External system checkpoint snapshot: guest's disk-state will be saved in
one file, its RAM & device-state will be saved in another new file.

Version Info
============

  $ uname -r ; rpm -q qemu-kvm libvirt libguestfs
  3.9.4-301.fc19.x86_64
  qemu-kvm-1.5.0-4.fc19.x86_64
  libvirt-1.0.5.1-1.fc19.x86_64
  libguestfs-1.21.38-1.fc19.x86_64


Snapshot creation
=================

1/ Start the guest:

  $ virsh start fed18
  Domain fed18 started


2/ List its block device in use:

  $ virsh domblklist fed18
  Target     Source
  ------------------------------------------------
  vda        /var/lib/libvirt/images/fed18.qcow2
 
3/ Make an XML file for snapshot:

  $ cat /var/tmp/ext-disk-ram-snap.xml 
  <domainsnapshot>
    <memory snapshot='external' file='/var/lib/libvirt/images/fed18.mem.snap2'/>
    <disks>
      <disk name='vda'>
        <source file='/var/lib/libvirt/images/fed18.disk.snap2'/>
      </disk>
    </disks>
  </domainsnapshot>

4/ Create snapshot (disk & memory state) w/ the the above XML file of the
   running guest:

  $ virsh snapshot-create fed18  \
    --xmlfile /var/tmp/ext-disk-ram-snap.xml --atomic
  Domain snapshot 1370930820 created from '/var/tmp/ext-disk-ram-snap.xml'


5/ List the snapshots:

  $ virsh snapshot-list fed18
   Name                 Creation Time             State
  ------------------------------------------------------------
   1370930820           2013-06-11 11:37:00 +0530 running


6/ Again, list the block device in use (note - the disk image file in use is the
   new qcow2 file we specified in the snapshot XML above):

  $ virsh domblklist fed18
  Target     Source
  ------------------------------------------------
  vda        /var/lib/libvirt/images/fed18.disk.snap2


7/ Find more information about the disk image (it can be seen, qcow2 overlays are
   being used):

  $ qemu-img info --backing-chain /var/lib/libvirt/images/fed18.disk.snap2
  image: /var/lib/libvirt/images/fed18.disk.snap2
  file format: qcow2
  virtual size: 20G (21474836480 bytes)
  disk size: 196K
  cluster_size: 65536
  backing file: /var/lib/libvirt/images/fed18.qcow2
  backing file format: qcow2
  
  image: /var/lib/libvirt/images/fed18.qcow2
  file format: qcow2
  virtual size: 20G (21474836480 bytes)
  disk size: 1.0G
  cluster_size: 65536
  Snapshot list:
  ID        TAG                 VM SIZE                DATE       VM CLOCK
  1         1370847718                0 2013-06-10 12:31:58   00:00:00.000


8/ List both files which has the snapshotted data:

  $ file /var/lib/libvirt/images/fed18.mem.snap2 \
    /var/lib/libvirt/images/fed18.disk.snap2 
  /var/lib/libvirt/images/fed18.mem.snap2:  data
  /var/lib/libvirt/images/fed18.disk.snap2: QEMU QCOW Image (v2), has backing file (path /var/lib/libvirt/images/fed18.qcow2), 21474836480 bytes