This section describes some guidelines for OpenVMS LUN 0.
The storage unit assigns LUN numbers using the lowest available number. The first storage unit volume that is assigned to a host is LUN 0, the next volume is LUN 1, and so on. When a storage unit volume is unassigned, the system reuses the LUN number that it had when assigning another volume to that host. You can verify the LUN number assignment with the DS CLI command showvolgrp -lunmap and modify the LUN number assignment with chvolgrp -lun.
OpenVMS considers the storage unit volume that becomes LUN 0 as the Command Console LUN (CCL), or pass-through LUN. The storage unit does not support CCL command functions. This storage unit volume (LUN 0 for OpenVMS) does not display when you issue the wwidmgr -show wwid AlphaServer console command. When OpenVMS is running and a UDID has been set for that storage unit volume, the storage unit volume LUN 0 displays as a GGA device type, not as a DGA device. Although OpenVMS does not strictly require a UDID for the CCL, the SHOW DEVICE GGA command displays CCL devices if you set a UDID. You can display the multiple paths and diagnose failed paths to the storage controller using the SHOW DEVICE/FULL GGA command.
Guidelines: The first storage unit volume that you assign to an OpenVMS system becomes LUN 0. However, the volume that you assign can be used by the system for only support capability and should be at the minimum size. An OpenVMS host cannot use the volume for any other purpose. Multiple OpenVMS hosts, even in different clusters, that access the same storage unit, can share the same storage unit volume as LUN 0, because there will be no other activity to this volume.
At the AlphaServer console, when you issue the wwidmgr -show wwid command, LUN 0 does not display. Only fibre-channel storage devices are listed in the output. The storage unit LUN 0 is presented as a CCL device, and therefore, is not shown. You can only see the LUN 0 from the operating system when a UDID had been assigned.