Data control interval size

You can either specify a data control interval size or default to a system-calculated control-interval size. If you do not specify a size, the system calculates a default value that best uses the space on the track for the average record size of spanned records or the maximum record size of nonspanned records.

If a CONTROLINTERVALSIZE value is specified on the cluster level, this value propagates to the component level at which no CONTROLINTERVALSIZE value has been specified.

Normally, a 4096-byte data control interval is reasonably good regardless of the DASD device used, processing patterns, or the processor. A linear data set requires a control interval size of 4096 to 32768 bytes in increments of 4096 bytes. However, there are some special considerations that might affect this choice:
  • If you have very large control intervals, more pages are required to be fixed during I/O operations. This could adversely affect the operation of the system.
  • Small records in a data control interval can result in a large amount of control information. Often free space cannot be used.
  • The type of processing you use can also affect your choice of control interval size:
    • When direct processing is predominant, a small control interval is preferable, because you are only retrieving one record at a time. Select the smallest data control interval that uses a reasonable amount of space.
    • When sequential processing is predominant, larger data control intervals can be good choices. For example, given a 16 KB data buffer space, it is better to read two 8 KB control intervals with one I/O operation than four 4 KB control intervals with two I/O operations.
    • If the processing is a mixture of direct and sequential (that is, mixed processing), a small data control interval with multiple buffers for sequential processing can be a good choice.

If you specify free space for a key-sequenced data set or variable-length RRDS, the system determines the number of bytes to be reserved for free space. For example, if control interval size is 4096, and the percentage of free space in a control interval has been defined as 20%, 819 bytes are reserved. Free space calculations drop the fractional value and use only the whole number.

To find out what values are actually set in a defined data set, issue the access method services LISTCAT command.