To plan and use a coupling facility configuration requires attention to the storage volatility of the coupling facility where the structures reside. The advantage of a nonvolatile coupling facility is that, if you lose power to a coupling facility that is configured to be nonvolatile, the coupling facility enters power save mode. This saves the data contained in the structures.

The safest environment is one in which there are two or more nonvolatile coupling facilities that are failure-independent from any of the exploiting MVS™ images, using dedicated processor resources.

One dedicated coupling facility plus a coupling facility LPAR provides the next safest environment for normal logging and lock-structure use. This environment has the same advantages of rebuilding with minimal impact to DFSMStvs instances that are running. Furthermore, MVS detects that the LPAR coupling facility is not in a failure-independent domain and causes the system logger to write log stream data to staging data sets for extra security.

The DFSMS cache structures are store-through caches, and no changed data resides in them. Store-through caching means that a user writes changed data to the cache structure and to permanent storage at the same time and under the same serialization. This way, the data in the cache structure always matches the data in permanent storage.

Because there is no chance of data loss, the DFSMS cache structures do not need to be in a nonvolatile coupling facility. The DFSMS lock structure has persistent connections and is a persistent structure. Data loss in the lock table requires a double failure. For example, a double failure occurs at the loss of the coupling-facility structure and the failure of one system in the sysplex at the same time. VSAM RLS also has a sharing-control data set to protect the integrity of the data even if there is a double failure. If this protection is not enough, the DFSMS lock structure can be placed in a nonvolatile coupling facility.

DFSMStvs caches all of its data in DFSMS cache structures that you define. The DFSMS lock structure and the system logger do all the logging for DFSMStvs. The IGWLOCK00 coupling-facility lock structure controls all VSAM RLS and DFSMStvs locks. DFSMStvs defines the names of its undo and shunt log streams, and you define the DFSMStvs log of logs and forward recovery log streams. RRS, which must be active for DFSMStvs to function, needs five log streams to be defined.

If you run with a single coupling facility, its failure would cause the system logger and any other users of the coupling facility to suspend normal operation until access to the coupling facility is restored. DFSMStvs would, effectively, be unusable in such a situation.

If a batch job attempts to communicate with RRS and the system logger goes down, RRS suspends the job until the logger comes back up. You cannot cancel the batch job because RRS has suspended it. If you cancel RRS or bring the system logger back up, however, RRS releases the batch job, which then ends; otherwise, the job remains suspended.

Recommendation: Do not perform a commit or backout through an RRS panel.

Unless you specify that the system logger is to use staging data sets, the recovery of log stream data depends on the MVS images remaining active. This is so that the system logger can use copies of log records that are held in storage to repopulate the coupling facility when it is again available. If you must run with a single coupling facility, specify the DUPLEXMODE(UNCOND) command to force the use of staging data sets.