SCSI - Device Identification

Device Identification

In modern SCSI transport protocols, there is an automated process for "discovery" of the IDs. SSA initiators "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value. Fibre Channel – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN (World Wide Name). For iSCSI, because of the unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example if an extra device is added.

On a parallel SCSI bus, a device (e.g. host adapter, disk drive) is identified by a "SCSI ID", which is a number in the range 0–7 on a narrow bus and in the range 0–15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator (host adapter). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let the operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID. The traditional SCSI ID for a host adapter is 7, as that ID has the highest priority during bus arbitration (even on a 16 bit bus).

The SCSI ID of a device in a drive enclosure that has a backplane is set either by jumpers or by the slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure's back plane delivers control signals to the drive to select a unique SCSI ID. A SCSI enclosure without a back plane often has a switch for each drive to choose the drive's SCSI ID. The enclosure is packaged with connectors that must be plugged into the drive where the jumpers are typically located; the switch emulates the necessary jumpers. While there is no standard that makes this work, drive designers typically set up their jumper headers in a consistent format that matches the way that these switches implement.

Note that a SCSI target device (which can be called a "physical unit") is often divided into smaller "logical units." For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit (more commonly, it is not that simple—virtual disk devices are generated by the subsystem based on the storage in those physical drives, and each virtual disk device is a logical unit). The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the logical unit number (LUN) identifies a disk device within the subsystem.

It is quite common, though incorrect, to refer to the logical unit itself as a "LUN." Accordingly, the actual LUN may be called a "LUN number" or "LUN id".

Setting the bootable (or first) hard disk to SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive while SCSI ID 3 is typically for a CD-ROM drive.