Modeling recycle: a case study in the industrial use of measurement and modeling

Large-scale data storage systems rely on magnetic tape cartridges to store millions of data objects. As these tapes age, the resident data objects become invalid; consequently, less and less of the tape potential capacity is effectively utilized. To address this problem, data storage systems have a facility, called 'recycle' in this paper, that transfers valid data objects from sparsely populated tapes onto new tapes, thus creating empty tapes for reuse. A high performance recycle process is needed to keep the number of tape cartridges to a minimum, and to maintain a continuous supply of empty tapes for storing newly created data objects. The performance of such processes is not easy to determine, and depends strongly on the data stored on the tapes, the speed and characteristics of the computer on which recycle is executed, and the nature of the algorithms themselves. This paper documents an extensive effort to evaluate a proposed recycle algorithm, using field workload data, laboratory measurements, and modeling. The results of the study were used to improve the recycle process, and were later verified through field trials. In addition yielding the results themselves, the effort illustrated that modeling and measurement, in an industrial setting, can indeed be used successfully in the design process.