Eecient Buuering for Concurrent Disk and Tape I/o

Tertiary storage is becoming increasingly important for many organizations involved in large-scale data analysis and data mining activities. Yet database management systems (DBMS) and other data-intensive systems do not incorporate tertiary storage as a rst-class citizen in the storage hierarchy. For instance, the typical solution for bringing tertiary-resident data under the control of a DBMS is to use operating system facilities to copy the data to secondary storage, and then to perform query optimization and execution as if the data had been in secondary storage all along. This approach fails to recognize the opportunities for saving execution time and storage space if the data were accessed on tertiary devices directly and in parallel with other I/Os. In this paper we examine issues in accessing secondary and tertiary storage in parallel and suggest buuering mechanisms for increasing the through-put of applications with concurrent, intensive I/O requirements. We rst identify several factors that determine the parallel I/O performance of secondary and tertiary storage devices. We discuss the performance characteristics of magnetic disks and magnetic tapes when used alone and when used concurrently, sharing the same I/O bus. We then describe alternative buuering schemes for parallel I/O and analyze their eeciency via an experimental implementation.