A shared catalog is a basic catalog structure that is eligible to be used by more than one system. It must be defined with SHAREOPTIONS(3 4), and reside on a shared volume. A DASD volume is initialized as shared using the MVS™ hardware configuration definition (HCD) facility. Note that the device must be defined as shared to all systems that access it. If some systems have the device defined as shared and some do not, catalog corruption will occur. Check with your system programmer to determine shared volumes. Note that it is not necessary that the catalog actually be shared between systems; the catalog address space assumes it is shared if it meets the criteria stated above. All VVDSs are defined as shared. Tape volume catalogs can be shared in the same way as other catalogs.

By default, catalogs are defined with SHAROPTIONS(3 4). You can specify that a catalog is not to be shared by defining the catalog with SHAREOPTIONS(3 3). Only define a catalog as unshared if you are certain it will not be shared. Place unshared catalogs on volumes that have been initialized as unshared. Catalogs that are defined as unshared and that reside on shared volumes will become damaged if referred to by another system.

If a catalog is shared, its catalog address control structures are refreshed when updates are made to the catalog from any system. This ensures that each system is using an up-to-date copy of the catalog at all times.