root@ubuntu:~# blockdev --getsz /dev/sda1
9767457
root@ubuntu:~# echo '0 9767457 snapshot /dev/sda1 /dev/ram7 N 1'|dmsetup -vvvvvvv create tstsnap
dm version   OF   [16384]
dm create tstsnap  OF   [16384]
tstsnap: Stacking NODE_ADD (252,0) 0:6 0660
dm reload tstsnap  OF   [16384]
device-mapper: reload ioctl failed: Device or resource busy
dm remove tstsnap  OF   [16384]
tstsnap: Stacking NODE_DEL (replaces other stacked ops)
Command failed
root@ubuntu:~# dmesg |tail
[ 2729.015213] device-mapper: table: 252:0: snapshot: Cannot get origin device
[ 2729.132113] device-mapper: ioctl: error adding target to table