LUN ID 0 issue

Background

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):

# ioscan -funNC ctl

Class     I  H/W Path  Driver S/W State   H/W Type     Description
==================================================================
ctl       4  64000/0xfa00/0x1e   esctl   CLAIMED     DEVICE       HP      OPEN-XPXP7
                      /dev/pt/pt4
ctl      32  64000/0xfa00/0x113  esctl   CLAIMED     DEVICE       HP      OPEN-XPXP7
                      /dev/pt/pt32
ctl      42  64000/0xfa00/0x165  esctl   CLAIMED     DEVICE       HP      OPEN-XPXP7
                      /dev/pt/pt42
ctl      24  64000/0xfa00/0x239  esctl   CLAIMED     DEVICE       HP      OPEN-XPXP7
                      /dev/pt/pt24
ctl      19  64000/0xfa00/0x945  esctl   NO_HW       DEVICE       IBM     2145
                      /dev/pt/pt19
ctl      30  64000/0xfa00/0xa01  esctl   NO_HW       DEVICE       IBM     2145
                      /dev/pt/pt30
ctl      40  64000/0xfa00/0xabd  esctl   NO_HW       DEVICE       IBM     2145
                      /dev/pt/pt40
ctl      41  64000/0xfa00/0xabe  esctl   NO_HW       DEVICE       IBM     2145
                      /dev/pt/pt41

Identification

LUN ID 0 devices can be identified by looking at the LUN ID field in the ioscan output:

# ioscan -m lun

Class     I  Lun H/W Path  Driver  S/W State   H/W Type     Health  Description
======================================================================
ctl       2  64000/0xfa00/0xc    esctl   CLAIMED     DEVICE       online  HP      OPEN-XPXP7
             0/0/1/0.0x50060e8007e2b160.0x0
             0/0/2/0.0x50060e8007e2b170.0x0
             0/0/5/0.0x50060e8007e2b131.0x0
             0/0/4/0.0x50060e8007e2b121.0x0
                      /dev/pt/pt2
disk      4  64000/0xfa00/0xd    esdisk  CLAIMED     DEVICE       online  HP      OPEN-V
             0/0/1/0.0x50060e8007e2b160.0x4000000000000000 <----
             0/0/5/0.0x50060e8007e2b131.0x4000000000000000 <----
             0/0/2/0.0x50060e8007e2b170.0x4000000000000000 <----
             0/0/4/0.0x50060e8007e2b121.0x4000000000000000 <----
                      /dev/disk/disk4      /dev/disk/disk4_p2   /dev/rdisk/disk4     /dev/rdisk/disk4_p2
                      /dev/disk/disk4_p1   /dev/disk/disk4_p3   /dev/rdisk/disk4_p1  /dev/rdisk/disk4_p3
disk      5  64000/0xfa00/0xe    esdisk  CLAIMED     DEVICE       online  HP      OPEN-V
             0/0/1/0.0x50060e8007e2b160.0x4003000000000000
             0/0/5/0.0x50060e8007e2b131.0x4003000000000000
             0/0/2/0.0x50060e8007e2b170.0x4003000000000000
             0/0/4/0.0x50060e8007e2b121.0x4003000000000000
                      /dev/disk/disk5   /dev/rdisk/disk5
disk      6  64000/0xfa00/0xf    esdisk  CLAIMED     DEVICE       online  HP      OPEN-V
             0/0/1/0.0x50060e8007e2b160.0x4001000000000000
             0/0/5/0.0x50060e8007e2b131.0x4001000000000000
             0/0/2/0.0x50060e8007e2b170.0x4001000000000000
             0/0/4/0.0x50060e8007e2b121.0x4001000000000000
                      /dev/disk/disk6   /dev/rdisk/disk6
disk      7  64000/0xfa00/0x10   esdisk  CLAIMED     DEVICE       online  HP      OPEN-V
             0/0/1/0.0x50060e8007e2b160.0x4002000000000000
             0/0/5/0.0x50060e8007e2b131.0x4002000000000000
             0/0/2/0.0x50060e8007e2b170.0x4002000000000000
             0/0/4/0.0x50060e8007e2b121.0x4002000000000000
                      /dev/disk/disk7      /dev/disk/disk7_p2   /dev/rdisk/disk7     /dev/rdisk/disk7_p2
                      /dev/disk/disk7_p1   /dev/disk/disk7_p3   /dev/rdisk/disk7_p1  /dev/rdisk/disk7_p3
disk      9  64000/0xfa00/0x18   esdisk  CLAIMED     DEVICE       online  HP      OPEN-V
             0/0/1/0.0x50060e8007e2b160.0x4004000000000000
             0/0/5/0.0x50060e8007e2b131.0x4004000000000000
             0/0/2/0.0x50060e8007e2b170.0x4004000000000000
             0/0/4/0.0x50060e8007e2b121.0x4004000000000000
                      /dev/disk/disk9   /dev/rdisk/disk9

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:

# ioscan -funN -C ctl

Class     I  H/W Path  Driver S/W State   H/W Type     Description
==================================================================
ctl       2  64000/0xfa00/0xc  esctl   CLAIMED     DEVICE       HP      OPEN-XPXP7
                      /dev/pt/pt2

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:

# scsimgr get_attr -H 64000/0xfa00/0xd -a wwid

        SCSI ATTRIBUTES FOR LUN : 64000/0xfa00/0xd

name = wwid
current = 0x60060e8007e2b1000030e2b100006a00
default =
saved =

# scsimgr get_attr -H 64000/0xfa00/0xc -a wwid

        SCSI ATTRIBUTES FOR LUN : 64000/0xfa00/0xc

name = wwid
current = 0x60060e8007e2b1000030e2b100006a00
default =
saved =

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).