Large Granularity Cache Coherence for Intermittent Connectivity

To function in mobile computing environments, distributed file systems must cope with networks that are slow, intermittent, or both. Intermittence vitiates the effectiveness of callback-based cache coherence schemes in reducing client-server communication, because clients must validate files when connections are reestablished. In this paper we show how maintaining cache coherence at a large granularity alleviates this problem. We report on the implementation and performance of large granularity cache coherence for the Coda File System. Our measurements confirm the value of this technique. At 9.6 Kbps, this technique takes only 4 – 20% of the time required by two other strategies to validate the cache for a sample of Coda users. Even at this speed, the network is effectively eliminated as the bottleneck for cache validation.