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[dm-devel] Snapshots on block devices

Documentation states: "Device-mapper allows you, without massive data
copying: *) To create snapshots of any block device i.e. mountable,
saved states of the block device which are also writable without
interfering with the original content;" - from
So, assuming I am reading right that it is possible to create snapshot
of say SATA disk partition or even md device. I'm trying to achieve
this but without much luck:

root ubuntu:~# blockdev --getsz /dev/sda1
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

root ubuntu:~# uname -a
Linux ubuntu 2.6.32-22-generic #33-Ubuntu SMP Wed Apr 28 13:28:05 UTC
2010 x86_64 GNU/Linux

root ubuntu:~# dmsetup targets
mirror           v1.12.0
snapshot-origin  v1.7.0
snapshot         v1.7.0
multipath        v1.1.0
striped          v1.3.0
linear           v1.1.0
error            v1.0.1

I guess i miss something but do have no idea what i'm missing

Best regards,

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