DFSMShsm keeps the backup copies of the control data sets as versions,
where each version is a set of CDS backup data sets and a journal
backup data set. The version number of each backup data set is shown
in the low-level qualifier of the data set names. Unless all backups
fail, the CDS backup version number is incremented by one, and the
new backup data sets are created and cataloged each time you back
up the control data sets.
Note: As a result of errors during
backup of an individual CDS, you may have one or more invalid backup
data sets in a backup version. If you back up your control data sets
to DASD and fail to attend to any CDS backup failures, you can eventually
lose the ability to recover the control data sets.
Whenever a new CDS backup version is created, the oldest backup
version is eliminated from the system (rolled off). If you back up
to tape, the oldest backup version is uncataloged. If you back up
to DASD, the oldest backup version is reused and renamed to the new
version.
You must decide how many versions to keep on your system, and then
tell DFSMShsm that number by using the BACKUPCOPIES subparameter of
the SETSYS CDSVERSIONBACKUP command. You should keep a minimum of
three CDS backup versions. Specify that number to DFSMShsm with the
following command:
SETSYS CDSVERSIONBACKUP(BACKUPCOPIES(3))