|Red Hat Cluster Suite: Configuring and Managing a Cluster|
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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
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.