The Backstore Engine › Externalization Server Subgroup Queue Management

Externalization Server Subgroup Queue Management

CA Vtape has 32 Externalization Groups. Each of these groups is broken into three subgroups. Each of these subgroups represents a unique queue of Virtual Volumes that need to be externalized. CA Vtape tries to efficiently manage the 96 subgroup queues to maximize the amount of data externalized while using the minimum amount of available system resources.

The Externalization Server checks the status of the subgroup queues in the following circumstances:

• A Virtual Volume is dismounted
• The SVTnD0908I cache warning threshold message is issued
• An Externalization Subgroup Queue is released
• Five minutes have passed
• The SVTn CHECK BACKstore console command is issued

If multiple Externalization Subgroup Queues have been released, the following factors are used to determine which queue should be processed first:

Drives-In-Use

CA Vtape will not start an Externalization subtask to a subgroup queue if there are not enough available physical drives or if the number of drives in use by Externalization would then exceed the threshold value established by the console command SVTn SET MAXDRIVES=nn.

Backstore-Priority

The BackstorePriority is a parmlib attribute assigned within the Group Definitions determines which group's Externalization Subgroup queues should be processed first.

Threshold-Priority

This is a dynamically calculated value based on cache utilization and the setting of the MB_Threshold attribute in the Group Definitions. It is used to determine the number of subtasks which will be started for a subgroup queue. Subgroup cache utilization divided by the MB_Threshold, minus the number of currently active subtasks for this queue, determines the Threshold-Priority or the number of additional subtasks required.

For example, if a subgroup queue currently has two active subtasks, still occupies 10000 MBs of cache, and has a MB_Threshold setting of 2000 MBs, its Threshold-Priority would be 3: int((10000/2000)) -2 = 3.

If Threshold Priority is zero, no additional subtasks are required to serve this queue.

Cache-Utilization

This is the amount of DASD buffer space occupied by the Virtual Volumes held in a subgroup queue.

Drives-Required

This value is determined by the Group Definitions in parmlib. The PRIMARY, DUPLEX and EXPORT esoteric attributes determine if a subgroup queue will require one, two, or three tape drives for Externalization.

The Backstore Engine selects the queue with the highest Backstore-Priority, Threshold-Priority, and Cache-Utilization, that has the lowest Drives-Required.