Optimising archive updates

This really isn't an issue, it is a question. Feel free to close it if you don't want this cluttering up your issues section..

I have been thinking about whether it is possible to automatically optimise updating archive copies. By this, I mean, sending the minimum changes to allow the archive to be updated to the current disk contents.

In the case I am looking at, I do regular btrbk snapshots and backups (to another disk but online on the same server). I also do occasional archive copies for offsite backup. I tend to do those using rsync, so they can be easily updated when I feel like it with only copying updated files.

I was wondering if it was possible for a script to use a btrfs snapshot to create the archive disk and then later, when the archive is next to be updated, just send the differences between the previous snapshot and a current snapshot. But without me (or the script) having to keep a reliable record or planning ahead (i.e. just from information in the archive snapshot itself).

I suppose the source disk would have to have kept a copy of the previous snapshot (or else it would end up having to work out the most recent parent that does still exist on both the archive disk and the source disk and use that!).

Is this even possible?