Updating games via rsync (or reverse rsync)?

[Not Mr Flıbble]

Could be useful, particularly for multi-gigabyte updates (such as today's Shadow of Mordor update) over slower 'net connections…

[aiusepsi]

Steam already does do delta updates, based on similar principles to rsync, although they don't use exactly the same algorithm as rsync; Steam's algorithm is tailored for things that are specific to Steam's use-cases, like it needs to work at scale, and to that end only requires dumb HTTP file servers. Steam's algorithm is also designed to cache really well; big LAN events typically run a proxy which means any given update only has to be downloaded once from the Internet, and thereafter is distributed from the onsite cache.

Rsync is a very chatty protocol, which is the sort of thing you'd want to avoid at scales like Steam's.

[Not Mr Flıbble]

Problem is that it appears that modified files are fetched in full. This doesn't scale well for individual users with slow connections and large files to be downloaded, as in the aforementioned update – there's 3.3GB of it (no idea of the size of the delta, though). Downloading that will take several hours.

[aiusepsi]

Steam's download system doesn't fetch files, it fetches chunks (about 1 MB) and assembles them into files. It reuses chunks where possible from your existing install, which is how the delta updating works, it only downloads the chunks it needs.

If the developer has packaged their files in a pathological way (e.g. modifications are interspersed at a low density but uniformly in a file, that can cause an overly-large number of chunks to get invalidated and need downloading. You can see here: http://the-witness.net/news/2012/08/fun-with-package-sorting/ for more of a detailed discussion about this from a game dev.