Concurrent image copy

IMS provides the capability to take an image copy of a database without taking that database offline. This means the database can be updated while the image copy is being taken and some, all, or none of the updates might appear in the image copy.

This image copy is called fuzzy because the copy represents the state of the database over a certain period of time rather than at one specific instant in time. It is also called a 'concurrent image copy' because the copy was taken while update processing is happening.

The ability to take image copies while the databases are being updated by IMS applications allows increased database availability. The offline image copy utilities, Database Image Copy and Database Image Copy 2, provide an option to take a concurrent image copy. Concurrent image copies can be created by both the concurrent copy and fast replication options of Database Image Copy 2. A database being copied by the Online Database Image Copy utility can be concurrently updated by the IMS subsystem in which the utility is running (but not by other IMS subsystems). Image copies created by HSSP processing are also 'fuzzy' copies because the areas are available for update processing while HSSP is running.

When a consistent image copy is input to database recovery, the recovery only requires logs from after the image copy job completed. A concurrent image copy, might not include updates that were made before the image copy process started or while it was executing. Therefore, when a concurrent image copy is input to recovery, logs from before the image copy process was started might have to be supplied to database recovery.