The SURE Approach to Reliability Analysis

Conclusions The SURE computer program is a reliability-analysis tool for ultrareliable computer-system architectures. SURE is based on computational methods developed at the NASA Langley Research Center. These methods provide an e%cient means for computing reasonably accurate upper and lower bounds for the death state probabilities of a large class of semiMarkov models. Once a semi-Markov model is described using a simple input language, SURE automatically computes the upper and lower bounds on the probability of system failure. A parameter of the model can be specified as a variable over a range of values, thus directing SURE to perform a sensitivity analysis automatically. This feature, along with the speed of the program, makes it an especially useful design tool. SURE is a flexible, user-friendly reliability-analysis tool. The program provides a rapid computational capability for semiMarkov models useful for describing the fault-handling behavior of fault-tolerant computer systems. The only modeling restriction imposed by the program is that the non-exponential recovery transitions must be fast in comparison to the mission time a desirable attribute of all fault-tolerant systems. The SURE reliability-analysis method uses a fast bounding theorem based on means and variances; the method yields upper and lower bounds on the probability of system failure. The upper and lower bounds are typieally within 5 percent of each other. Techniques have been developed to enable SURE to solve models with loops and calculate the operational-state probabilities. The computation is extremely fast, and large statespaces can be directly solved; a pruning technique enables SURE to process extremely large models.