A storage control apparatus in cooperation with a plurality of controlled storage apparatus arranged in a cascade of point-in-time copy relationship. The apparatus includes: a rate control component operable to set a desired rate for cleaning a first target one of the plurality of controlled storage apparatus; a comparator component in communication with the rate control component to identify discrepant bits associated with first and second targets; and a copy initiator, responsive to the comparator component to initiate a copy operation responsive to the identified discrepant bits.