And are you saying if it is over 2TB you cannot thin provision?
That's the issue, thin provisioning is limited currently to 2TB or smaller volumes (VHDs.)
Wait, is it 2 TB volumes as in ext3 partitions or is it 2 TV VHDs?
That was another thing people are always clamoring about with XS ... the lack of VHDX.
Which I think is ridiculous that it's not in 7. VHDX is over 4 years old. Even worse, QCOW2 is something like 10 (or more) years old, and has almost a 9 EB limit.
The limitation on the partitions sizes is because of the use of snapshot functionality. Imagine having a 9EB array and partition, and then trying to perform a snapshot on it.
VHDX has the same limit (or similar). No one would really have an image that size, but it would be compatible with every other system. And snapshots wouldn't take any longer. It creates a new file and writes to it and reads from the old one.