1. A computer-readable, non-transitory medium storing a program that causes a computer to execute a procedure, the procedure comprising: acquiring manufacture date information set as numeric values corresponding to dates of manufacture of a plurality of storage apparatus from the plurality of storage apparatus; calculating a difference between numeric values indicative of the manufacture date information for each combination of two storage apparatus on the basis of the manufacture date information acquired; calculating a simultaneous failure occurrence degree which becomes smaller with an increase in the difference between the numeric values indicative of the manufacture date information, and storing in a memory the simultaneous failure occurrence degree in association with each combination of two storage apparatus; combining two of the plurality of storage apparatus, thereby generating a plurality of combination patterns; acquiring, from the memory, the simultaneous failure occurrence degrees corresponding to combinations of storage apparatus for each of the plurality of combination patterns generated, and calculating a total of the simultaneous failure occurrence degrees acquired; determining that a combination pattern for which the total of the simultaneous failure occurrence degrees is the smallest is a duplication combination pattern; and sending a mirroring device that performs mirroring by the use of the plurality of storage apparatus instructions to perform mirroring by the use of combinations of two storage apparatus indicated in the duplication combination pattern determined.