Because the master catalog is vital to the functioning of a z/OS® system, you should create an alternate master catalog that can be used in a system initial program load (IPL) if the regular master catalog becomes damaged.

At minimum, an alternate master catalog contains entries for the system data sets necessary to IPL the system. After IPL, the original master catalog can be repaired or recovered, and the system can again be IPLed with the newly recovered master catalog.

The simplest procedure for creating an alternate master catalog is to use the access method services REPRO command to copy the master catalog into a defined new master catalog. After you have copied the master catalog into the newly defined master catalog, complete the following steps:

1. Use the newly defined catalogs as the master and the old master catalog as the alternate master catalog. This is necessary because the REPRO process changes the VVDS pointers to the output catalog. Although the system can still be IPLed with the old master, allowing you to recover the new master catalog if necessary, the new master should be used under normal circumstances.
2. Define the alternate master on a different volume than the volume of the original master. Otherwise, if the original master's volume is damaged, both the original master and alternate master are unavailable. If you desire volume IPL, initialize the new volume with IPL text, and copy the required system data sets to the new volume. Allocate the new system data sets with a different high level qualifier than SYS1 (e.g. SYS2), then rename them after they are cataloged in the alternate master. This is necessary because some system data sets are always in use. The alternate master then has entries for the same data sets defined in the original master.
3. To manually create a minimum alternate master catalog without using REPRO, create the catalog and define all the system data sets in the alternate master. Then define new storage index and page data sets.

The newly defined catalog should then be used as the master catalog, and the old master should be used as the alternate master. The alternate master should be defined on a different volume than the volume of the original master: otherwise, if the original master's volume is damaged, both the original master and the alternate master are unavailable.

After the alternate is created, changes to the entries for the system data sets are only reflected in the master catalog that is in use. If you IPL the system with the new master, changes are not reflected in the old master (now the alternate master). For example, if a system data set is moved to a different volume, update the alternate master by recreating it or recataloging the data set.

After defining the new master and copying the original master into it, create a SYSCATxx member in SYS1.NUCLEUS to identify the alternate master. To use the alternate master catalog, specify at IPL time the two-character identifier of the SYSCATxx member that contains the entry identifying the alternate master catalog. You can also define the alternate master in a LOADxx member in SYS1.PARMLIB.

If you want to maintain the SYSCATLG member as the member identifying your current master catalog, and the SYSCATLG member points to the old master catalog, copy the member to a different SYSCATxx member. Then update the SYSCATLG member to point to the catalog you just created.

The following example shows the procedure for creating an alternate master catalog. The new master catalog created is SYS1.ICFCAT.NEWMASTR. The old master catalog SYS1.ICFCAT.MASTER is used as the alternate master catalog, and is identified in the SYSCATAL member of SYS1.NUCLEUS. Both the new master and the old master use a multilevel alias search level of 1, no SYS% conversion, and the default number of CAS tasks.