A Simulation Framework and Evaluation for Optimistic Replicated Filing Environments

Optimistic replication of data is becoming increasingly popular in distributed systems. However, the performance characteristics of such systems are not well understood. We have developed a simulation to evaluate optimistically replicated file systems in terms of their operating costs (e.g., computing resources) and service quality (e.g., consistency of data perceived by users). The simulation built-in language and library components ease the specification of various system settings. The traffic generator captures the inverse relationship between sharing and write accesses, and the traffic is based on a three-month long trace collected at Locus Computing Corporation. Our result shows that the widely used conflict rate metric is misleading: Both extremes of data propagation frequencies result in near zero conflict rates. Also, conflict resolutions can generate many intermediate versions of data that are in conflicts, or meta-conflicts. We have proposed alternative service quality metrics and ways to reduce meta-conflicts. Our study further demonstrates that the effectiveness of a data propagation topology depends on the propagation distance between replicas as well as the number of propagation processes that can occur in parallel. Optimistic replicated filing scales well; further exploiting user sharing patterns can achieve further economies. Per user adjustment of data propagation frequencies based on the work cycle hurts the service quality of overall system. Finally, we have observed the tradeoffs between data propagation rate and service quality, and traffic loads on shared replicated data.