## DFSMS VOLREF processing 

DFSMS VOLREF  services are started during locate processing if VOL=REF=  is present in a DD statement for each data set that contains a volume reference to a cataloged data set. DFSMS VOLREF  services determine whether the data set referenced by a VOL=REF=  parameter is DFSMS managed. VOL=REF=  now maps to the same SG for a DFSMS managed data set, but not necessarily to the same volume. DFSMS VOLREF  services also collect information about the job’s resource requirements. 

## Fetch messages 

The TS3500 tape library supports the following features: 

GLYPH<SM590000> Identifies the DDs that are TS3500 tape library-managed mountable entries 

GLYPH<SM590000> Obtains the associated device pool names list 

GLYPH<SM590000> Selects the LDG that best matches the names list 

GLYPH<SM590000> Provides the LDG name to JES3 for setup 

GLYPH<SM590000> Indicates to JES3 that the mount is deferred until execution 

When tape library volumes are mounted and unmounted by the library, fetch messages to an operator are unnecessary and can be confusing. With this support, all fetch messages ( IAT5110 ) for TS3500 tape library requests are changed to be the non-action informational USES  form of the message. These messages are routed to the same console destination as other USES  fetch messages. The routing of the message is based on the UNITNAME . 

## JES3 allocation and mounting 

JES3 MDS controls the fetching, allocation, and mounting of the tape volumes that are requested in the JCL for each job to be run on a processor. The scope of MDS tape device support is complex-wide, unlike z/OS job resource allocation, whose scope is limited to one processor. 

Another difference between JES3 MDS allocation and z/OS allocation is that MDS considers the resource requirements for all the steps in a job for all processors in a loosely coupled complex. z/OS allocation considers job resource requirements one step at a time in the running processor. 

MDS processing also determines which processors are eligible to run a job based on resource availability and connectivity in the complex. 

z/OS allocation interfaces with JES3 MDS during step allocation and dynamic allocation to get the JES3 device allocation information and to inform MDS of resource deallocations. z/OS allocation is enhanced by reducing the allocation path for mountable volumes. 

JES3 supplies the device address for the tape library allocation request through a subsystem interface (SSI) request to JES3 during step initiation when the job is running under the initiator. This support is not changed from previous releases. 

DFSMS and z/OS provide all of the tape library support except for the interfaces to JES3 for MDS allocation and processor selection. 

JES3 MDS continues to select tape units for the tape library. MDS no longer uses the UNIT parameter for allocation of tape requests for tape library requests. DFSMS determines the appropriate LDG name for the JES3 setup from the SG and DC assigned to the data set, and replaces the UNITNAME  from the JCL with that LDG name. Because this action is after the ACS routine, the JCL-specified UNITNAME  is available to the ACS routine. 

