Abstract
New applications require online access to many terabytes of data, but a magnetic disk storage system this large requires thousands of drives. Magnetic tape is be a good alternative, except that the application demand for transparent data retrieval is not met by current tape systems because of their high access latency. This latency can be significantly improved by good retrieval scheduling. A fundamental prerequisite to efficient scheduling is the ability to estimate the amount of time required for tape positioning operations (the locate time ). For serpentine tape, which is the most common mass storage tape technology, this estimation is subtle and complex. The main contribution of this paper is a locate-time model for a DLT4000 tape drive. The accuracy of the model is evaluated by measurements, and the utility of the model is demonstrated through a model-driven simulation of retrieval scheduling, validated by measurements and sensitivity testing. In brief, the locate-time model is accurate to within a few percent, which enables the production of efficient schedules.
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