HP-UX requires at least one LUN device to be present with LUN or SCSI ID 0 in order to scan the corresponding SCSI bus. This LUN ID 0 device will be used as a controller device to query the underlying devices. On a typical SAN environment this will mean that the controller device will be used as an array controller device and that multiple of such devices will be present (depending on how the storage system is presenting the LUNs).
In no case should these devices be removed (unless it serves as a duplicate). Removing these devices will cause tools like ioscan to fail to see corresponding hardware/LUNs (reporting as NO_HW or offline state):
LUN ID 0 devices can be identified by looking at the LUN ID field in the ioscan output:
In the above example hardware device 64000/0xfa00/0xd or disk /dev/disk/disk4 has a LUN ID of 4000 which translates in HP-UX speak to 0 (4000 minus 4000).
In order to see the corresponding controller device:
How do we know that both the disk & controller device map to the same LUN? By looking at their WWID. Both devices will have the same WWID:
Very often the boot disk is the LUN ID 0 device because it is the first device to be ever presented to the host and the last one to be removed during a decommissioning. But this may not always be the case!
Multiple LUN ID 0 devices
In complex SAN environments it is not unusual to have multiple LUN ID 0 devices. If boot disks are not used, then special purpose LUNs should be used. For example: for small devices that will not be used for any other purpose but being a controller device. This also means that it should not be used to store any other data on (e.g. do not make it into a LVM physical volume or add it to a LVM volume group). It is therefore good practice to provision these LUNs with a very small size (e.g. 1 GB).