Probabilistic modeling of computer system availability

System availability is becoming an increasingly important factor in evaluating the behavior of commercial computer systems. This is due to the increased dependence of enterprises on continuously operating computer systems and to the emphasis on fault-tolerant designs. thus, The authors expect availability modeling to be of increasing interest to computer system analysts and for performance models and availability models to be used to evaluate combined performance/availability (performability) measures. Since commercial computer systems are repairable, availability measures are of greater interest than reliability measures. Reliability measures are typically used to evaluate nonrepairable systems such as occur in military and aerospace applications. The authors discuss system aspects which should be represented in an availability model; however, their main focus is a state of the art summary of analytical and numerical methods used to solve computer system availability models. They consider both transient and steady-state availability measures and for transient measures, both expected values and distributions. The authors are developing a program package for system availability modeling and intend to incorporate the best solution methods.