Many of the system's services use DASD calculation services (DCS) to determine optimum block sizes for non-VSAM data sets and optimum control interval sizes for VSAM data sets. You can use the DASD calculation services installation exits to influence the size chosen. The precalculation exit routine, IGBDCSX1, indicates to DCS which value to use when calculating the optimum block or control interval size. The postcalculation exit routine, IGBDCSX2, lets you override the DCS-calculated optimum block or control interval size with your own value.