Installation guide

Appendix A. Supplementary Hardware Information 145
A cluster supports LVD (low voltage differential) buses. The maximum length of a single-
initiator LVD bus is 25 meters. The maximum length of a multi-initiator LVD bus is 12
meters. According to the SCSI standard, a single-initiator LVD bus is a bus that is con-
nected to only two devices, each within 0.1 meter from a terminator. All other buses are
defined as multi-initiator buses.
Do not connect any single-ended devices to an LVD bus; doing so converts the bus single-
ended, which has a much shorter maximum length than a differential bus.
A.3.4. 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 as-
signed 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 identifica-
tion number. At this point, it is possible to enable or disable the adapter’s internal termina-
tion, as needed, and disable SCSI bus resets. Refer to Section A.3.2 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.