The big difference is that xdelta (much like any diff program) needs both the old and the new versions to create a patch. With zsync, the server only needs to h

The big difference is that xdelta (much like any diff program) needs both the old and the new versions to create a patch. With zsync, the server only needs to have the new version (which is the only one of interest), and the clients then gets only the parts they want, because they only have the old version. zsync also does all the fetching stuff directly.