System logging and processing continuity

To protect the integrity of the data, the online IMS™ uses both external security checking and various internal techniques to record the transactions entered into the system and the database update activity.

The principal tool for recording online system activity is IMS system logging. Data stored on various logging data sets contains information used for restart, recovery, statistics, and audit purposes.

IMS log data is recorded in four kinds of data sets:

• Online log data set (OLDS)
• Write ahead data set (WADS)
• Restart data set (RDS)
• System log data set (SLDS)

The online system uses a minimum of three OLDSs, one WADS, and a single RDS, all residing only on DASD. When one or more online log data sets are filled, you can archive them to system log data sets using the IMS Log Archive utility. DASD or tape media can be used for SLDSs. Batch systems use system log data sets and are able to log to either tape or DASD.

The online system uses the OLDSs in wrap-around fashion. If dual logging of the OLDSs is an installation requirement, a pair of data sets (primary and secondary) must be assigned. The DD names for the OLDSs begin with the character string DFSOLP for the primary data set and DFSOLS for the secondary data set. A unique suffix (00 through 99), called an OLDS identifier, completes the 8-character DD name. Either single logging or dual logging is performed, as determined by DD statements during system initialization or by instructions included in the DFSVSMxx IMS.PROCLIB member.

A WADS is a small data set containing a copy of log records that are in OLDS buffers but have not yet been written to the OLDSs. When logging to DASD (required for online processing), fixed-length blocks make direct retrieval easier. A WADS allows large fixed-length blocks (in variable blocked format) to be written to the OLDSs without the requirement to rewrite blocks. When log data has been written to the OLDSs, the WADS is reused.

If a system failure occurs, the log data in the WADS is used to close the OLDSs. The close process occurs as part of an emergency restart or as an option of the Log Recovery utility.

IMS allows up to ten WADS, only one or two of which are active while the rest are spares. When a write error is detected, a spare WADS replaces the WADS that encountered the error. Dual WADS logging is also supported if it is required to have backup in the event of a read error while closing the OLDSs from the WADS.

The online IMS system controls the log data sets that are used for startup. It makes use of entries in the checkpoint identification table written on the restart data set, and of log data set information recorded in the DBRC RECON data set. If you are using automatic restart, the ⁄START IMS command issued from the system console causes the appropriate kind of restart. Normally, this restart results in the use of OLDSs records and in a normal restart that completes without the use of prior system logs. If restart processing abnormally terminates before the initial checkpoint, the appropriate restart for automatic restart is the same type (⁄NRESTART or ⁄ERESTART) as the aborted restart.