Google “Build in the cloud” – http://google-engtools.blogspot.com/2011/08/build-in-cloud-how-build-system-works.html
Google has a FUSE filesystem that keeps track of digests of files as an attribute. This is something that needs to be default in all filesystems. http://google-engtools.blogspot.com/2011/06/build-in-cloud-accessing-source-code.html. Also, they build everything from head.
http://www.cs.virginia.edu/~dww4s/articles/build_systems.html