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 which 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.