Random I/O scheduling in online tertiary storage systems

Abstract

New database applications that require the storage and retrieval of many terabytes of data are reaching the limits for disk-based storage systems, in terms of both cost and scalability. These limits provide a strong incentive for the development of databases that augment disk storage with technologies better suited to large volumes of data. In particular, the seamless incorporation of tape storage into database systems would be of great value. Tape storage is two orders of magnitude more efficient than disk in terms of cost per terabyte and physical volume per terabyte; however, a key problem is that the random access latency of tape is three to four orders of magnitude slower than disk. Thus, to incorporate a tape bulk store in an online storage system, the problem of tape access latency must be solved. One approach to reducing the latency is careful I/O scheduling. The focus of this paper is on efficient random I/O scheduling for tape drives that use a serpentine track layout, such as the Quantum DLT and the IBM 3480 and 3590. For serpentine tape, I/O scheduling is problematic because of the complex relationships between logical block numbers, their physical positions on tape, and the time required for tape positioning between these physical positions. The results in this paper show that our scheduling schemes provide a significant improvement in the latency of random access to serpentine tape.