I guess they probably use a rolling hash, that would be the smart thing to do. I think I first heard of them with casync. The gist of it is that you hash file

I guess they probably use a rolling hash, that would be the smart thing to do. I think I first heard of them with casync. The gist of it is that you hash file contents in a sliding window, and use a condition on the resulting hash to chunck your file. That way, blocks are determined by the file content, not an arbitrary offset, and this allows to propagate deltas even with insertions or removals.