Backup

Backup a set of files, putting in a placeholder for large files "easily re-downloadable from the network". The latter is a matter of user-configuration, though I'm imagining something like "server:optional-authentication:hash". (Do NOT use a broken hash like SHA-1 or MD5!) One use case is upgrading a Linux distribution. All the old OS files should be on the internet somewhere, and may be safely replaced with the hash. One can also think of this as an extreme form of file compression, where the compression program is very large, containing an internet-full of dictionaries of files (or substrings) it knows about.

Next, make it filesystem-transparent; that is, the filesystem automatically goes and fetches the file if it needed at sometime in the future.