The correct available capacity is 20 GB. Both 20 GB LUNs are space-reserved, so ONTAP reserves their full provisioned capacity rather than only the 10 GB currently consumed in each LUN. The two LUN reservations therefore account for 40 GB. Because the volume uses a 100% fractional reserve and a Snapshot copy exists, ONTAP also reserves sufficient overwrite space for 100% turnover of the reserved objects. That represents another 40 GB. Consequently, 80 GB of the 100 GB volume is committed: 40 GB for the two space-reserved LUNs and 40 GB to guarantee their overwrites. This leaves 20 GB available. NetApp describes fractional reserve 100 as reserving 100% of the space necessary for overwrites of reserved objects. Relevant topics include LUN space reservation, fractional reserve, thick provisioning, and Snapshot overwrite guarantees.