We have enabled both options  discard-after delete and wipe after delete but nothing is working , I have tried to creating small disk ( like 1 GB ). after deleting that disk space has reclaimed succsuuufully. 
Seems this happens for  larger size disk 

On Mon, Jul 12, 2021 at 1:10 AM Nir Soffer <nsoffer@redhat.com> wrote:
On Sun, Jul 11, 2021 at 5:03 PM Ranesh Tharanga Perera
<raneshtharanga@gmail.com> wrote:
>
> I have attached  7TB disk ( pre allocated)  for VM ( Redhat 8 ).. Due to some space issue we have unattached disk from VM and deleted it.

Did you enable the "discard after delete" option in the storage domain
advanced options?

If  you did not, the logical volume is deleted, but the blocks allocated on the
server side from this logical volume are not discarded.

Since you deleted the logical volume, it is not possible to discard it
now, but you
do this:

Try this:
- Put the storage domain to maintenance
- Enable "discard-after delete"
- Active the storage domain
- Create new raw preallocated disk filling the entire domain
- Delete the disk

Nir