The Backstore Engine › Automatic Recall Source Switching

Automatic Recall Source Switching

The IO engine of CA Vtape reads and writes Virtual Volumes from the DASD Buffer (cache). The Backstore Engine stacks the Virtual Volumes to a Primary and optional Duplex dataset on tape. When USS Backstore is active, the Backstore Engine writes and reads Virtual Volumes to the Triplex copy on USS. These Backstore copies are the Recall Sources. When the licensed CA Vtape Peer-To-Peer Option is configured, Backstore copies on Remote systems are included as additional Recall Sources.

Automatic Recall Source Switching improves data availability by automatically switching between Recall Sources when a recall failure occurs. This feature is only available when Dynamic Cache Management is active.

When this feature is active CA Vtape maintains counters for the Recall Sources and establishes a hierarchy for automatically moving through the Sources when a recall attempt fails. The order in which the Sources are used can be influenced by the SVTn SET RECALL console command and the OffsiteBackstoreCopy and RecallAttemptsThreshold attributes in the Group Definitions.

Note: If the last Source becomes excluded, then the Virtual Volume is mounted empty, and the application will take an SX13 ABEND.

The OffsiteBackstoreCopy attribute determines if any Backstore copies should be excluded from Recall processing. This attribute facilitates sending copies of Virtual Volumes offsite so only the available copy is considered for Recalls.

RecallAttemptsThreshold limits the number of times a Recall is attempted from a single Source. Once the threshold is exceeded, if another eligible Source is present, Recall will attempt to use the alternate Source. Once all Sources have exceeded the threshold, the volume will be mounted as an empty Virtual Volume. If the attribute is set to zero, automatic switching is disabled. The Recall will always use the Backstore copy after applying the OffsiteBackstoreCopy exclusion rule.

We recommend setting RecallAttemptsThreshold=2.

Each time a recall is required the Integrated Catalog Facility (ICF) is interrogated for Primary and Duplex dataset entries. When USS Backstore is active, the USS file system is also interrogated regardless of whether the group is set to write Triplex copies. This is because the group may have been changed over time. The OffsiteBackstoreCopy works by excluding from consideration the data set entries that have been sent off-site. Next, the RecallAttemptsThreshold counters for the eligible Sources are examined. Any Source whose threshold has been exceeded will be eliminated from consideration. The recall order set by the SVTn SET RECALL console command will then be considered.

When the licensed CA Vtape P2P Option is configured, local Sources take precedence over remote Sources to minimize network traffic.