Image copies and the IMS image copy utilities

Backup copies of databases are called image copies. You can create image copies by using one of the image copy utilities provided by IMS. Depending on which utility you use, you can create image copies while databases are online, offline, or quiesced.

IMS provides three utilities that you can use to make image copies:
  • Database Image Copy utility (DFSUDMP0)
  • Online Database Image Copy utility (DFSUICP0)
  • Database Image Copy 2 utility (DFSUDMT0)
Restriction: You cannot run these utilities against a HALDB partition that is being reorganized by HALDB Online Reorganization (OLR).

These utilities create image copies of recoverable and nonrecoverable databases. The image copy utilities operate on data sets.

The frequency of creating image copies is determined by your recovery requirements. The minimum requirement is that an image copy be created immediately after a database is reorganized, reloaded, or initially loaded. Because database recovery is done on a physical replacement basis, a reloaded data set is not physically the same as it was before unload.

With the Database Image Copy utility (DFSUDMP0) and the Online Database Image Copy utility (DFSUICP0), if a database includes multiple data sets or areas, you must supply the utility with multiple specifications. With the Database Image Copy 2 utility, however, you can copy multiple database data sets in one execution of the utility. You can specify a group name to represent the collection of database data sets that are to be copied in a single execution.

Use the DBRC commands INIT.DB and CHANGE.DB to identify recoverable databases to DBRC. DBRC works similarly with all of the image copy utilities.

The output data sets from the Database Image Copy utility (DFSUDMP0) and the Online Database Image Copy utility (DFSUICP0) have the same format. The rules for predefinition and reuse of image copy data sets apply to the data sets that are used by both of these utilities.

If you use the Database Image Copy 2 utility, the format of the output data sets depends on which image copy option you use. Image copies that are produced by using the concurrent copy function of the Database Image Copy 2 utility are in DFSMSdss dump format. Image copies that are produced by using the fast replication function of the Database Image Copy 2 utility are exact copies of the original data set.

If you use the Database Image Copy 2 utility, the DBRC rules for predefinition and reuse of image copy data sets apply only to the data sets that are used for the concurrent copy function. The fast replication function of the Database Image Copy 2 utility does not support the predefinition and reuse of image copy data sets.

All of these utilities call DBRC to verify the input (DBRC allows them to run only if the input is valid), and they call DBRC to record information in the RECON data set about the image copy data sets that they create. An image copy record in the RECON data set has the same format regardless of which utility created its corresponding image copy data set. The Online Database Image Copy utility, however, has its own PDS member of skeletal JCL, and you use a separate DBRC command, GENJCL.OIC, to generate the job for the Online Database Image Copy utility.

Recommendation: Copy all data sets or areas for a database at the same time. When you recover a database to a prior state, you must recover all data sets that belong to the database, as well as all logically related databases (including those related by application processing) to the same point to avoid data integrity problems.
When using the image copy utilities, you have the option of creating one or more output image copies, unless you are using the fast replication function of DFSUDMT0. Advantages of making two or more copies are:
  • If an I/O error occurs on one copy, the utility continues to completion on the others.
  • If one copy cannot be read, you can perform recovery using another.

The trade-off in deciding whether to make more than one copy is that the performance of the image copy utility is degraded by the time required to write the other copies.

If you are using the fast replication option of the Database Image Copy 2 utility, the utility can make only a single output image copy of each source database data set during each execution of the utility.

The utilities enforce a minimum output record length of 64 bytes; thus, it is possible that an image copy of a database with a very short logical record length can require more space than the original database.

Recommendation: Take an image copy immediately after running batch jobs that update the database without IMS logging. Even when pool sizes are identical and your system uses VSAM background write, updates made by batch jobs are not bit-for-bit repeatable. Taking an image copy allows you to maintain the integrity of your database if a recovery is required.

If you recover using log tapes up to the start of a batch job, and then reprocess the batch job, the resulting database might not be bit-for-bit identical to the database after the previous batch run, although it is logically identical. If the database is not bit-by-bit identical, log tapes created after the previous execution of the batch jobs would not be valid after the reprocessing. Therefore, do not attempt a recovery by starting with an image copy, applying log tapes, reprocessing unlogged batch executions, and then applying more log tapes.

Image copies that are not created by one of the IMS image copy utilities are called nonstandard image copies or user image copies.