Description of problem:

When data is deleted from inside virtual machine the underlying block devices in the RHEV Storage Domain do not know this and blocks are not actually freed. An Example (similar to) mechanism is currently found in: http://ceph.com/docs/master/rbd/qemu-rbd/, section "Enabling Discard/TRIM"

Qemu does support trim - but RHEV 3.5 doesn't seem to enable it:

host# virsh -r dumpxml VM_NAME | grep -B3 -A5 sda
    <disk type='block' device='disk' snapshot='no'>
      <driver name='qemu' type='qcow2' cache='none' error_policy='stop' io='native'/>
      <source dev='/rhev/data-center/00000002-0002-0002-0002-000000000237/1312236c-5b8d-4d60-8c84-7e92c19ec064/images/19b6d07c-becd-4cbe-8bf6-5b1bcd37de7a/3b8ddefb-68b1-4b1c-9fc6-47f2f040b07f'/>
      <target dev='sda' bus='scsi'/>
      <serial>19b6d07c-becd-4cbe-8bf6-5b1bcd37de7a</serial>
      <boot order='1'/>
      <alias name='scsi0-0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

In order to enable it, <driver> element should contain "discard='unmap'"
parameter. qemu in RHEL7 can free TRIMmed space taken by qcow2-file-backed
disks and even earlier version of qemu should be able to mark TRIMmed
space in qcow2 for further allocation.

Version-Release number of selected component (if applicable):
rhev-3.5