Okay, good to know it's being considered. For a non-optimized diff then (zfs-diff, which just prints which files have changed, not what about them have changed), it would be the same approach? Scan all the metadata looking for keys with versions between the two snapshots, and print out the associated file names? Would that simple approach be likely to be faster than whatever rsync does to find updated files when doing a backup?
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.
