1. A computer-implemented method for testing a virtualised storage system, the method comprising: receiving, by a computer, data defining one or more configuration features of the virtualised storage system; receiving, a set of one or more predetermined rules defining interpretation of disk exercising commands, the interpretation being dependent on one or more of the configuration features, wherein at least one of the configuration features is a grain, and wherein a selected rule enables a selected number of grains that is fewer than all grains to be exercised; receiving, a first disk exercising command; interpreting the first disk exercising command in accordance with the selected rule that enables the selected number of grains that is fewer than all grains to be exercised to produce a second disk exercising command; and sending the second disk exercising command to the virtualised storage system, wherein the selected rule is a first rule, and wherein a second rule enables a randomly selected subset of all grains to be exercised.