Linux-Client, dattobd and device-mapper

I get a strange behavior using /usr/local/share/urbackup/dattobd_create_snapshot. I am not sure, if it is a problem of the combination datto / device-mapper or a problem of the way urbackup is using datto.

As I understand urbackup works the following way

  1. urbackup makes a datto-snapshot, resulting in a block device e.g. /dev/datto0
  2. urbackup creates a dummy-snapshotfile
  3. it combines the datto-snapshotfile /dev/datto0 with the dummy-snapshotfile to a new /dev/mapper/wsnap…
  4. it tries to mount the new mapping-device

My problem is, that the new mapping-device is not mountable while the datto-snapshot-device /dev/datto0 is mountable; the new mapping-device /dev/mapper/wsnap… differs in some bytes different from /dev/datto0.

So it could be a problem with datto and the device-mapper. But what is the reason, urbackup does the “double-mapping” and not using /dev/datto0 directly?