How VSAM Adjusts Control Interval Size

The control interval sizes you specify when the data set is defined are not necessarily the ones that appear in the catalog. VSAM makes adjustments, if possible, so that control interval size conforms to proper size limits, minimum buffer space, adequate index-to-data size, and record size. VSAM makes the following adjustments when your data set is defined.

1. Specifies data and index control interval size. After VSAM determines the number of control intervals in a control area, it estimates whether one index record is large enough to handle all control intervals in the control area. If not, the size of the index control interval is increased, if possible. If the size cannot be increased, VSAM decreases the number of control intervals in the control area.
2. Specifies maximum record size as 2560 and data control interval size as 2560, and have no spanned records. VSAM adjusts the data control interval size to 3072 to permit space for control information in the data control interval.
3. Specifies buffer space as 4K, index control interval size as 512, and data control interval size as 2K. VSAM decreases the data control interval to 1536. Buffer space must include space for two data control intervals and one index control interval at DEFINE time. For more information about buffer space requirements see Determining I/O Buffer Space for Nonshared Resource.