. Future versions
> will reject this parameter. Please update your scripts.

This warning is not the real problem, but a patch to libvirt to avoid it
might be nice, if it hasn't already been patched in newer libvirt.

> qemu-system-x86_64: -drive
> file=/var/lib/one/vm/56/images/disk.0,if=none,id=drive-virtio-disk0,boot=on,format=qcow2,cache=none,boot=on:
> could not open disk image /var/lib/one/vm/56/images/disk.0: Invalid argument

You mentioned Ubuntu - do you have appArmor running?  This could be a
case of the apparmor settings on your machine preventing qemu from
opening the backing file.  I don't have Ubuntu experience myself to tell
you how to resolve it (I tend to work with SELinux on Fedora as my
security mechanism), but suspect that it might be a failure along the
lines of an over-strict security policy.

> 
> So...something isn't working. Is it possible to do this, or should I
> give up on this path?

Libvirt definitely supports what you want to do, but I don't know what
to suggest to help you get further.

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

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