Staging data sets for coupling facility logstreams

MVS usually keeps a second copy of the data written to the coupling facility in a data space, for use when rebuilding a coupling facility log if an error occurs.

This backup is satisfactory if the coupling facility is failure-independent from MVS; that is, in a separate CPC and non-volatile.

When the coupling facility is in the same CPC, or uses volatile storage, the MVS system logger supports staging data sets for copies of logstream data that might otherwise be vulnerable to failures that affect both the coupling facility and the MVS images.

When defining logstreams, consider the following tasks:
  • Define STG_DUPLEX(YES) and DUPLEXMODE(COND) for those logstreams that are associated with the system log. These definitions ensure that the MVS system logger automatically copies to staging data sets if it detects that the coupling facility is not failure-independent and is a single point of failure, and is therefore vulnerable to permanent log data loss. `

    A connection to a logstream contains a single point of failure if the coupling facility is volatile or it resides on the same CPC as the MVS system connecting to it. For example, if there are two CPCs, CPC1 and CPC2, and CPC1 has an MVS LPAR and a coupling facility, but CPC2 has only MVS LPARs, the connections from the MVS LPAR in CPC1 to the coupling facility are failure-dependent. If you lose CPC1, you lose both MVS and its local buffers and the coupling facility. However, the connections from CPC2 are failure-independent, because the system logger local storage and buffers are in a physically separate CPC from the coupling facility, and the data is lost only if both fail. With DUPLEXMODE(COND), failure-dependent connections result in staging sets, but failure-independent connections are not allocated staging data sets.

  • If you are operating with only a single coupling facility, define STG_DUPLEX(YES) and DUPLEXMODE(UNCOND) for those logstreams associated with the system log.
  • Define STG_DUPLEX(YES) and DUPLEXMODE(COND) for those logstreams associated with forward recovery logs. If you do not, and a failure causes loss of data from the logstream, you need to take a new image copy of the associated VSAM data sets. For the consequent period of time until this operation is complete, the data sets are not fully protected.
  • If you operate a nonvolatile, stand-alone coupling facility for normal logging, with a PR/SM LPAR configured as a coupling facility acting as backup, define all logstreams with STG_DUPLEX(YES) and DUPLEXMODE(COND).
  • Define each staging data set to be at least the same size as the logstream share of the coupling facility, but round the average block size up to 4 KB.
    For example, you can calculate the staging data set size that corresponds to the basic coupling facility space requirement for each CICS® system logstream (DFHLOG) by using the following formula: 
    staging data set size = number_of_entries *  rnd-avg-buf /  4096
    where rnd-avg-buf = avgbufsize (rounded up to 4K)

    To calculate a value for number_of_entries, see number of entries calculation.