discard-support

1.sh

[root@localhost ~]$ rpm -q qemu-kvm
qemu-kvm-1.6.2-1.fc20.x86_64
 
[root@localhost ~]$ rpm -q libvirt-daemon
libvirt-daemon-1.1.3.4-4.fc20.x86_64

dd.sh

[root@localhost ~]$ mount -o discard /dev/sda1 /sda
[root@localhost ~]$ mount /dev/sdb1 /sdb 
[root@localhost ~]$ for _ in {1..3}; do
    /bin/dd if=/dev/urandom of=/sda/file.$RANDOM bs=1M count=100 && sync
    /bin/dd if=/dev/urandom of=/sdb/file.$RANDOM bs=1M count=100 && sync
    /bin/rm -f /sda/file.* /sdb/file.* && sync
done

discard.xml

  <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none' discard='unmap'/>
      <source file='/disk/fedora20.data1.img'/>
      <target dev='sda' bus='scsi'/>
  </disk>

  <controller type='scsi' index='0' model='virtio-scsi'>
  </controller>

du.sh

[root@localhost ~]$ du -h sd{a,b}.img
34M  sda.img
138M sdb.img

vm1.xml

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none' discard='unmap'/>
      <source file='/mnt/sda.img'/>
      <target dev='sda' bus='scsi'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/mnt/sdb.img'/>
      <target dev='sdb' bus='scsi'/>
    </disk>