The Reliability of Regeneration-Based Replica Control Protocols
Proceedings of the International Conference on Distributed Computing Systems
Abstract
The accessibility of vital information can be enhanced by replicating the data on several sites and employing a consistency control protocol to manage the copies. For many applications, the reliability of a system is a more important measure of its performance than its availability. These applications are characterized by the property that interruptions of service are intolerable and often involve interaction with real-time processes. The reliability of a replicated data object depends on maintaining a viable set of current replicas. When storage is limited, it may not be feasible to simply replicate a data object at enough sites to achieve the desired level of reliability. If new replicas of a data object can be created faster than a system failure can be repaired, better reliability can be achieved by creating new replicas on additional sites in response to changes in the system configuration. Several strategies for replica maintenance are considered, and the benefits of each are analyzed. Formulas describing the reliability of the replicated data object are presented, and closed-form solutions are given for the tractible cases. Numerical solutions, validated by simulation results, are used to analyze the trade-offs between reliability and storage cost. With estimates of the mean times to site failure and repair in a given system, the numerical techniques presented here can be applied to predict the fewest number of replicas required to provide the desired level of reliability.