[libvirt-users] Snapshot Error

Hey When I want to take a snapshot for a Domain by using virDomainSnapshotCreateXML. The Error as below happen: libvir: QEMU error : Requested operation is not valid: Disk '/var/lib/libvirt/images/fedora-qcow.qcow2' does not support snapshotting The snapshot Configuration is : <domainsnapshot> <description>Snapshot of OS install and updates</description> <disks> <disk name='/var/lib'> <source file='/var/lib-snapshot'/> </disk> <disk name='vdb' snapshot='no'/> </disks> </domainsnapshot> And /var/lib/libvirt/images/fedora-qcow.qcow2 is the disk for the domain (use qemu-img to create). And I had tried another Disk format (*.img), but the error still happen. how can i deal with this error?? PS:qemu version is 0.14 & libvirt version is 0.9.4. Pls help. :-(