Communications of the ACM
Communications of the ACM
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When a file is loaded into a direct access storage device using key-to-address transformations, the number and size of storage blocks can be selected. In this study, a selection that minimizes the combined cost of storage space and accesses to the storage device is determined for the case where no record additions or deletions occur after loading. The analysis is based on the assumption that for a given set of keys, a transformation exists that gives a uniform probability distribution over the available addresses. Under this assumption, formulas are derived for the average number of overflow records and for the average number of accesses required to retrieve a record. Given these formulas, the costs are expressed as a function of storage used, number of accesses, cost per unit of storage, and cost per access. Minima are computed foar range of block sizes and operational conditions. The results seem to indicate that current file design practices are abundant with storage space. Finally, the results are condensed in an easy to use approximate formula.