D.5. SCSI Identification Numbers

Each device on a SCSI bus must have a unique SCSI identification number. Devices include host bus adapters, RAID controllers, and disks.

The number of devices on a SCSI bus depends on the data path for the bus. A cluster supports wide SCSI buses, which have a 16-bit data path and support a maximum of 16 devices. Therefore, there are sixteen possible SCSI identification numbers that can be assigned to the devices on a bus.

In addition, SCSI identification numbers are prioritized. Use the following priority order to assign SCSI identification numbers:

7 - 6 - 5 - 4 - 3 - 2 - 1 - 0 - 15 - 14 - 13 - 12 - 11 - 10 - 9 - 8 

The previous order specifies that 7 is the highest priority, and 8 is the lowest priority. The default SCSI identification number for a host bus adapter is 7, because adapters are usually assigned the highest priority. It is possible to assign identification numbers for logical units in a RAID subsystem by using the RAID management interface.

To modify an adapter's SCSI identification number, use the system BIOS utility. When the system boots, a message is displayed describing how to start the utility. For example, a user may be prompted to press [Ctrl]-[A] and follow the prompts for setting the SCSI identification number. At this point, it is possible to enable or disable the adapter's internal termination, as needed, and disable SCSI bus resets. Refer to Section D.3 SCSI Bus Termination for more information.

The prioritized arbitration scheme on a SCSI bus can result in low-priority devices being locked out for some period of time. This may cause commands to time out, if a low-priority storage device, such as a disk, is unable to win arbitration and complete a command that a host has queued to it. For some workloads, it is possible to avoid this problem by assigning low-priority SCSI identification numbers to the host bus adapters.