Placement of the communications data set

Each TDMF session must use a unique communication data set (COMMDS) for reasons of history logging, audit trails, diagnostics, and messages. The COMMDS must be placed on a volume with low activity and this volume must not be defined to TDMF as a migration volume.

The COMMDS must not be placed on a volume that is subject to physical device reserves by other software subsystems. For this reason, a volume that contains other control data sets (for instance, CA-MIM, StorageTek) is not suitable.

TDMF periodically initiates a reserve macro for the device where the COMMDS exists. Other applications that use the device are affected if the reserve is not converted to a global ENQ (enqueue) by GRS or CA-MIM. The resource major name that is used by TDMF is TDMFRESV. The minor name is equal to the data set name.

The appropriate adjustments to MIM or GRS ensure that the reserve macro results in a physical device reserve or is converted to a global resource ENQ. See CA-MIM resource sharing and Global resource serialization.

The COMMDS can be defined as a generation data group (GDG), which can ease the tracking of these data sets. In this case, each new COMMDS generation would be defined before the master and agent jobs were submitted. Thus, reference is allowed by relative generation number zero in the JCL.

Specify the History data set through the GTDSOPTN job when GDGs are used. Reviewing migration status from the individual data sets can then begin. On the Past Sessions: Display Communication Data Set History window of the TDMF TSO Monitor. Individual generations can be selected for viewing.

Some benefits are obtained in a large migration session (many volumes and many agent systems) if the COMMDS is allocated on a PAV capable volume. TDMF completes each interval read and write as two concurrent I/O requests. One I/O request uses Cache Fast Write.