IGBDCSX1 gains control before DCS calculates the optimum block size or control interval size. You can use IGBDCSX1 to either bypass or limit the DCS-calculated optimum block or control interval size. IGBDCSX2 gains control after DCS calculates the statistics you requested. You can use it to override the DCS-calculated optimum block or control interval size with a value of your own.

DCS retrieves DASD data set information, performs calculations, and returns statistics to its caller. These statistics are primarily provided for display by ISMF (Interactive Storage Management Facility). The values returned are designated in kilobytes or bytes, rather than cylinders or tracks, to eliminate device dependency.

IGBDCSX1 and IGBDCSX2 allow you to exercise control over the values returned. However, because the access methods limit maximum block size to 32,760, if an exit routine returns an override or limit greater than this, DCS sets the block size to 32,760. DCS also verifies that exit-supplied control interval size override values do not violate VSAM restrictions.