Ok, but this approach is pushing the complexity down the stack into the kernel/filesystem. And a general-purpose performance-oriented snapshotting filesystem i

Ok, but this approach is pushing the complexity down the stack into the kernel/filesystem. And a general-purpose performance-oriented snapshotting filesystem is going to be making a lot of compromises that aren't really ideally suited to backup duties. Plan9's Venti/Fossil immediately come to mind as an example of simplicity in this vein.