Using structure alter for CQS

Structure alter is a z/OS® process supported by CQS that can be used to alter the structure size or to redistribute the objects within the structure. CQS supports structure alter for primary queue structures, overflow queue structures, and resource structures. CQS allows you to dynamically change the size of a primary or overflow structure.

To enable structure alter, activate a CFRM policy and define the INITSIZE and SIZE parameters in this policy. For information on structure size, see IMS Version 15.5 System Definition.

To change the structure size, enter the following XES command: 

SETXCF START,ALTER,STRNAME=strname,SIZE=size

The value of size must be within the range of values between INITSIZE and SIZE in the CFRM policy.

Automatic structure alter is a z/OS function that can automatically alter the structure size or the element to entry ratio when the structure full threshold is reached. CQS supports automatic structure alter for queue structures and resource structures. To enable automatic structure alter, activate a CFRM policy defined with INITSIZE, SIZE, ALLOWAUTOALT(YES).

Important: A structure enabled with automatic structure alter is a candidate to be contracted in size by z/OS, if the coupling facility storage becomes constrained. Be careful when enabling automatic structure alter for queue structures. If z/OS contracts the queue structure size, it might cause the queue structure to go into overflow unnecessarily. To prevent this happening, define the CFRM policy with a MINSIZE (minimum size), below which z/OS will not contract the structure.

The FULLTHRESHOLD parameter in the CFRM policy structure definition specifies the percent full threshold for the structure. This threshold is the percent full that coupling facility resources such as entries or elements must reach before auto alter processing is triggered. The default value for the FULLTHRESHOLD parameter is 80 percent. The CQSSGxxx PROCLIB member parameter OVFLWMAX specifies the percentage of elements in the structure that must be in use to trigger CQS overflow processing. The default value for OVFLWMAX is 70 percent.

Recommendation: If you are using ALLOWAUTOALT(YES), change one or both of these default values so that the OVFLWMAX value is larger than the FULLTHRESHOLD value by at least five percent. This is to give z/OS a chance to alter the entry-to-element ratio and structure size, and potentially avoid CQS overflow processing. Also, setting these parameters to different values reduces the chance of both auto alter and CQS overflow processing occurring at the same time and causing extra overhead.

For more information on automatic Structure Alter, see z/OS MVS Setting Up a Sysplex.