Allocate and the Data Class DVC

Products

• Allocate DASD Space and Placement

Issue/Introduction

How does Allocate support the IBM DF/SMS DATA CLASS "DYNAMIC VOLUME COUNT (DVC)" parameter?

 

Resolution

 

The IBM DF/SMS DATA CLASS "DYNAMIC VOLUME COUNT (DVC)" parameter allows eligible data sets to automatically expand to multiple volumes. The IBM publication z/OS DFSMS Storage Administration Reference SC26-7402 has complete details on this parameter. In general, to be eligible for DVC processing, the data set has to be SMS-managed, and also be defined with a DATA CLASS that has the "SPACE CONSTRAINT RELIEF=Y" parameter specified.

When an eligible data set is defined with a DATA CLASS that has a DVC value greater than the current volume count, and space is exhausted on the current volume, then IBM End-of-Volume (EOV) processing will recognize the DVC as a candidate volume, and automatically allow the data set to expand to an additional volume. For non-VSAM SMS-managed data sets, CA Allocate has always recognized the existence of the DVC as a candidate volume, and does not enter the "EOV" ASR environment.

Allocate now recognizes DVC candidate volumes for eligible SMS-managed VSAM data sets.
This additional flexibility is available in the "EOV_VSAM" ASR environment.

If PLSOPT24 (N) is set in the Vkgparms member of the parmlib, the EOV_VSAM ASR environment will continue to be entered even if a DVC candidate volume is available. However, the ASR variable &CANDIDATE_VOLUMES (alias is &CAND_VOLS) will have a value of "Y" if there are any DVC candidate volumes available. To allow the DATA CLASS DVC parameter to perform the volume addition, the ASR does not need to have a SET &SG='SMSEOV' or a SET &POOLSUB='Y'. For example, an ASR code snippet like this:

    IF &VAMENVIR = 'EOV_VSAM' THEN
    IF &CAND_VOLS = 'Y' THEN EXIT CODE(0) 

will allow the DATA CLASS DVC parameter to add an additional volume to the data set. Existing ASR code that performs other processing can be left in place, but to allow the DATA CLASS DVC to perform the volume addition requires not having a SET &SG='SMSEOV' or a SET &POOLSUB='Y' present if &CAND_VOLS = 'Y'. Once the DVC candidate volumes have been used up (&CAND_VOLS = 'N'),  Allocate will continue to add volumes.

If PLSOPT24 (Y) is set in Vkgparms, the EOV_VSAM ASR environment will not be entered if a DVC candidate volume is available. This is useful in reducing Allocate overhead if the EOV_VSAM ASR does not need to be entered.

For DB2 linear data sets that are shared among multiple DB2 systems across a sysplex, Allocate Support is recommending that these data sets be defined with the DVC parameter, thus allowing IBM EOV processing to handle the adding of a volume to these data sets.