Does the segment fit on KSDS?

You want most database records to fit on the primary data set, because additional read and seek operations are required to access those parts of a database record on the overflow data set. Additional reads and seeks degrade performance. If, however, most of the processing you do against a database record occurs on segments in the primary data set (in other words, your high-use segments fit on the primary data set), these considerations might not be as important.