Control interval size for auxiliary temporary storage

You specify the control interval size for the auxiliary temporary storage data set with the CONTROLINTERVALSIZE parameter in the VSAM CLUSTER definition. Because a control interval contains one or more temporary storage records, take the temporary storage record size into account when choosing the control interval size.

A temporary storage record is a single numbered item in a temporary storage queue, which might be written by CICS or an application. A temporary storage record must have the following space:
  • 36-bytes for the temporary storage header.
  • The length of the data in the temporary storage record (the item in the temporary storage queue). If you are using BMS with 3270 support, the data length of the record is at least as large as the 3270 buffer size. For 3270 terminals with the alternate screen size facility, the data length is the larger of the two sizes. Make sure that you allow sufficient space for the data length used by large-screen devices.
The total number of bytes allocated for a temporary storage record (including the 36-byte header) must be rounded up to a multiple of 64 for control interval sizes less than, or equal to, 16 KB (16 384-bytes), or a multiple of 128 for larger control interval sizes.

CICS can process temporary storage records that exceed the control interval size, but performance might degrade in this situation. Choose a control interval size large enough to hold at least one instance of the largest normally occurring temporary storage record, together with the VSAM control information for the control interval.

Typically, a control interval contains more than one temporary storage record, and the records might be of different sizes. The control interval size affects transfer efficiency: a smaller size can improve performance if access to temporary storage is random, and a larger size can improve performance if applications tend to use items in temporary storage in a sequential way. In general, the larger the queues and write to read ratio, the more sequential the usage tends to be.

Select your exact control interval size following these rules:

  • The maximum control interval size is 32 KB.
  • A control interval size less than, or equal to, 16 KB (16 384-bytes) must include space for 64-bytes of VSAM control information in addition to the space allowed for temporary storage records.
  • A control interval size greater than 16 KB (16 384-bytes) must include space for 128-bytes of VSAM control information in addition to the space allowed for temporary storage records.
  • A control interval size smaller than 8 KB must be a multiple of 512-bytes.
  • A control interval size equal to or larger than 8 KB must be a multiple of 2 KB.

Example

If you use BMS to write a 24 x 80 character screen to temporary storage, the data written occupies 1920-bytes. You require 36-bytes for the CICS temporary storage header, giving a total of 1956-bytes. Rounding this up to a multiple of 64 gives 1984-bytes. Finally, adding a further 64-bytes of VSAM control information gives a control interval size of 2048-bytes to hold a single record. You can select a control interval size larger than 2048-bytes to hold several records that might differ in size.