libvirt/tests/qemublocktestdata/xml2json/file-backing_basic-unmap.xml

<disk type='file' device='disk'>
  <driver name='qemu' type='qcow' discard='unmap'/>
  <source file='/var/lib/libvirt/images/a'>
    <privateData>
      <nodenames>
        <nodename type='storage' name='node-a-s'/>
        <nodename type='format' name='node-a-f'/>
      </nodenames>
    </privateData>
  </source>
  <backingStore type='file' index='1'>
    <format type='qed'/>
    <source file='/var/lib/libvirt/images/b'>
      <privateData>
        <nodenames>
          <nodename type='storage' name='node-b-s'/>
          <nodename type='format' name='node-b-f'/>
        </nodenames>
      </privateData>
    </source>
    <backingStore type='network' index='2'>
      <format type='vmdk'/>
      <source protocol='gluster' name='images/c'>
        <host name='test.org'/>
        <privateData>
          <nodenames>
            <nodename type='storage' name='node-c-s'/>
            <nodename type='format' name='node-c-f'/>
          </nodenames>
        </privateData>
      </source>
      <backingStore type='file' index='3'>
        <format type='raw'/>
        <source file='/var/lib/libvirt/images/d'>
          <privateData>
            <nodenames>
              <nodename type='storage' name='node-d-s'/>
              <nodename type='format' name='node-d-f'/>
            </nodenames>
          </privateData>
        </source>
        <backingStore/>
      </backingStore>
    </backingStore>
  </backingStore>
  <target dev='vda'/>
</disk>