Using MODIFY CATALOG, you can specify that a catalog be closed, or that the catalog be allocated or unallocated from the catalog address space. You cannot, however, permanently close a catalog or prevent a user from accessing a catalog, nor can you explicitly open a catalog, with the MODIFY CATALOG command. To prevent users from accessing a catalog, use the access method services ALTER LOCK command.

Catalogs are allocated to the catalog address space when the first request for the catalog is processed. However, you can use the ALLOCATE parameter with the NOISC or NOVLF parameters to prevent the catalog from using the specified cache.

When you unallocate a catalog from CAS, the catalog is closed and all CAS private storage used by the catalog is freed. The device on which the catalog resides is also unallocated from CAS. However, CAS storage related to the catalog remains allocated. This space can only be freed by deleting the catalog. Common service area space for the catalog is not freed.

You can also close a catalog. When you use CLOSE, the catalog is closed but remains allocated to CAS. All the CAS private storage associated with the catalog is freed.

Closing or unallocating a catalog might be useful when the control blocks for a catalog become corrupted. When the catalog is opened again, new control blocks are built for the catalog.

If a catalog is using the CDSC, all CDSC space used by the catalog is freed when the catalog is closed or unallocated. This space is then available for the use of other catalogs in the CDSC.

If you alter a catalog's attributes (for example, the catalog's share options), you can close the catalog, and when it is reopened, the new values are recognized. Thus, catalog attributes can be changed without cancelling jobs or performing a system IPL.

If your installation has little or no catalog activity once your system is up and running, you might consider closing all the catalogs to free up CAS private storage associated with the closed catalogs.

You can also close a VVDS using the VCLOSE parameter. When you close a VVDS, the CAS private storage used by the VVDS is freed. The next request that uses the VVDS reopens the VVDS, and new control blocks are built for the VVDS.

Use VUNALLOCATE to unallocate all VVDSs. This might be necessary to allow the VARY command to vary a volume offline. VVDSs remain unallocated until you issue MODIFY CATALOG,NOVUNALLOCATE.