If random access and/or standard FS operations (i.e. ls) across the WAN are desired, GlusterFS is a good choice. It's what I used in a past project with success

If random access and/or standard FS operations (i.e. ls) across the WAN are desired, GlusterFS is a good choice. It's what I used in a past project with success and what Facebook uses internally. If you know exactly where your resource lives and don't need access a bucket as if it were a directory then S3 is awesome. For random access or FS like use, S3 (native or via FUSE) is pretty laggy.