High availability clusters are becoming increasingly common in IT environments today, and the level of availability offered by such systems is a key factor used in their evaluation. However, no systematic and consistent methodology exists to perform such an assessment. Thus there is a critical need to establish cluster availability benchmarks. This paper proposes a cluster availability benchmark, SRB-X, that measures the automatic recovery time of a cluster framework in the event of a cluster node failure. SRB-X is a repeatable and portable benchmark, that measures a common outage mode for high availability clusters. After listing the main requirements of the underlying system recovery benchmark framework, this paper describes the various properties of SRB-X. The calculation of the SRB-X metric is illustrated through two examples.