Consolidating logged database changes for recovery

You can use the Database Change Accumulation utility (DFSUCUM0) to extract and consolidate database change records from SLDSs or RLDSs. The Database Change Accumulation utility optimizes the database change records for recovery and saves them to a change accumulation data set. You can use the change accumulation data set as input to the Database Recovery utility (DFSURDB0).

As IMS runs, the number of SLDSs or RLDSs increases. You can use these data sets to recover a lost or damaged database, but to use them without change would be inefficient for the following reasons:

• Each SLDS or RLDS contains a record of activities of the entire IMS subsystem and of all the data sets for all the databases. Yet when you are recovering a database, you usually only recover a single data set. Thus, much of what is in the SLDS and RLDS does not apply.
• The SLDS and RLDS chronologically stores each change to any single database record. If a record changes 100 times since the last backup of the data set, the SLDS or RLDS includes all 100 changes. Yet, during recovery, you are only interested in the value the data had at the moment the data set was lost; the other 99 changes are irrelevant.

You can use the Database Change Accumulation utility to sort through your accumulated SLDSs and RLDSs in advance and condense and streamline them. This utility:

• Picks out only those log records relating to recovery of databases
• Sorts these records by data set within a database
• Finds the most recent change in each part of an individual record

As the utility creates the change accumulation data set, IMS compresses repeated single characters, such as blanks and zeros. IMS expands the data again during recovery.

Running the Database Change Accumulation utility is not required, but using it periodically speeds database recovery. Alternatively, you can run the Database Change Accumulation utility only when the need for recovery arises (just before running the Database Recovery utility). Running these two utilities instead of just the Database Recovery utility can reduce the total time needed for recovery, depending on how much unaccumulated log information exists.