[libvirt-users] Snapshot without volumes temporarily

[Eric Blake]

On 03/28/2013 03:46 AM, Hubert Chiang wrote:
> Hello,
> 
> I want to do a snapshot which the VM has volumes.
> 
> But I want to skip the volume check when I do the snapshot. (Which means I
> want to do a snapshot without detaching the volume)

If I understand you, you have a VM with two <disk> devices, but you only
need a snapshot of the contents of one of those disks, so you want to
skip the other one during snapshot creation.

> 
> So I try to write a snapshot XML refer by
> http://libvirt.org/formatsnapshot.html as following:
> 
> vda is my VM basic disk (qcow2)
> vdb is a volume (raw)
> ================ t1.xml ===============
> <domainsnapshot>
> <disks>
> <disk name='vda' snapshot='internal'>
> <source file='/var/lib/libvirt/VM1/disk' />
> </disk>

Mixing snapshot='internal' and <disk> was not possible until libvirt
1.0.1; furthermore, use of snapshot='internal' requires that ALL of your
<disk>s are qcow2, based on limitations still present in qemu 1.4.  What
you are asking for is not possible with the current tools, although we
might be able to support it in future releases.

> <disk name='vdb' snapshot='no' />
> </disks>
> </domainsnapshot>
> =====================================
> 
> with this command: # virsh snapshot-create VM1 --xmlfile t1.xml
> 
> and get the message: # error: argument unsupported: unable to handle disk
> requests in snapshot
> 
> My environment is Libvirt 0.9.8, Qemu 1.0.0 on Ubuntu12.04.02

Correct - the version of libvirt that you are using is too old to mix
<disks> with internal snapshots.  To use <disk name='vdb'
snapshot='no'/> with your version of libvirt, you _have_ to pass the
--disk-only flag and take an external snapshot, but then you are no
longer able to capture the VM memory state (which was not possible until
libvirt 1.0.1).

> 
> I try to do this action before at libvirt 0.9.2 QEMU 0.14.1 on Ubuntu 11.10
> before with following commands, It's success.

No, what was really happening with libvirt 0.9.2 is that the <disks>
element of your snapshot was completely ignored.  <disks> support wasn't
added until 0.9.5.  You were taking an internal snapshot of ALL disks,
whether you realized it or not, because that old of libvirt didn't know
to use <disks>.

> 
> Step1. I copy the XML from /etc/libvirt/qemu/VM1.xml to VM1.xml.backup

Dangerous.  Copying files straight from /etc/libvirt is NOT guaranteed
to be reliable; instead, you should use 'virsh dumpxml $dom' if you want
to back up a domain's XML.

> Step2. I edit the VM1.xml to remove the disk tag which is create by volume

I'm not sure I follow.  It would help if you showed a before-and-after
of the .xml file that you are editing, and a bit more explanation of
what you hoped to accomplish with your edit.

> Step3. do virsh command: virsh snapshot-create VM1
> Step4. Move VM1.xml.backup to VM1.xml

Even more dangerous.  While it is possible to copy files out of /etc
without too much risk (the worst that can happen is that you copied
inconsistent state), writing files _into_ /etc goes behind libvirt's
back and is liable to break things.

> 
> It's success on libvirt 0.9.2. But on 0.9.8 It doesn't work. Because
> libvirt will check the setups from memory not the XML file in
> /etc/libvirt/qemu/
> 
> What could I do with libvirt 0.9.8?

I'm not sure what you are really wanting to do.  Do you want an internal
snapshot (requires qcow2, but the snapshot lives in the same file as the
original) or an external snapshot (which creates a second qcow2 around
your original file which is now frozen in time)?  Do you want this while
your domain is running or offline?  Are you willing to upgrade to new
enough libvirt and qemu to get things properly supported (at least
libvirt 1.0.1 and qemu 1.2 or better)?

It sounds like maybe you have a situation where your disk for vdb is not
qcow2, so you can't do an internal snapshot while it is attached.  If
that is the case, then the only supported way of doing an internal
snapshot of a running domain, while using your current versions of
libvirt and qemu, is to use 'virsh suspend VM1' to suspend the domain,
'virsh detach-disk ...' to hot-unplug the vdb disk, 'virsh
snapshot-create ...' to create your snapshot, 'virsh attach-disk ...' to
hot-plug the vdb disk back in, then 'virsh resume VM1' to resume things.

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

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