Empirical estimates of software availability of deployed systems

We consider empirical evaluation of the availability of the deployed software. Evaluation of real systems is more realistic, more accurate, and provides higher level of confidence than simulations, testing, or models. We process and model information gathered from a variety of operational and service support systems to obtain estimates of software reliability and availability. The three principal quantities are the total runtime, the number of outages, and the duration of outages. We consider methods to assess the quality of information in customer support systems, discuss advantages and disadvantages of various sources, consider methods to deal with missing data, and ways to construct bounds on measures that are not directly available. We propose a method to assess empirically software availability and reliability based on information from operational customer support and inventory systems and use a case study of a large communications system to investigate factors affecting software reliability. We find large variations among platforms and releases and find the failure rate to vary over time.