Replication policies for layered clustering of NFS servers

Layered clustering offers cluster-like load balancing for unmodified NFS or CIFS servers. Read requests sent to a busy server can be offloaded to other servers holding replicas of the accessed files. This paper explores a key design question for this approach; which files should be replicated? We find that the popular policy of replicating read-only files offers little benefit. A policy that replicates read-only portions of read-mostly files, however, implicitly coordinates with client cache invalidations and thereby allows almost all read operations to be offloaded. In a read-heavy trace, 75% of all operations and 52% of all data transfers can be offloaded.