find out which are the BE filesystems:
$ zfs list
NAME USED AVAIL REFER MOUNTPOINT
tank 59.2G 840G 88K none
tank/ROOT 16.5G 840G 88K none
tank/ROOT/11.0-RC3 8K 840G 3.66G /
tank/ROOT/12.0-CURRENT-up-20161003_121021 60.6M 840G 7.08G /
tank/ROOT/12.0-CURRENT-up-20161006_054232 12.0G 840G 9.97G /
tank/ROOT/12.0-CURRENT-up-20161017_004530 4.43G 840G 9.04G /