On Switching Policies for Modular Redundancy Fault-Tolerant Computing Systems

The objective of fault-tolerant computing systems is to provide an error-free operation in the presence of faults. The system has to recover from the effects of a fault by employing certain recovery procedures like program rollback, reload, and restart, etc. However, these recovery procedures, result in interruptions in the system´s operation, thus reducing the availability of the system for user applications. Fault-tolerant systems for critical applications include, therefore, standby spares that are ready to replace active modules which fail to recover from the effects of a fault. A standby spare may also be used to replace a module suffering from frequent fault occurrences resulting in too many repetitions of the recovery process, in order to increase the availability of the system for user applications. In this case a module switching policy is needed indicating upon a fault occurrence, whether to retry a failing module or switch it out and replace it by a spare, considering the remaining mission time and the probability of a system crash. A module switching policy for dynamic redundancy systems is presented in this paper and the improvement in application-oriented availability due to the use of this policy is illustrated.