Section 2.5. LUNs | Inside Windows Storage: Server Storage Technologies for Windows 2000, Windows Server 2003 and Beyond

2.5 LUNs

The storage units, located behind the controller in the storage subsystem depicted in Figure 2.2, need to be accessed in some fashion. These units are called LUN s, which is short for "logical unit numbers ." As far as a storage application or Windows NT is concerned , three separate identifiers work together to uniquely identify each LUN. Note that SCSI literature uses the phrase "per SCSI target." I prefer to use "per SCSI identifier" because a SCSI device can be both a target and an initiator. This distinction will become even more important with the advent of iSCSI, where a SCSI device can be an initiator in one case, but can itself be the target in another case.

A SCSI command is directed at a SCSI device that is uniquely identified by a set of triple values:

The SCSI bus (an HBA may support multiple buses, or a Windows NT server may have multiple HBAs installed). Windows NT supports up to 8 buses per HBA.
The SCSI device identifier on the bus . Windows NT supports up to 128 SCSI identifiers per bus.
The SCSI LUN identifier . Windows NT SP4 and higher versions of Windows NT support up to 254 LUNs per SCSI identifier. This is sometimes referred to as Large LUN support. Prior versions of Windows NT supported up to 8 LUNs per SCSI identifier. SCSI-2 specifies 8 LUNs per SCSI identifier. SCSI-3 specifies a 64-bit LUN value per SCSI identifier, but the actual number supported depends on the device implementation.

Support for this larger number (more than 8) of LUNs depends on the HBA driver that is providing the support, as well as the SCSI devices that are properly implementing the Report LUNs (SCSI) command. Newer drivers support the higher number by default. Older drivers may not support LUNs at all, or they may require Windows NT registry changes to support this feature. Windows NT discovers the presence of these devices by sending a Report LUNs command to each initial LUN (LUN 0). To enable Windows NT to work with more LUNs, the device must respond properly to this Report LUNs command.

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