echo "0 21474836480 zero" | dmsetup create testdev
lvcreate -L 500M -n cowdev vgs
echo " 0 21474836480 snapshot /dev/mapper/testdev /dev/vgs/cowdev P 16" | dmsetup create realtestdev

dmsetup remove realtestdev
dmsetup remove testdev
lvremove vgs/cowdev