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The use of server replication to increase the availability of remote servers in a system of workstations is discussed. Tradeoffs between consistency of replication and caching versus availability are considered for the Echo distributed file system which uses two different replication techniques, one at the upper levels of the hierarchical name space, the name service, and another at the lower levels of the name space, the file volume service. The two replication techniques provide different guarantees of consistency between their replicas and, therefore, different levels of availability. Echo also caches data from the name service and file volume service in client machines (e.g. workstations), with the cache for each service having its own cache consistency guarantee that mimics the guarantee on the consistency of the replicas for that service. The replication and caching consistency guarantees provided by each service are appropriate for its intended use. > |
