Optimizing Control Interval Size

You can let VSAM select the size of a control interval for a data set, you can request a particular control interval size in the DEFINE command, or you can specify data class in DEFINE and use the CISIZE attribute assigned by your storage administrator. You can improve VSAM's performance by specifying a control interval size in the DEFINE command, depending on the particular storage and access requirements for your data set. See Control Intervals for information about the structure and contents of control intervals.

Control interval size affects record processing speed and storage requirements in the following ways:

Buffer space. Data sets with large control interval sizes require more buffer space in virtual storage. For information about how much buffer space is required, see Determining I/O Buffer Space for Nonshared Resource.
I/O operations. Data sets with large control interval sizes require fewer I/O operations to bring a given number of records into virtual storage; fewer index records must be read. It is best to use large control interval sizes for sequential and skip-sequential access. Large control intervals are not beneficial for keyed direct processing of a key-sequenced data set or variable-length RRDS.
Free space. Free space is used more efficiently (fewer control interval splits and less wasted space) as control interval size increases relative to data record size. For more information about efficient use of free space, see Optimizing Free Space Distribution.