Taking snapshots on KVM with libvirt

There is an interesting feature related to the KVM virtualization, the snapshots, which take the disk, memory and device state of a concrete domain at a specific moment. This can have many use cases, such as saving a copy of a virtual machine, preserving a domain's state before a potentially dangerous operation, etc. Snapshots are identified with an unique name and can be taken with the virtual machine turned on.

This feature can only be used with the virsh command and not with the graphical tool (the version of Virtual Machine Manager included on Kubuntu 10.10 is the 0.8.4).

So as to create a snapshot for a particular domain, we must set a XML file with at least a name and description for the snapshot.

javi@javi-kubuntu:~$ cat UbuntuServer_10.10-ss.xml
<domainsnapshot>
   <name>UbuntuServer_10.10-16032011</name>
   <description>Snapshot of OS install and updates</description>
</domainsnapshot>

javi@javi-kubuntu:~$ virsh snapshot-create UbuntuServer_10.10 UbuntuServer_10.10-ss.xml
error: Requested operation is not valid: Disk '/var/lib/libvirt/images/UbuntuServer_10.10.img' does not support snapshotting

It is possible that as in the previous output, we get an error message about the virtual disk does not support snapshotting. What is the problem in this case?

The sort of the disk image is raw (default format when you create a virtual machine), which is very simple and does not allow snapshotting. In contradistinction to raw, qcow2 (QEMU image format) supports to have smaller images, encryption, compression, snapshots and so on.

In order to make a qcow2 virtual disk, we can run the following command. In this case, we have reserved all the space for the virtual disk. If we do not want to preallocate the image with metadata, we must take away the "preallocation" option.

javi@javi-kubuntu:~$ sudo qemu-img create -f qcow2 -o preallocation=metadata /var/lib/libvirt/images/UbuntuServer_10.10.qcow2 8G

javi@javi-kubuntu:~$ sudo chown libvirt-qemu:kvm /var/lib/libvirt/images/UbuntuServer_10.10.qcow2

If we want to convert an existing image (for example raw to qcow2), we can use the next order.

javi@javi-kubuntu:/var/lib/libvirt/images$ sudo qemu-img convert -f raw -O qcow2 -o preallocation=metadata UbuntuServer_10.10.raw UbuntuServer_10.10.qcow2

javi@javi-kubuntu:/var/lib/libvirt/images$ sudo chown libvirt-qemu:kvm UbuntuServer_10.10.qcow2

In this moment, we will be able to set a snapshot up with the virsh snapshot-create command. This operation will generate a new XML file.

javi@javi-kubuntu:~$ ls -l /var/lib/libvirt/qemu/snapshot/UbuntuServer_10.10/UbuntuServer_10.10-16032011.xml
-rw------- 1 root root 307 2011-03-16 11:40 /var/lib/libvirt/qemu/snapshot/UbuntuServer_10.10/UbuntuServer_10.10-16032011.xml

javi@javi-kubuntu:~$ virsh snapshot-dumpxml UbuntuServer_10.10 UbuntuServer_10.10-16032011
<domainsnapshot>
   <name>UbuntuServer_10.10-16032011</name>
   <description>Snapshot of OS install and updates</description>
   <state>shutoff</state>
   <creationTime>1300272022</creationTime>
   <domain>
      <uuid>d19af827-30ca-b3a0-8304-231f8cf5dd8b</uuid>
   </domain>
</domainsnapshot>

So as to list all the available snapshots for a certain domain, we can type the following order.

javi@javi-kubuntu:~$ virsh snapshot-list UbuntuServer_10.10
Name                        Creation time             Status
---------------------------------------------------------------
UbuntuServer_10.10-16032011 2011-03-16 11:40:22 +0100 shutoff

If at any point we want to bring back a snapshot, we must run the next command.

javi@javi-kubuntu:~$ virsh snapshot-revert UbuntuServer_10.10 UbuntuServer_10.10-16032011

And finally, we also have the option to remove a snapshot.

javi@javi-kubuntu:~$ virsh snapshot-delete UbuntuServer_10.10 UbuntuServer_10.10-16032011