Efficient Data Distribution in a

A novel locking protocol maintains data consistency in distributed and clustered file systems that are used as scalable infrastructure for Web server farms. High-performance Web sites relyon Web server “farms”—hundredsof computers serving the same content—for scalability, reliability, and low-latency access to Internet content. Deploying these scalable farms typically requires the power of distributed or clus-tered file systems. Building Web server farms on file sys-tems complements hierarchical proxy caching.1 Proxy caching replicates Web content throughout the Internet, thereby reducing latency from network delays and off-loading traffic from the primary servers. Web server farms scale resources at a single site, reducing latency from queuing delays. Both technologies are essential when building a high-perfor-mance infrastructure for content delivery. In this article, we present a cache con-sistency model and locking protocol cus-tomized for file systems that are used as scalable infrastructure for Web server farms. The protocol takes advantage of the Web’s relaxed consistency semantics to reduce latencies and network overhead. Our hybrid approach preserves strong consistency for concurrent write sharing with time-based consistency and push caching for readers (Web servers). Using simulation, we compare our approach to the Andrew file system and the sequen-tial consistency file system protocols we propose to replace. Data Consistency File system design carries assumptions about workloads and application consis-tency needs that, when applied to Web serving, lead to inferior performance. File system designers assume that data are shared infrequently and that such data require strong (sequential) consistency.2 Consistency describes how processors in a parallel or distributed system view shared data; the sidebar on related work in data consistency presents examples. For Web serving, data are widely shared among many servers, as Figure 1 shows, and strong consistency guarantee