A new algorithm combines a coverage model with a combinatorial model to compute system unreliability. Its advantage is that for a class of computer systems, it is simpler than current algorithms. The method applies to combinatorial models which can generate cutsets for the system. This set of cutsets is augmented with cutsets representing the uncovered failures of the system. The resulting set is manipulated by combining standard multi-state and sum-of-disjoint products solution techniques. It is possible to compute the exact unreliability of the system using this algorithm. If the size of the system and the time required for the analysis become prohibitive, however, the solution can be truncated and bounds on the system unreliability computed. The authors' algorithm is important because it adapts standard combinatorial solution techniques to a problem that was previously thought to require a Markov solution. The ability to model a fault-tolerant computer system completely within a combinatorial model allows results to be calculated more quickly and accurately, and thus to impact system design. This new technology is easily integrated into existing design/analysis methodologies. Coverage provides a more accurate picture of system behavior, and gives more faith in reliability estimates.
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