>What happens if you clone a source VM which is using "thin" ?
The VM cloned from a thin templated base VM will be created with a full
copy of the disk (as in "Clone" when creating from template)
"
but I have not tried myself recently with 4.0/4.1 versions
Instead I did test some days ago with 4.0.6 this scenario below as I wanted to convert a source thin provisioned VM to a preallocated template:
VM is powered on
create a snapshot of the VM
select the snapshot line and select clone
in resource allocation section of the new vm screen change allocation policy for the desired disk/s from thin provision to preallocated
Then you can decommission old VM and use the cloned one
Verified today in 4.1 too.
Or from the snapshot line you can also "make template" if you plan to use it as a base for more VMs.
But in this case (tested in 4.1) your options are shown in a "Format" column, not "Allocation Policy" column; and you can choose between QCOW2 (that should be thin) and Raw (that should be preallocated).