The UltraSAN modeling environment

Model-based evaluation of computer systems and networks is an increasingly important activity. For modeling to be used effectively, software environments are needed that ease model specification, construction, and solution. Easy to use, graphical methods for model specification that support solution of families of models with differing parameter values are also needed. Since no model solution technique is ideal for all situations, multiple analysis and simulation-based solution techniques should be supported. This paper describes UltraSAN, one such software environment. The design of UltraSAN reflects its two main purposes: to facilitate the evaluation of realistic computer systems and networks, and to provide a test-bed for investigating new modeling techniques. In UltraSAN, models are specified using stochastic activity networks, a stochastic variant of Petri nets, using a graphical X-Window based interface that supports large-scale model specification, construction, and solution. Models may be parameterized to reduce the effort required to solve families of models, and a variety of analysis and simulation-based solution techniques are supported. The package has a modular organization that makes it easy to add new construction and solution techniques as they become available. In addition to describing the features, capabilities, and organization of UltraSAN, the paper illustrates the use of the package in the solution for the unreliability of a fault-tolerant multiprocessor using two solution techniques.