This is perhaps a bit of an unrealistic idea, so feel free to close the
bug if it's unlikely to happen. Or possibly this should be a bigger
thing than just debmirror.

zsync is an implementation of the rsync algorithm, which works over
HTTP without requiring a zsync or rsync server on the remote end.
apt-sync is a similar tool for .debs specifically. Both require the
server to have some additional files for computing deltas between
the remote and local ends.

When updating a mirror with debmirror, you probably already have the
old version of a package. Having support for zsync and/or apt-sync
might reduce the amount of data that needs to be downloaded. This could
be a big benefit for those living in bandwidth-starved locations
(most of the world).

To implement this, the Debian mirrors would have to add the special
files required for zsync/apt-sync. In the simplistic implementation, this
would result in a huge number of new files on the mirrors, and that's
not acceptable, so something should be done about that.

Ubuntu has considered this a bit:

* https://wiki.ubuntu.com/AptSyncInKarmicSpec
* https://lists.ubuntu.com/archives/ubuntu-devel/2009-July/028529.html
* https://wiki.ubuntu.com/apt-sync

I don't know what the status of these things in Ubuntu are, nor what
the status of using, say, apt-sync in Debian is.

It might be useful to have support for both zsync and apt-sync:
zsync for .orig.tar.gz and apt-sync for .deb.