[ovirt-users] effectiveness of "discard=unmap"

Hi, i'm sorry to bother you again with my ignorance of the DISCARD feature for block devices in general. after finding several ways to enable "discard=unmap" for oVirt disks (via standard GUI option for iSCSI disks or via "diskunmap" custom property for Cinder disks) i wanted to check in the guest for the effectiveness of this feature. to my surprise i couldn't find a difference between Linux guests with and without "discard=unmap" enabled in the VM. "lsblk -D" reports the same in both cases and also fstrim/blkdiscard commands appear to work with no difference. Why is this? Do i have to look at the underlying storage to find out what really happens? Shouldn't this be visible in the guest OS? thx matthias