Buffer space sizings

Use the buffer space parameters to define buffers of a specific size in the VSAM definition for the global catalog data set.

You must ensure that the REGION parameter on your CICS jobs is high enough to cope with the increase in buffer storage used for the global catalog, because this storage comes out of region storage not EDSA.

By default, VSAM uses a buffer space equal to twice the control interval size of the data component and the control interval size of the index. In the sample job, this calculation gives a default of 69632 bytes. A larger minimum buffer size can improve cold start and warm restart times, and might significantly reduce CICS shutdown times.

You can use two parameters to control the buffer size. The BUFFERSPACE parameter defines the minimum buffer space size that is allowed. The BUFSP parameter defines the maximum buffer size. You can add either parameter to the sample job to set an appropriate buffer size.

For performance reasons, CICS defines a STRNO (number of strings) value of 32. Based on the sample job, the minimum value of BUFSP is calculated as follows:

BUFND = (STRNO + 1)
BUFNI = STRNO
BUFSP = 33 * 32768 (BUFND * CI size) + 32 * 1024  (BUFNI * CI size) =
        1114112 bytes

Another way to define buffer space for the global catalog data set is to use the AMP parameter on the DD statement in the CICS startup job stream, which you can use to override the default or defined value. If the AMP BUFSP specifies fewer bytes than the BUFFERSPACE parameter of the access method services DEFINE command, the BUFFERSPACE number overrides the BUFSP number.