HP StorageWorks HSG80 ACS Solution Software V8.8 for OpenVMS Installation and Configuration Guide (AA-RV1PA-TE, March 2005)

Planning a Subsystem
36 HSG80 ACS Solution Software V8.8 for OpenVMS 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 Multiple-Bus Failover Mode
In multiple-bus failover mode, the ADD UNIT command creates a unit for host
connections to access. All unit numbers (0 through 199) are potentially visible on
all four controller ports, but are accessible only to those host connections for
which access path is enabled and which have offsets in the unit's range.
The LUN number a host connection assigns to a unit is a function of the
UNIT_OFFSET switch of the ADD
(or SET) CONNECTIONS command. The
default offset is 0. 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 (number the host sees the unit as)
— Unit number is relative to the controller (number the controller sees the
unit as)
For example, unit D7 would be visible to a host connection with an offset of 0 as
LUN 7 (unit number of 7 minus offset of 0). Unit D17 would be visible to a host
connection with an offset of 10 as LUN 7 (unit number of 17 minus offset of 10).