Understanding the backup-while-open (BWO) facility

Many CICS® applications depend on their data sets being open for update over a long period of time. Normally, you cannot make a backup of the data set while the data set is open.

If a failure occurs that requires forward recovery, all updates that have been made to the data set since it was opened must be recovered. This means that you must keep all forward recovery logs that have been produced since the data set was opened. A heavily used data set that has been open for update for several days or weeks might require many log records to be applied during forward recovery.

The backup-while-open (BWO) function allows DFSMSdss to make a backup when applications are running in continuous operation while the data set is open for update with full data integrity of copied data. This is only feasible for CICS VSAM file control data sets for which CICS creates forward recovery logs. Only the updates that have been made since the last backup copy was taken need to be recovered. This could significantly reduce the amount of forward recovery that is needed. Long running transactions, automated teller machines, and continuously available applications require the database to be running when the backup is being taken.

DFSMSdss is an optional feature of z/OS® and is the primary data mover. When used with supporting hardware, DFSMSdss also provides concurrent copy capability. Concurrent copy lets you copy or backup data while that data is being used; even when control-area and control interval splits and data set additions, new extents or add-to-end, are occurring for VSAM key sequenced data sets.