The Backstore Engine › Subgroup Automation

Subgroup Automation

When Virtual Volumes are created or updated, the space they occupy on DASD cannot be reassigned or used until they have been Externalized by an active Externalization Server. If DASD buffer usage reaches 100%, Virtual Volumes cannot be mounted or recalled until:

• Virtual Volumes are Externalized.
• Virtual Volumes that were previously Externalized and are currently in use are closed and their space freed.
• More space is added to the DASD buffer.

The console command, SVTn Display Groups, provides a concise display of cache utilization information.

Note: For more information on the display commands, see the chapter “Console Commands” in the Administration Guide.

The ISPF Interface Tape Devices and Virtual Volumes options also display DASD buffer information.

Note: For more information on the ISPF Interface, see the chapter “ISPF Interface” in the Administration Guide.

AutomatedSubgroups is a parmlib attribute assigned within the Group Definitions. It determines which subgroup queues within a group are automatically held or released for Externalization based on DASD buffer utilization thresholds. The CacheWarningThreshold and CacheAutoReleaseHighThreshold attributes must both be greater than zero to enable subgroup automation.

The following parmlib attributes can be found in the DynamicOptions Section in the VTPARMS member.

• CacheWarningThreshold is a parmlib attribute that establishes the DASD buffer percentage value where alert messages are issued. These messages are issued once a minute and appear highlighted on the console (WTO DESC=7,11) until DASD buffer utilization subsequently decreases below the following value:
  • SVTnD0908I SVTS Cache Shortage - 085% Utilization Reached
  • SVTnD0912I SVTS Cache Shortage Relieved

  If the CacheWarningThreshold is zero, Subgroup Automation is disabled and no alert messages are issued. Use the SVTn SET BACKstore=Hold|Release console commands to manually hold or release all subgroup queues. Since no alert messages are issued, you should keep the subgroup queues released at all times. This will allow the Externalization Server to externalize Virtual Volumes as soon as they are dismounted and will maximize reusable DASD buffer space at the expense of more frequent physical tape mount requests.

  If the CacheWarningThreshold is greater than zero, Subgroup Automation can be enabled. When the Backstore Engine starts or restarts it places all groups and subgroups on hold status. When Subgroup Automation is not enabled, use the SVTn SET BACKstore=Hold|Release,Group=nn,SubGroup=x console command to hold or release subgroup queues.

• CacheAutoReleaseHighThreshold is a parmlib attribute that establishes the DASD buffer utilization percentage value where the queues for automated subgroups are to be released for processing by the Externalization Server. The following console message is issued when Externalization Server subgroup queues are released:

  SVTnD0916I Cache at 072% >= 065%, Automated subgroup queues released
  

  If the CacheAutoReleaseHighThreshold is set to zero, Subgroup Automation is disabled. The Externalization Server will not dynamically hold or release the Externalization subgroup queues. The SVTn SET BACKstore=Hold| Release,Group=nn,SubGroup=x console commands must be issued to hold or release all subgroup queues or specific subgroup queues as needed.

  When the DASD buffer utilization percentage is calculated, the percentage is rounded up to the next highest whole number. This means that a single DASD buffer LDS that needs to be externalized will cause the percentage to be equal to one. Consequently, setting CacheAutoReleaseHighThreshold=1 means that whenever there is anything to be Externalized, the automated subgroups are eligible for release.

• CacheAutoHoldLowThreshold is a parmlib attribute that establishes the DASD buffer utilization percentage value where automated subgroups are to be held, not processed by the Externalization Server. The following console message is issued when the subgroups are held:

  SVTnD0920I Cache at 020% <= 020%, Automated subgroup queues held
  

  Note: After the automated subgroups are released, as each subgroup queue is emptied (that is, the number of Virtual Volumes waiting for externalization reaches zero), the subgroup queue is automatically held. This policy helps minimize the number of Backstore mounts and drive allocations in scenarios where a subgroup queue is quickly externalized but the DASD buffer utilization does not reach the CacheAutoHoldLowThreshold.

• CacheAutomationSchedule is a parmlib attribute that defines up to four time periods in a 24-hour period when the Externalization Server is allowed to release automated subgroups. Outside of these time periods the Externalization Server will place all automated subgroup queues in hold status. The following console message is issued when automated subgroup queues are held:

  SVTnD0924I Cache at 054% Automated subgroup queues held due to automation schedule
  

  CA Vtape checks DASD buffer utilization once a minute.