Log files: an extended file service exploiting write-once storage

A log service provides efficient storage and retrieval of data that is written sequentially (append-only) and not subsequently modified. Application programs and subsystems use log services for recovery, to record security audit trails, and for performance monitoring. Ideally, a log service should accommodate very large, long-lived logs, and provide efficient retrieval and low space overhead. In this paper, we describe the design and implementation of the Clio log service. Clio provides the abstraction of log files : readable, append-only files that are accessed in the same way as conventional files. The underlying storage medium is required only to be append-only; more general types of write access are not necessary. We show how log files can be implemented efficiently and robustly on top of such storage media—in particular, write-once optical disk. In addition, we describe a general application software storage architecture that makes use of log files. This work was supported in part by the Defense Advanced Research Projects Agency under contracts N00039-84-C-0211 and N00039-86-K-0431, by National Science Foundation grant DCR-83-52048, and by Digital Equipment Corporation, Bell-Northern Research and AT&T Information Systems.