zfs native works like a charm with openvz containers and using compression and deduplication. I feel its much faster than ext and uses just a bit of space.
However under KVM I experienced heavy performance impacts and my KVMs are running extremely slow and unstable. Possibly to much IO that cant be handled by my "undersized" lab host. For ZFS you need loads of memory and a good sized CPU. Be careful using zfs with a single shared SAN/iscsi disk. Its not designed for that type of storage.
