The Echo system is no longer under development or in use, but we speak of it in the present tense throughout this paper to avoid awkwardness. Echo was in active use from November 1990 to November 1992, when the evolution of our laboratory s hardware and software base led us to retire the system. Thus, replication of persistent storage is beneath the EchoBox file system layer. The log is essential in keeping the replicas consistent. During crash recovery, the log reveals which data was in the middle of being updated, and thus may be different on the different replicas.
Recovery uses the log to bring the replicas back into agreement.
Granularity and semantic level of replication in the Echo distributed file system . IEEE Computer Society, November 1990. Our algorithm for bringing a replica back up to date while still providing service is described in a later section of this paper. Hisgen, A., et al. Granularity and Semantic Level of Replication in the Echo Distributed File System .