Increasing file system availability through second-class replication

High availability in the Coda file system is achieved through two types of data replication: first-class replication, in which entire servers are replicated, and second-class replication, otherwise known as client caching. The two are complementary: server replication increases the availability of the entire file store, whereas caching permits operation in the event of total disconnection. A particularly valuable use of disconnected operation is the graceful integration of portable computers into the system. The use of second-class replication and its contribution to the system availability and disconnected operation are discussed. Transparent disconnected operation dictates a much-expanded role for client cache managers, effectively turning them into pseudoservers. Three phases of pseudoserver operation-hoarding, server emulation and reintegration-are described