A Petri Net model for Service Availability in Redundant Computing Systems

In this paper we present and analyse a coloured stochastic Petri net model of a redundant fault-tolerant system. As our measure of interest we are interested in a dependability metric, i.e., service availability. Service availability is defined as the number of successfully completed jobs relative to the total number of arrived jobs. This paper is the first step towards a comprehensive comparison of redundancy and rejuvenation, i.e., the preventive restart of servers when studying service availability. The question we strive to answer in this paper is whether and to what degree additional redundant servers can increase service availability in all load scenarios. We find that the first redundant server improves service availability by almost 90 % in a highly loaded system, while adding a second and third redundant server yields further but much lower improvement. Under low load the benefit of additional servers is not as pronounced.