Changing the Maximum Number of Catalogs and Tasks in CAS

When you IPL a system, the maximum number of catalogs that can be open in the catalog address space is set at 9999. The maximum number of CAS service tasks available for user requests is set to either 180 (the default) or 90% of the value optionally specified in the SYSCATxx member of SYS1.NUCLEUS. You can specify the number of catalogs and tasks as follows:

Specify the service task lower limit in SYSCATxx (SYS1.NUCLEUS), or LOADxx (parmlib).
Specify the maximum number of concurrent user service tasks in the TASKMAX parameter of the IGGCATxx parmlib member. The value of TASKMAX in IGGCATxx should be no more than 90% of the maximum number of concurrent Catalog requests specified in SYSCATxx.

You can also change these values temporarily using the CATMAX or TASKMAX parameters on the MODIFY CATALOG command. Changing these values can help you manage or limit the amount of storage used by CAS to perform catalog functions.

When you use CATMAX to change the maximum number of catalogs that can be open in CAS and the new limit is lower than the previous limit, all open catalogs are closed. This does not unallocate catalogs. Catalogs remain allocated to CAS, but in restart status. All the storage associated with the catalogs that were closed is freed.
If a request for a closed catalog must be processed after the limit for open catalogs is reached, the least-recently used catalog is closed and the required catalog is opened.

Limiting the number of open catalogs affects catalog performance. However, if space is a primary consideration, you might need to set a maximum.
When you use TASKMAX to change the maximum number of CAS service tasks, first determine the current number of CAS tasks. The REPORT parameter shows this value in the "# ATTACHED SERVICE TASKS" field.
When the upper limit for tasks is reached, any new requests that require catalog resources wait until another task is finished. No user jobs fail because the limit is reached, but if the limit is set too low, it might cause a significant performance degradation for jobs on the system.

Catalog management reserves 10% of the total number of tasks for its exclusive use, with the remaining tasks being available for both catalog management use and user requests. The reserved tasks are only allocated if all the other tasks are in use, and catalog management requires the use of a CAS service task. Thus, the highest number of service tasks listed for the catalog address space might be as high as 180 (if the default value is used) or 360 (if the largest possible value is specified in SYSCATxx).