Unable to activate "Discard" checkbox (qcow2)

[encore]

Hi,

I am not able to enable the Discard Feature on my KVM VPS stored on qcow2 images (stored in an ext4 directory):
http://prntscr.com/mwsbuy
Virtio SCSI is enabled.

Any ideas?

UPDATE: My storage was VIRTIO Block instead of SCSI, works now.