A storage controller includes a cache destage controller that determines write cache destage rates for each of a plurality of storage arrays as a function of available system resources. When the available system resources are sufficient to satisfy the optimal destage rates for all arrays, the destage rate for each array is set to its optimal rate. When the available system resources are insufficient to satisfy the array rates of all of the arrays, each array is allocated destage rate that is a percentage of the array rates based on the available resources. When the available system resources are insufficient to satisfy the optimal destage rates for all arrays, but sufficient to satisfy the array rates of some of the arrays, a destage rate is determined for each array based on mathematical relationships between sums of the array rates, global rates, optimal rates, and sums of these across arrays.