Coda: A Highly Available File System for a Distributed Workstation Environment

Coda is a file system for a large-scale distributed computing environment composed of Unix workstations. It provides resiliency to server and network failures through the use of two distinct but complementary mechanisms. One mechanism, server replication, involves storing copies of a file at multiple servers. The other mechanism, disconnected operation, is a mode of execution in which a caching site temporarily assumes the role of a replication site. Disconnected operation is particularly useful for supporting portable workstations. The design of Coda optimizes for availability and performance, but provides the highest degree of consistency attainable in the light of those objectives. Measurements from a prototype show that the performance cost of providing high availability in Coda is reasonable.