こんな風に動くみたい。

# zfs create -o copies=1 zwimming/single
# zfs create -o copies=2 zwimming/dual
# zfs create -o copies=3 zwimming/triple
# cp -rp /usr/share/man1 /zwimming/single
# cp -rp /usr/share/man1 /zwimming/dual
# cp -rp /usr/share/man1 /zwimming/triple
# zfs list -r zwimming
NAME              USED  AVAIL  REFER  MOUNTPOINT
zwimming         48.2M   310M  33.5K  /zwimming
zwimming/dual    16.0M   310M  16.0M  /zwimming/dual
zwimming/single  8.09M   310M  8.09M  /zwimming/single
zwimming/triple  23.8M   310M  23.8M  /zwimming/triple