Analysis of Typical Fault-Tolerant Architectures using HARP

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Abstract
HARP (the Hybrid Automated Reliability Predictor) is a software package that implements advanced reliability modeling techniques. We present an overview of some of the problems that arise in modeling highly reliable fault-tolerant systems; the overview is loosely divided into model construction and model solution problems. We then describe the HARP approach to these difficulties, which is facilitated by a technique called behavioral decomposition. The bulk of this paper presents examples of the dependability evaluation of some typical fault-tolerant systems, including a local-area network, two well-known fault-tolerant computer systems (C.mmp and SIFT), and an example of a flight control system. HARP has been used to solve very large models. A system consisting of 20 components distributed among 7 stages produced a Markov chain with 24 533 states and over 335 000 transitions (without coverage). Depending on the system used to run this example, the run time took anywhere from 4 to 8 hours. HARP is undergoing beta testing at approximately 20 sites. It is written in standard FORTRAN 77, consists of nearly 30000 lines of code and comments, and has been tested under several operating systems. The graphics interface (written in C) runs on an IBM PC AT, and produces text files that can be used to solve the system on the PC (for very small systems), or can be uploaded to a larger machine. HARP is accompanied by an Introduction and Guide for Users. For information on obtaining a copy of HARP, contact one of the authors.

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