[koverstreet]

If you just want to know what files have changed, there's no reason to scan all metadata - you'd just want to scan all the inodes (and we'd need to add a version number field to the inode itself that covers all the contents; easily done). And since just prior to snapshots inodes have backpointers back to dirents, so given an inode we can easily get a path.

So yeah, that would be easy and a lot faster than what rsync does. Dunno about plumbing to anything in userspace, that's outside the scope of what I can think about right now. But if anyone wanted to work on such a feature, I'd happily help them get started.