Mailing the story:

This tutorial shows how to combine four single storage servers (running Ubuntu 12.04) to a distributed replicated storage with GlusterFS. Nodes 1 and 2 (replication1) as well as 3 and 4 (replication2) will mirror each other, and replication1 and replication2 will be combined to one larger storage server (distribution). Basically, this is RAID10 over network. If you lose one server from replication1 and one from replication2, the distributed volume continues to work. The client system (Ubuntu 12.04 as well) will be able to access the storage as if it was a local filesystem.