Report ID
1997-07
Report Authors
Sunil Prabhakar, Divyakant Agrawal, and Amr El Abbadi
Report Date
Abstract
The large storage requirements of many commerical and scientific applicationscannot be met by magnetic disks due to their high cost and low storagedensity. Consequently, cheaper and more dense tertiary storage systems arebeing intergrated into the storage hierarchies of these applications. Althoughtertiary storage can accomodate large amounts of data, the access latency isvery 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 ofseconds. In order to reduce this high latency research efforts have focussedon minimizing media exchanges under the assumption that it is always beneficialto eliminate exchanges. We analyze the validity of this assumption. It isshown that there are instances when the assumption does not hold. It isfurther shown that various factors influence the effect of an exchange, makingit difficult to establish simple heuristics for scheduling media exchanges.Simulation is used to validate the theoretical results.
Document
1997-07.ps487.67 KB