DFSMSdss supports backup-while-open serialization, which can perform backups of data sets that are open for update for long periods of time. It can also perform a logical data set dump of these data sets even if another application has them serialized. Backup-while-open is a better method than using SHARE or TOLERATE(ENQFAILURE) for dumping Customer Information Control System (CICS®) VSAM file-control data sets that are in-use and open for update. When you dump data sets that are designated by CICS as eligible for backup-while-open processing, data integrity is maintained through serialization interactions between CICS (database control program), CICSVR (CICS VSAM Recovery), VSAM record management, DFSMSdfp, and DFSMSdss. When you dump data sets that are designated by DFSMStvs as eligible for backup-while-open processing, data integrity is maintained through serialization interactions between DFSMStvs, VSAM record management, DFSMSdfp, and DFSMSdss.

Although the BWO(TYPECICS) parameter applies to both CICS and DFSMStvs, DFSMStvs enables you to back up a data sets while they are open whether or not you are running CICS.

Figure 1 shows the backup-while-open serialization for dumping CICS data sets that are open for update. Backup-while-open processing also ensures that any update activity that may invalidate the dump is detected. Simultaneous recovery or deletion of the data set while it is being dumped is also prevented.

Figure 1. Block diagram for backup-while-open serialization

In Figure 1, a VSAM file-control data set (7) is allocated for CICS (1) through MVS™ allocation services (3) using JCL or dynamic allocation methods. This results in serialization through an enqueue on the name of the data set and the resource name SYSDSN (4). When the VSAM data set is opened (3), another level of serialization occurs through an enqueue on the names of the components of the VSAM data set and the resource name SYSVSAM (4). For eligible data sets, CICS uses DFSMSdfp (5) to set a status in the backup-while-open indicators (6) in the catalog entry for the data set.

For a dump operation, DFSMSdss (8) attempts to acquire the SYSDSN, SYSVSAM, and backup-while-open (9) enqueues for the data set. When the enqueue on the cluster name of the data set and the resource name of BWODSN is acquired, but not the enqueues for both SYSDSN and SYSVSAM, DFSMSdss uses DFSMSdfp to get the backup-while-open indicators and starts to dump the open data set if it were eligible for backup-while-open processing.

The backup-while-open enqueue is used to prevent more than one DFSMSdss operation, such as a simultaneous dump and restore (10), and to prevent the data set from being deleted while it is being dumped by DFSMSdss.

While the data set is being dumped, a database application program might update the data set in a manner that invalidates the data set. For example, a control-interval or control-area split might occur. If this happens, VSAM record management uses DFSMSdfp to change the backup-while-open status.

When the backup of the open data set is completed, DFSMSdss obtains the current backup-while-open indicators. If the indicators are different from when the dump was started, DFSMSdss invalidates the dump of the data set. When concurrent copy is used, updates made while the data set is being dumped do not cause the dump to be invalidated.