Using the Basic Components › Backstore Engine for Externalization and Recalls

Backstore Engine for Externalization and Recalls

All processes related to stacking on and recalling from the Backstore Tapes are controlled by the Backstore Engine running in its own address space (default name SVTSAS.SVT1PT). The Backstore Engine can be restarted without impacting Virtual Device processing.

The Backstore Engine can be configured in the following different ways:

• Perform Externalization just for its owning subsystem
• Perform Recalls just for its owning subsystem
• Perform Externalization and Recalls for its owning subsystem
• Perform Externalization and Recalls for all subsystems in a CA Vtape Complex
• Not perform Externalization and Recalls.

Backstore Engines that perform Externalization are referred to as Externalization Servers. Backstore Engines that perform Recalls are referred to as Recall Servers. Backstore Engines that perform Externalization and Recalls are referred to as Externalization and Recall Servers. Backstore Engines that cannot perform these functions are referred to as Clients.

Note: For more information, see the chapter “The Backstore Engine” in the Configuration Guide.

Recalls are independent of the requestor, meaning utilities such as IEFBR14 will finish processing and release the initiator even when all or part of the recalls generated during its execution are still in progress. Canceled or failing jobs will finish and the Recall process will continue. When the job is executed again, the Virtual Volume will be found in the DASD buffer.

When a mount requires the Virtual Volume to be recalled, depending on the CA Vtape Recall Server configuration, applications may need to wait until the Virtual Volume is fully recalled to the DASD buffer. If the CA Vtape Subsystem which mounted the Virtual Volume is performing the Recall, the application can access the data while it is being recalled. If the CA Vtape Subsystem which mounted the Virtual Volume is not performing the Recall, the application can access the data when the Virtual Volume is fully recalled to the DASD buffer.

Recalls, for Virtual Volumes opened for update, stop when the block to be read has been Recalled. Data or tape marks beyond the block to be updated are not recalled because they will be rewritten.

When a Recall is in progress by the Recall Server, all of the other subsystems in the CA Vtape Complex will be notified. Incoming Recalls for the same Virtual Volume on any of those subsystems will be queued and automatically completed when the first Recall completes.

The Backstore Engine provides support for shops managing their tape devices through CA MIM or IBM GRS in a sysplex or a non-sysplex environment and also to nonshared implementations where tape devices are dedicated to each LPAR. CA Vtape will query the operating system for the status of the desired devices and drive an allocation request depending on the setting of the BypassOfflinePhysicalDevices parameter in the VTPARMS parmlib member.

You can safely and immediately stop Externalization and Recall tasks using console commands. Using these commands, tasks will stop even when hardware errors are impeding the proper access to tape devices or output media. If all active tasks must be immediately stopped, you can restart the Backstore Engine. In all these scenarios, CA Vtape Virtual Devices are not affected.

Note: For more information about stopping tasks and restarting the Backstore Engine, see the SVTn STOP PTaskid and Restart Backstore console commands in the chapter “Console Commands.”