There is information necessary to communicate changes in a basic catalog structure (BCS) to other systems that are sharing the catalog. This information is stored in a special record in the VVDS on the volume the catalog is defined on. The information is used to ensure the consistency of the catalog records that are cached on any sharing subsystem. It is also used to update the BCS control block structure in those cases where a sharing system has extended beyond the current high used value or to a new extent. In addition, it is used to invalidate BCS data and index buffers when they have been updated from a sharing system.

The storing and retrieval of this information requires additional I/O to the volume containing the catalog. In some cases, this I/O overhead can become significant and have a noticeable impact on system or sysplex performance. VVDS Mode sharing is the default mode of sharing.