It is possible to specify the number of concurrent read requests for a BCS with the STRNO attribute. You can have from 2 to 255 concurrent read requests. Only one write request is allowed at a given time.

Initially define the catalog with STRNO(3). You can monitor the adequacy of this value by using the Resource Measurement Facility™ (RMF™) to watch for enqueues on the resource SYSZRPLW.catname, and I/O contention on catalog volumes. SYSZRPLW is the major name of the resource, and catname (the name of the catalog) is the minor name. An indication of enqueue contention is given by the I/O service times, not necessarily the number and frequency of enqueues on this resource.

You can change the STRNO attribute with the access method services ALTER command. This will take effect following a close and subsequent reopen of the catalog. You can close the catalog using the MODIFY CATALOG,CLOSE console command. The next request to the catalog will cause the catalog to be reopened.

If a catalog request is for update, an ENQ with exclusive control is issued on the BCS itself. However, most requests to the BCS are read requests, and those result in shared enqueues on the BCS.

Because requests for the VVDS are provided for dynamically, the number of concurrent requests is variable. There can be multiple updates to the VVDS concurrently, if the updates do not change the length of the VVDS records, and if the updates occur in different control intervals. An update to a record gets an exclusive enqueue on a control interval, and if the update changes the length of the VVDS record, the update request gets an exclusive enqueue on the VVDS itself.