A single-initiator SCSI bus has only one node connected to it, and
provides host isolation and better performance than a multi-initiator
bus. Single-initiator buses ensure that each node is protected from
disruptions due to the workload, initialization, or repair of the other
nodes.
When using a single- or dual-controller RAID array that has multiple
host ports and provides simultaneous access to all the shared logical
units from the host ports on the storage enclosure, the setup of the
single-initiator SCSI buses to connect each cluster node to the RAID
array is possible. If a logical unit can fail over from one controller
to the other, the process must be transparent to the operating
system. Note that some RAID controllers restrict a set of disks to a
specific controller or port. In this case, single-initiator bus setups
are not possible.
A single-initiator bus must adhere to the requirements described in
Section A.3.1 SCSI Configuration Requirements.
To set up a single-initiator SCSI bus configuration, perform the
following steps:
Enable the onboard termination for each host bus adapter.
Enable the termination for each RAID controller.
Use the appropriate SCSI cable to connect each host bus
adapter to the storage enclosure.
Setting host bus adapter termination is done in the adapter BIOS
utility during system boot. To set RAID controller termination, refer to
the vendor documentation. Figure A-3 shows a
configuration that uses two single-initiator SCSI buses.
Figure A-4 shows the termination in a
single-controller RAID array connected to two single-initiator SCSI
buses.
Figure A-5 shows the termination in a
dual-controller RAID array connected to two single-initiator SCSI
buses.
SCSI devices must adhere to a number of configuration requirements
to operate correctly. Failure to adhere to these requirements
adversely affects cluster operation and resource availability.
The following is an overview of SCSI configuration
requirements:
Buses must be terminated at each end. Refer to Section A.3.2 SCSI Bus Termination for more information.
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. Refer to Section A.3.3 SCSI Bus Length for more
information.
All devices (host bus adapters and disks) on a bus must have
unique SCSI identification numbers. Refer to Section A.3.4 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
example, the utility prompts 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. Refer to Section A.3.2 SCSI Bus Termination and Section A.3.4 SCSI Identification Numbers for more information.
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 cause the bus to operate
incorrectly. Termination for a SCSI bus can be provided by the devices
connected to the bus or by external terminators, if the internal
(onboard) device termination can be disabled.
Testing has shown that external termination on HBAs that run at
speeds greater than 80MB/second 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 disconnected.
To disconnect a host bus adapter from a single-initiator bus,
first disconnect the SCSI cable from the RAID controller and then
from the adapter. This ensures that the RAID controller is not
exposed to any erroneous input.
Protect connector pins from electrostatic discharge while the
SCSI cable is disconnected by wearing a grounded anti-static wrist
guard and physically protecting the cable ends from contact with
other objects.
Do not remove a device that is currently participating in any
SCSI bus transactions.
To enable or disable an adapter's internal termination, use the
system BIOS utility. When the system boots, a message is displayed
describing how to start the utility. For example, many utilities
prompt users to press [Ctrl]-[A]. Follow
the prompts for setting the termination. At this point, it is also
possible to set the SCSI identification number, as needed, and disable
SCSI bus resets. Refer to Section A.3.4 SCSI Identification Numbers for more
information.
To set storage enclosure and RAID controller termination, refer to
the vendor documentation.
A SCSI bus must adhere to length restrictions for the bus
type. Buses that do not adhere to these restrictions do not operate
properly. The length of a SCSI bus is calculated from one terminated
end to the other and must include any cabling that exists inside the
system or storage enclosures.
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 connected 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.
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 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.
