A Simulation Study of Replication Control Protocols Using Volatile Witnesses

Voting protocols guarantee the consistency of replicated data objects by disallowing all access requests that cannot gather a sufficient quorum of replicas. The performance of voting protocols can be greatly enhanced by adding to the replicas small independent entities that hold no data but can attest to the state of the replicated data object. It has been recently proposed to store these witnesses in volatile storage. Volatile witnesses repond faster to write requests than those stored in stable storage. They can also be more easily regenerated as many local area net-works contain a majority of diskless sites. We present a simulation study of the availability afforded by two voting protocols using volatile witnesses and investigate the impact of access rates, network topol-ogy and witness placement on the availability of the repli-cated data