Howtoforge posted a tutorial about combine four single storage servers to one large storage server
This tutorial shows how to combine four single storage servers (running Ubuntu 11.10) to one large storage server (distributed storage) with GlusterFS. The client system (Ubuntu 11.10 as well) will be able to access the storage as if it was a local filesystem. GlusterFS is a clustered file-system capable of scaling to several peta-bytes. It aggregates various storage bricks over Infiniband RDMA or TCP/IP interconnect into one large parallel network file system. Storage bricks can be made of any commodity hardware such as x86_64 servers with SATA-II RAID and Infiniband HBA.Distributed Storage Across Four Storage Nodes With GlusterFS 3.2.x On Ubuntu 11.10