Disconnected operation in the Coda File System

Reviewer: Andrew Robert Huber

The simple but elegant idea behind disconnected operation in Coda is that caching of files can be used to enhance availability as well as performance. Disconnected operation implemented this way provides the benefits of shared files and allows continued operation during a server or other network failure or when using a portable computer. This paper shows that disconnected operation is feasible and can be made efficient and practical. The authors' experience to date in an academic and research environment shows that users can work for as long as one or two days in disconnected mode with only about 50 megabytes of local storage, yet propagating the changes that were made when the user reconnects takes only about one minute. The key to the design is the cache manager running on each client. Two implementation decisions are crucial: caching entire files, and optimistic copy control. Caching entire files means that a cache miss can only occur upon opening a file, when it can conveniently be handled. Optimistic copy control allows reads and writes to the cached copies during disconnected operation. One important contribution of the work is the reported data showing how little conflicting usage of files occurs in practice. The weaknesses in the approach are also due to the basic mechanisms that make it work. First, whole file caching means users must plan their work to take full advantage of disconnected operation (although this is less of a problem in reality than it seems). Second, caching entire files and relying on having few write conflicts limits the applicability of the solution, since it rules out both applications with a large database or several large files (tens or hundreds of megabytes) and online transaction processing applications where many writes are made to files. It remains an open question how well this approach would work in many commercial applications. The paper is well written and well organized. It was a pleasure to read, as the results are interesting, important, and thought-provoking. It is mandatory reading for researchers in distributed file systems and high availability, and highly recommended for everyone else.