Draft Document for Review February 7, 2020 3:15 pm 8464axJES3.fm DFSMS VOLREF processing Fetch messages JES3 allocation and mounting 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. 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 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.  Appendix C. JES3 examples and information  843