The Parameter Library (PARMLIB) › VTPOOLS Parmlib Member › VolumePools Section

VolumePools Section

This section contains the description of the VolumeSize attribute and its valid value.

VolumeSize=

Defines the maximum size of the Virtual Volumes in a Virtual Volume Pool. This attribute can be added to each Virtual Volume Pool. A different size can be defined in each pool.

During customization a default Virtual Volume Size is entered into an ISPF panel and initialized into the Local VCAT. Pools that do not have a VolumeSize= attribute coded use the default size. The default size can be displayed with the SVT1 Display Status console command.

When a scratch mount is performed, the Virtual Volume chosen for the mount is assigned a maximum size. If the VolumeSize= attribute is updated with a different value, active Virtual Volumes in the modified pool continue to use the size they were assigned during their scratch mount. The maximum size does not change until they scratch and are reused.

To use this attribute, all CA Vtape Subsystems in the same CA Vtape Complex must be running with their CacheManagement attributes set to Dynamic.

To define VSAM linear data sets to hold Virtual Volumes larger than 2 GBs, the VSAM extended addressability attribute must be defined in the SMS data class used for these allocations. The Data Class that CA Vtape uses can be displayed with the SVT1 Display Cache console command or by executing an IDCAMS LISTCAT for one of the Virtual Volume LDSs.

Some things to keep in mind about Volume Size is that the size or uncompressed capacity of a Virtual Volume can affect some aspects of CA Vtape performance.

Using a larger value for the Volume Size can allow applications to store more data per Virtual Volume. For applications writing large amounts of tape data, this results in fewer tape mounts allowing them to finish faster. This also results in fewer Virtual Volume DASD cache and Backstore Data Set catalog and tape management entries. This could also result in better Backstore and Recycle performance as fewer tape mounts, opens, and closes are performed per Backstore Tape.

Another aspect to consider is how uncompressed Virtual Volume capacities might affect the number of Virtual Volumes that can concurrently reside in the same amount of DASD cache space. For example, 500GB of DASD cache could probably house 225 to 250 fully utilized 2GB Virtual Volumes. Increasing VolumeSize= to 16GB could reduce the total number of Virtual Volumes housed and cache to 30 fully utilized 16GB Virtual Volumes. Without adding more DASD cache space, Virtual Volumes might need to be Externalized and Recalled more frequently. Finally while a 2GB Virtual Volume might only take a few seconds to RECALL, a 16GB Virtual Volume might take 5 to 6 times longer, delaying the time to mount.

Valid Values: DEFAULT, 400M, 800M, 2G, 4G, 8G, 16G

Section: <VolumePoolDefinitions>

Feature: Volume Pooling

Note: VolumeSize= allows the uncompressed capacity of a Virtual Volume to exceed the Default Virtual Volume Size. Default Virtual Volume Size is the maximum uncompressed capacity supported for Static Cache. You cannot convert Virtual Volumes with uncompressed capacities larger than the Default Virtual Volume Size back to Static Cache. These volumes are lost as a Static Cache Subsystem cannot mount the volumes. The volumes must be carefully analyzed and deleted before attempting to convert back to Static Cache.