Impact of Media Exchanges in Robotic Libraries

Abstract

The large storage requirements of many commerical and scientific 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 efforts have focussed on minimizing media exchanges under the assumption that it is always beneficial 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 influence the effect of an exchange, making it difficult to establish simple heuristics for scheduling media exchanges. Simulation is used to validate the theoretical results.