[ovirt-users] Changing from thin provisioned to preallocated?

I am currently running a ZFS server (Running RaidZ2) and I have been experimenting with NFS and shares to host my guests. I am currently running oVirt 4.2.8 and using a RaidZ2 NFS mount for the guests. ZFS definitely is definitely pretty awful (At least in my experience so far) for hosting VMs. I believe this is due to the synchronous writes being performed. However, I think running an iSCSI target with Synchronization disabled over a 10Gb connection might do the trick. (I have a couple of mirroed SSD drives for performance if I need it, but the RaidZ2 crawls for disk speed). When I tried to migrate a thin provisioned guest to iSCSI, I keep getting an "Out of disk space error" which I am pretty sure is due to the block style storage on the iSCSI target. Is there a way to switch from Thin to Preallocated? Or is my best bet to try and take a snapshot and clone this into a pre-allocated disk?