Logical Unit Number(LUN) 0, a SCSI Requirement (Doc ID 1017707.1)

Last updated on JULY 17, 2023

Applies to:

Goal

This article is intended to explain the importance of having a Logical Unit Number(LUN) 0 on any storage array.

A few issues in the field have been reported, where, depending on the array, problems have been seen on hosts connected to arrays without a LUN 0. While this Technical Instruction shows examples of Sun StorEdge[TM] 3510 and SE6x20 arrays specifically, it can apply to other storage. However, that still needs to be verified.

LUN 0, is a Small Computer Systems Interface(SCSI) protocol requirement, and needs to exist for proper communication with the driver.

This requirement is explicitly stated in :

SCSI Architecture Model - 2 (SAM-2)

4.7.5 SCSI task router

The task router routes tasks and task management functions to the selected logical unit. Any task that is sent to a
logical unit that is not known to the task router is handled as described in 5.9.3. Any task management function that
is not sent to a specific logical unit shall be broadcast to all logical units known to the task router.

---
4.9.2 LUN 0 address

All SCSI devices shall accept LUN 0 as a valid address. For SCSI devices that support the hierarchical addressing
model the LUN 0 shall be the logical unit that an application client addresses to determine information about the
SCSI target device and the logical units contained within the SCSI target device.

To address the LUN 0 of a SCSI device the peripheral device address method shall be used.

▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬

SCSI Architecture Model - 3 (SAM-3)

4.7.5 SCSI task router

The task router routes tasks and task management functions to the selected logical unit. Any task that is sent to a
logical unit that is not known to the task router shall be routed to a default logical unit (e.g., LUN 0). Any task
management function that is not sent to a specific logical unit shall be broadcast to all logical units known to the
task router.

---
4.12.2 LUN 0 address

All SCSI devices shall accept LUN 0 as a valid address. For SCSI devices that support the hierarchical addressing
model the LUN 0 shall be the logical unit that an application client addresses to determine information about the
target and the logical units contained within the target.

To address the LUN 0 of a SCSI device the peripheral device address method shall be used.
---

Also, from Sun's very first Hardware RAID array, the RSM2000 / Sun StorEdge[TM] A3000 and onward, the array management software has always depended on LUN 0 being available. Not having a LUN 0 was unqualified and has typically always led to unexpected behavior.

As an example, with the Sun StorEdge [TM] 351x and SE99xx families of arrays that are able to present multiple (SCSI-) targets on a single host-channel, it is even more important to realize that every attached host MUST be able to communicate with a LUN 0 on EVERY presented target in order to ensure proper operation.

In some cases this requirement even leads to having to map multiple different LUN 0's on the same (SCSI-) target, most commonly when LUN security (World Wide Number(WWN)) filtering is being used.

Solution