Low-cost access time model for serpentine tape drives

When a serpentine tape drive is used as a slow random access device, the I/O performance can be substantially improved by clever re-ordering of the I/O requests. This kind of re-ordering relies on a scheduling algorithm and a model of the access time. In this paper, we propose a low-cost access time model for serpentine tape drives, which is not a trivial task due to the complex data layout of serpentine tape. This model provides a way to estimate the physical positions on the tape for any logical data block, provides cost functions to estimate the seek time between two physical tape positions, and computes the transfer time of a data request. Our experiments show that the mapping from logical address to physical position has to be instrumented once for each tape cartridge. Algorithms are given to do this at a low cost. The accuracy of the model is assessed by measurements on rape drives and by use in scheduling of I/O requests. Experiments show that the model estimates are good enough to facilitate efficient scheduling of I/O requests.