Why the cow path must be a path on the block device?

[zekyang]

Hi, I have a question. why the cow path must be a path on the block device? If I want to snapshot a disk and save the backup data in the another disk, dattobd can do?

[yito24]

Due to the way COW algorithm, it's best to create it on a block device.
The same is true for Windows VSS.

dattobd is a partition-unit snapshot, not a disk.
You can mount the snapshot and copy the data.
Or you can create an image from the snapshot and restore it to a partition on a different disk.