Impact of Media Exchanges in Robotic StorageLibraries

The large storage requirements of many commerical and scientiic applications cannot be met by magnetic disks due to their high cost and low storage density. Consequently, cheaper and more dense tertiary storage systems are being intergrated into the storage hierarchies of these applications. Although tertiary storage can accomodate large amounts of data, the access latency is very high due to the need to load and unload media from the read/write drives. These media exchanges are very slow with typical times in the order of tens of seconds. In order to reduce this high latency research eeorts have focussed on minimizing media exchanges under the assumption that it is always beneecial to eliminate exchanges. We analyze the validity of this assumption. It is shown that there are instances when the assumption does not hold. It is further shown that various factors innuence the eeect of an exchange, making it diicult to establish simple heuristics for scheduling media exchanges. Simulation is used to validate the theoretical results.