Fast Reconciliations in Fluid Replication

• Venue: ICDCS '01 Proceedings of the The 21st International Conference on Distributed Computing Systems
• Year: 2001

Abstract: Mobile users can increasingly depend on high speed connectivity. Despite this, using distributed file services across the wide area is painful. Past approaches sacrifice one or more of safety, visibility, and consistency in the name of performance. Instead, we propose Fluid Replication, the ability to create replicas where and when needed. These replicas, called WayStations, maintain consistency with home servers through periodic reconciliations. Two techniques make reconciliation fast; this is crucial to the success of Fluid Replication. First, we defer propagation of updates, and only invalidate files during a reconciliation. Second, rather than depend on operation logs, we provide the subtrees in which all updates have occurred. These subtrees, named by their least common ancestors, or LCAs, can be constructed incrementally, and reduce the burden of checking serializability during a reconciliation. While these techniques provide better performance, they are not without risk. Bulk invalidation can lead to false sharing, optimistic updates are subject to conflict, and deferred updates may cause performance problems if they are needed elsewhere. To address these concerns, we performed a trace-based evaluation of our algorithms. Because reconciliations can be frequent, no update conflicts were produced. Less than 0.2% of all file operations required a spurious revalidation, and less than 4% of all updates were needed at remote sites, resulting in substantial performance benefits.