One of the challenges with thinly provisioned volumes in ZFS is that they grow somewhat unabated as files are deleted and as the disk generally fragments; ZFS doesn’t have any meta awareness of the NTFS filesystem which is living on the thinly provisioned volume and which is made available through COMSTAR iSCSI. One of the possible remedies is to run a tool like Eraser, which has an option to write binary zeros to the ununsed space on an NTFS volume. Once the space is zeroed, either ZFS compression will efficiently reduce the occupied size — or so the theory goes.