An Exact Algorithm for the Linear Tape Scheduling Problem
Valentin Honoré, Bertrand Simon and Frédéric Suter
Abstract: Magnetic tapes are often considered as an outdated storage
technology, yet they are still used to store huge amounts of
data. Their main interests are a large capacity and a low price
per gigabyte, which come at the cost of a much larger file access time than on disks. With tapes, finding the right ordering
of multiple file accesses is thus key to performance. Moving
the reading head back and forth along a kilometer long tape
has a non-negligible cost and unnecessary movements thus
have to be avoided. However, the optimization of tape request
ordering has rarely been studied in the scheduling literature,
much less than I/O scheduling on disks. For instance, minimizing
the average service time for several read requests on a
linear tape remains an open question.
Therefore, in this paper, we aim at improving the quality of
service experienced by users of tape storage systems, and not
only the peak performance of such systems. To this end, we
propose a reasonable polynomial-time exact algorithm while
this problem and simpler variants have been conjectured NP-hard. We also refine the proposed model by considering U-turn penalty costs accounting for inherent mechanical accelerations. Then, we propose a low-cost variant of our optimal
algorithm by restricting the solution space, yet still yielding
an accurate suboptimal solution. Finally, we compare our algorithms to existing solutions from the literature on logs of
the mass storage management system of a major datacenter.
This allows us to assess the quality of previous solutions and
the improvement achieved by our low-cost algorithm. Aiming
for reproducibility, we make available the complete implementation of the algorithms used in our evaluation, alongside
the dataset of tape requests that is, to the best of our knowledge, the first of its kind to be publicly released.
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