Secondary host promotion

DFSMShsm allows secondary hosts to take over functions for a failed primary host. This failure can be either an address space failure or an entire z/OS image failure. In addition, DFSMShsm allows another host to take over secondary space management (SSM) from a failed host, which can either be the primary or a secondary host. Secondary host promotion ensures continuous availability of DFSMShsm functions. Host promotion occurs without users who have to interact with other programs, receive or interpret console messages, or issue commands from batch jobs.

The following definitions are key to understanding the concept of secondary host promotion:

An original host is a host that is assigned to perform primary host or SSM responsibilities.
A secondary host is a host that is not assigned to perform primary host or SSM responsibilities.
A primary host is a host that performs primary level functions.
The primary host is the only host that performs the following functions:
- Hourly space checks (for interval migration and recall of non-SMS data)
- During autobackup: Automatic CDS backup
- During autobackup: Automatic movement of backup versions from ML1 to tape
- During autobackup: Automatic backup of migrated data sets on ML1
- During autodump: Expiration of dump copies
- During autodump: Deletion of excess dump VTOC copy data sets
An SSM host is generally the only host that performs SSM functions.
A host is said to be promoted when that host takes over the primary or SSM (or both) host responsibilities from an original host.
A host is said to be demoted when it has had its primary or SSM (or both) host responsibilities taken over by another host. There is always a corresponding promoted host for each demoted host, and vice versa.