Phase 1: Backing up the control data sets

The first phase of automatic backup is the backing up of the DFSMShsm control data sets (CDS). This process is called CDS version backup, and it is controlled by the CDSVERSIONBACKUP parameter of the SETSYS command.

Before backing up the control data sets and the journal data set, DFSMShsm issues an exclusive enqueue against the control data sets. In a multiple DFSMShsm-host environment, if the control data sets are not being accessed with RLS, DFSMShsm reserves the volumes containing the journal and the control data sets. If the control data sets are being accessed using RLS, DFSMShsm does an exclusive enqueue. If XCF is available in the multi-host environment, the CDS backup host will notify other hosts through XCF that the enqueue is needed.

The use of the concurrent copy function will reduce the duration of complex-wide serialization of the DFSMShsm CDS resources when performing CDS version backup and, when SETSYS JOURNAL(RECOVERY) is specified, non-intrusive journal backup is used to further reduce the duration (see Using non-intrusive journal backup). This time reduction increases the availability of DFSMShsm functions and non-intrusive journal backup allows DFSMShsm activity to continue during the journal backup. To achieve the highest level of DFSMShsm availability, you should place the control data sets on SMS-managed devices that support the concurrent copy function, and you should use DFSMSdss to back up the control data sets. If DFSMSdss backs up to tape, each CDS (or each cluster if multi-cluster control data sets are used) is copied to an individual tape drive (in parallel). Use of concurrent copy is required in order to use non-intrusive journal backup.

If CDS backup is invoked while the control data sets are accessed in RLS mode, DFSMSdss must be the datamover. If the backup is directed to tape, the PARALLEL parameter of the SETSYS CDSVERSIONBACKUP(BACKUPDEVICECATEGORY) must be specified. Automatic backup overrides the settings if they are not correct and issues error message ARC0793I. CDS backup fails.

Using the CDSVERSIONBACKUP parameter, you can specify how many versions of the CDS backups be kept active. Each time CDS version backup runs, DFSMShsm assigns a new version number to the newly-created backup data sets and uses that version number as the final qualifier of the CDS backup data set names.

When backing up control data sets to tape, DFSMShsm always creates and catalogs the new version and expires the oldest version, so that only the latest versions are available. When backing up the control data sets to DASD, the oldest existing version is reused to record the new backup, and when the backup is complete, that oldest version is renamed and recataloged as the newest version.

DFSMShsm optionally invokes the CDS backup installation exit (ARCCBEXT) that allows you to access or validate the backup versions after DFSMShsm creates them.

When the CDS backup is complete, DFSMShsm releases the volumes containing the journal data set and each control data set, and removes the exclusive enqueue against the control data sets, allowing other DFSMShsm functions to proceed.