When a key-sequenced data set is defined, unused space can be scattered throughout the data set to permit records to be inserted or lengthened. The unused space is called free space. When a new record is added to a control interval (CI) or an existing record is lengthened, subsequent records are moved into the following free space to make room for the new or lengthened record. Conversely, when a record is deleted or shortened, the space given up is reclaimed as free space for later use. When you define your data set, use the FREESPACE parameter to specify what percentage of each CI is to be set aside as free space when the data set is initially loaded.

Within each CA, reserve free space by using free CIs. If you have free space in your CA, it is easier to avoid splitting your control area when you want to insert additional records or lengthen existing records. When you define your data set, specify what percentage of the control area is to be set aside as free space, using the FREESPACE parameter.

For information about specifying the optimal amount of CI and CA free space, see Optimizing Free Space Distribution.