Optimizing Control Area Size

You cannot explicitly specify control-area size. Generally, the primary and secondary space allocation amounts determine the control-area size:

If either the primary or secondary allocation is smaller than one cylinder, the smaller value is used as the control-area size. If RECORDS is specified, the allocation is rounded up to full tracks.
If both primary and secondary allocations are equal to or larger than one cylinder, the control-area size is one cylinder, the maximum size for a control area.

The following examples show how the control-area size is generally determined by the primary and secondary allocation amount. The index control-interval size and buffer space can also affect the control-area size. The available control area sizes are 1, 3, 5, 7, 9 and 15 tracks. The following examples are based on the assumption that the index CI size is large enough to handle all the data CIs in the CA. The buffer space is large enough not to affect the CI sizes:

CYLINDERS(5,10)—Results in a 1-cylinder control-area size.
KILOBYTES(100,50)—The system determines the control area based on 50 KB, resulting in a 1-track control-area size.
RECORDS(2000,5)—Assuming 10 records would fit on a track, results in a 1-track control-area size.
TRACKS(100,3)—Results in a 3-track control-area size.
TRACKS(3,100)—Results in a 3-track control-area size.

A spanned record cannot be larger than the size of a control area minus the size of the control information (10 bytes per control interval). Therefore, do not specify a primary or secondary allocation that is not large enough to contain the largest spanned record.

Note: If space is allocated in kilobytes, megabytes, or records, the system sets the control area size equal to multiples of the minimum number of tracks or cylinders required to contain the specified kilobytes, megabytes, or records. Space is not assigned in units of bytes or records.

If the control area is smaller than a cylinder, the size of the control area is an integral multiple of tracks, and the control area can span cylinders. However, a control area can never span an extent of a data set, which is always composed of a whole number of control areas. The available control area sizes are 1, 3 ,5, 7, 9 and 15 tracks. 16 tracks is also a valid control area size if the number of stripes is equal to 16. For requests where either the primary or secondary allocation amount is smaller than one cylinder, the system might adjust the primary and secondary quantity. The system might also select a CA that is different than what is selected from a prior release. For example, a TRK(24,4) request results in a control area of 5 tracks, a primary and secondary amounts of 25, and 5 tracks respectively. For more information about allocating space for a data set, see Allocating Space for VSAM Data Sets.