Installation guide

Section A.3:SCSI Bus Termination 161
• Buses must be terminated at each end. See Section A.3, SCSI Bus Termination for more informa-
tion.
• Buses must not extend beyond the maximum length restriction for the bus type. Internal cabling
must be included in the length of the SCSI bus. See Section A.4, SCSI Bus Length for more
information.
• All devices (host bus adapters and disks) on a bus must have unique SCSI identification numbers.
See Section A.5, SCSI Identification Numbers for more information.
• The Linux device name for each shared SCSI device must be the same on each cluster system.
For example, a device named /dev/sdc on one cluster system must be named /dev/sdc on
the other cluster system. One way to ensure that devices are named the same is by using identical
hardware for both cluster systems.
Use the system’s configuration utility to set SCSI identification numbers and enable host bus adapter
termination. When the system boots, a message is displayed describing how to start the utility. For ex-
ample, the utility will prompt the user to press
[Ctrl]-[A], and follow the prompts to perform a particular
task. To set storage enclosure and RAID controller termination, refer to the vendor documentation.
See Section A.3, SCSI Bus Termination and Section A.5, SCSI Identification Numbers for more infor-
mation.
See http://www.scsita.org and the following sections for detailed information about SCSI bus require-
ments.
A.3 SCSI Bus Termination
A SCSI bus is an electrical path between two terminators. A device (host bus adapter, RAID controller,
or disk) attaches to a SCSI bus by a short stub, which is an unterminated bus segment that usually
must be less than 0.1 meter in length.
Buses must have only two terminators located at opposing ends of the bus. Additional terminators,
terminators that are not at the ends of the bus, or long stubs will cause the bus to operate incorrectly.
Termination for a SCSI bus can be provided by the devices connected to the bus or by external termi-
nators, if the internal (onboard) device termination can be disabled.
Testing has shown that external termination on HBAs that run at speeds greater than 80MB/sec. does
not work reliably.
When disconnecting a device from a single-initiator SCSI bus follow these guidelines:
• Unterminated SCSI cables must not be connected to an operational host bus adapter or storage
device.
• Connector pins must not bend or touch an electrical conductor while the SCSI cable is discon-
nected.