As Virtual Volumes are stacked, CA Vtape catalogs the location of the primary and duplex copies using a generated data set name that contains the virtual VOLSER.
The stacking is performed by one or more Externalization tasks, each one processing a subgroup queue. Each task is dedicated to a subgroup queue. When that queue is empty the task stops. A new task is started to process a different subgroup queue. Multiple tasks can be started to process the same subgroup queue, depending on the amount of data in that queue. The maximum number of tasks that can be active for a subgroup queue is eight.
You can control the number of concurrent tasks for a subgroup by setting a threshold parameter for the corresponding group. The total number of concurrent tasks can also be controlled by limiting the number of drives Externalization is allowed to use.
Note: When writing to a USS file system the number of concurrent tasks may also be controlled.
Limiting the number of concurrent tasks that can be active increases the amount of time it takes for Externalization to stack a given amount of Virtual Volume data. This limits the amount of DASD buffer space that Externalization can make available for reuse to support new virtual mount requests.
When a subgroup queue is empty and a task stops, its drive or drives and the corresponding tapes are released. The VOLSERs and the last-used file sequence numbers are saved for use when new work is queued for this subgroup. At that time, a task is started, the tapes are remounted, and the next file sequence number is used. This pattern is repeated until the tapes are full.
The parameters that control the Externalization tasks can be found in the Group Definitions in parmlib. These parameters can be modified and refreshed dynamically.
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