| It’d be useful for more than just the Packages file – you could use any older .debs you have cached as a source for matching data in newer versions of the same package. The assumption is that between close versions of a package, there are often large binary similarities. (This of course, is preconditioned on the packaged being built with the ‘rsyncable-gzip’).
It’s been suggested several times before, and has so far been shot down every time. See the debian-devel list archives. The main reasons seem to be:
1) the patch that gives gzip an ‘rsyncable’ option isn’t standard – and there doesn’t appear to be a current maintainer of gzip to accept it. (Personally I think this isn’t a problem, because it would just have to be patched into the debian packaged gzip)
2) The current rsync algorithm puts most of the CPU load onto the server side. This obviously isn’t popular with server administrators.
3) There are rumoured to be patent issues with the rsync algorithm. (I’ve never seen any evidence of this – just rumour, and I’m inclined to discount it until I see something to convince me otherwise).
I’ve had some ideas in the direction of number 2 – I think it’s possible to move the CPU load onto the client – and in fact, not even require a new daemon on the server at all. I perceive a lot of built-up hostility to the idea of an rsync-like algorithm in apt-get, so I’m reluctant to argue the point until I at least have some proof-of-concept code to back myself up.