The z/VM Agent for TDMF for z/OS is implemented as a VM server
virtual machine that can run multiple concurrent migration sessions.
Each migration session uses a single dedicated COMMDS (or SYSCOM data set) that is stored on an MVS formatted volume
(containing a valid VTOC) which is shared among all host systems that participate in a
migration.
When you allocate the COMMDS, special considerations must be taken when MVS is
running under VM or when a TDMF agent is run on z/VM. Also see MVS running under VM for details.
On z/VM, the TDMF Agent uses real Reserve/Release CCWs
to serialize access to the COMMDS. To provide a consistent serialization mechanism, the z/OS Master
and Agents must then also use Reserve/Release. Therefore, the TDMFRESV
resource for the COMMDS that is used in a migration with VM agents must not be converted to global
ENQs. For further information, refer to Global resource serialization.
If the TDMF Master on z/OS does not use Reserve/Release CCWs, the TDMF Server virtual
machine on z/VM issues message GTD1286S on its virtual
console:
GTD1286S Communications Dataset serialization error.
Note: Message GTD1286S will not be visible to the TDMF master on z/OS or the TDMF monitor running on
z/OS, as the TDMF Agent on z/VM cannot update the COMMDS because of the
serialization problem. However, the message is issued on the TDMF
Server virtual machine’s console. The console log can be obtained with the GETCONS command.
The COMMDS must be allocated on a volume that is defined as SHARED in the z/VM
system configuration (the CP SET SHARED command can be used to dynamically change this attribute for
a device). It is recommended that you place the COMMDS on a volume which is online and free on the
z/VM system. The volume is automatically attached to the
TDMF Server
virtual machine when a session is started by using the RDEV or VOLSER parameters of the SESSION
command. The volume is automatically detached after the last migration session that is using it
completes. Because real Reserve/Release is used to serialize access to the COMMDS, it is also highly
recommended that you do not put any other
data sets on the COMMDS volume. For the
same reason avoid placing multiple COMMDS used by concurrent migration sessions onto the same
volume.
Note: If you place the COMMDS on a fullpack minidisk defined for the z/VM Agent for TDMF Server Virtual machine, the underlying real device must be defined as
SHARED in the z/VM system configuration and the V suffix must be used on the MDISK statement in the
user directory in order for VM to issue real Reserve/Release CCWs.