A Quantitative Comparison of Reactive and Proactive Replicated Storage Systems

Replicated storage systems allow their stored data objects to outlive the life of the nodes storing them through replication. In this paper, we focus on durability, and more specifically on the concept of an object's lifetime, i.e., the duration of time between the creation of an object and when it is permanently irretrievable from the system. We analyze two main replication strategies: reactive, in which replication occurs in response to failures, and proactive, in which replication occurs in anticipation of failures. Our work presents a quantitative analysis that compares reactive and proactive through analytical models and simulations, considering exponentially distributed failures and reactive repairs, and periodic proactive replications. We also present a derivation of the analytical formula for the variance of the lifetime in the reactive model. Our results indicate that a proactive strategy leads to multiple times higher storage requirements than a reactive strategy. In addition, reactive systems are only moderately bursty in terms of bandwidth consumption, with rare peaks of at most five times the bandwidth consumption in proactive systems (given input parameter values that are compatible with real systems). Finally, for both strategies, the standard deviation is very close to the expected lifetime, and consequently, the lifetimes close to being exponentially distributed.