HP StorageWorks HSG80 ACS Solution Software V8.8 for IBM AIX Installation and Configuration Guide (AA-RV1HA-TE, March 2005)

Planning a Subsystem
46 HSG80 ACS Solution Software V8.8 for IBM AIX Installation and Configuration Guide
Assigning Unit Numbers
The controller keeps track of the unit with the unit number. The unit number can
be from 0–199 prefixed by a D, which stands for disk drive. A unit can be
presented as different LUNs to different connections. The interaction of a unit and
a connection is determined by several factors:
■ Failover mode of the controller pair
■ The ENABLE_ACCESS_PATH and PREFERRED_PATH switches in the
ADD UNIT (or SET unit) commands
■ The UNIT_OFFSET switch in the ADD CONNECTIONS (or SET connections) commands
■ The controller port to which the connection is attached
■ The SCSI_VERSION switch of the SET THIS_CONTROLLER/OTHER_CONTROLLER
command
The considerations for assigning unit numbers are discussed in the following
sections.
Matching Units to Host Connections in Transparent Failover Mode
In transparent failover mode, the ADD UNIT command creates a unit for host
connection to access and assigns it to either port 1 or port 2 of both controllers.
Unit numbers are assigned to ports as follows:
■ 0–99 are assigned to host port 1 of both controllers.
■ 100–199 are assigned to host port 2 of both controllers.
For example, unit D2 is on port 1, and unit D102 is available through port 2.
The LUN number that a host connection assigns to a unit is a function of the
UNIT_OFFSET switch of the
ADD (or SET) CONNECTIONS command. The relationship of
offset, LUN number, and unit number is shown in the following equation:
LUN number = unit number – offset
Where...
— LUN number is relative to the host (what the host sees the unit as)
— Unit number is relative to the controller (what the controller sees the unit
as)
If no value is specified for offset, then connections on port 1 have a default offset
of 0 and connections on port 2 have a default offset of 100.