One of the most important differences is that Colossus and GFS are user-space filesystems for the client. You're just making RPCs to a service. There is no kernel filesystem. Upgrading the client and the server is just software upgrade, no kernel changes. And the operations were intentionally restricted- for example, instead of the debacle that is locking in NFS, locking is part of a different (and even more fundamental) tech at Google (chubby, which is NOT a filesystem, or so they say).