[libvirt-users] Snapshots - can't take a snapshot with running VM

Hi I'm quite new to KVM/libvirt0 so hope you can help I'm using Ubuntu Lucid 10.04 and libvirt0 0.8.1 When I try a :- snapshot create vm_name I get an error, ie 'error -5 while writing' Googling doesnt bring anything up. Taking snapshots of a vm when it isnt running works fine. Can I really only create snapshots of a vm when it isnt running ? -- Kind regards Julian