Access Time Modeling of a MLR1 Tape Drive

Traditionally, digital tape has been used by applications accessing the tape mostly sequentially. Applications having a random access pattern to the data are much better off storing the data on magnetic disks. Today we see new application areas, where the need for huge amounts of digital storage does not make it cost-effective to store all the data on magnetic disk. One example of such an application is digital video archives. In this paper, we present an access model for a Tandberg MLR1 serpentine tape drive. For such a drive, there is no direct relation between the logical block address and the corresponding physical position on the tape. This makes it difficult to optimize the usage of the tape drive when we have concurrent accesses to the tape. The paper presents a generic model of a serpentine tape drive together with several algorithms for characterizing individual tapes to improve the accuracy of the model. Based on measurements using the MLR1 tape drive the generic model is improved. This extended model makes it possible to estimate seek times with an average error of about two seconds.