If a catalog is defined with share options (3 4), and if it resides on a shared device, catalog management considers the catalog a shared catalog. A catalog is considered a shared catalog if it meets both of these conditions, even if it is not actually being shared among systems. Before each physical access to a shared catalog, special checking is performed to ensure that the ISC or CDSC contains current information. Checking also ensures that the access method control blocks for the catalog are updated in the event the catalog has been extended, or otherwise altered from another system. This checking maintains data integrity. It also affects performance because the VVR for a shared catalog must be read before using the ISC or CDSC version of the BCS record.

Recommendation: To avoid catalog corruption, define a catalog volume on a shared UCB and set catalog share options to (3 4) on all systems sharing a catalog.

A single catalog request can involve many physical references to the catalog, so the effect of this additional access to the VVDS can be significant. The volume containing the VVDS will be reserved, and I/O will be performed. If the catalog has been defined to support Enhanced Catalog Sharing (ECS) by the ECSHARING attribute, most of this overhead will be eliminated.

Changes to shared catalogs are handled differently depending on whether the catalog uses the ISC or the CDSC.

If a catalog uses the ISC and a sharing system updates a record (any record, even if the record is not cached in this system's ISC), catalog management releases the entire ISC for the catalog and creates a new ISC for the catalog. Individual records changed by a sharing system are not identified and updated for ISC catalogs.

The CDSC, however, can identify individual records that a sharing system has updated. Thus, when a sharing system updates a record, the CDSC space used by the catalog is not necessarily released. CDSC space for a catalog is only "invalidated" (marked unusable and given back to the CDSC as "free space" which any catalog can use) if so many changes were made by a sharing system that catalog management could not maintain a record of all the changes. Otherwise, all changes made by the sharing system can be made to the CDSC record by record.

If a catalog is not really shared with another system, move the catalog to an unshared device or alter its share options to (3 3). To prevent potential catalog damage, never place a catalog with share options (3 3) on a shared device.