Impact of Correlated Failures on Dependability in a VAXcluster System

Abstract

This paper addresses the issue of correlated failures and their impact on system dependability. Measurements are made on a VAXcluster system and validated analytical models are proposed to calculate availability and reliability for simple systems with correlated failures. A correlation analysis of the VAXcluster data shows that errors are highly correlated across machines (average correlation coefficient ρ = 0.62) due to sharing of resources. The measured failure correlation coefficient, however, is not high (0.15). Based on the VAXcluster data, it is shown that models that ignore correlated failures can underestimate unavailability by orders of magnitude. Even a small correlation significantly affects system unavailability. A validated analytical model, to calculate unavailability of 1-out-of-2 systems with correlated failures, is derived. Similarly, reliability is also significantly influenced by correlated failures. The joint failure rate of the two components, λ _f , is found to be the key parameter for estimating reliability of 1-out-of-2 systems with correlated failures. A validated relationship between ρ and λ _f , is also derived.