HP StorageWorks HSG80 ACS Solution Software V8.8 for HP-UX Installation and Configuration Guide (AA-RV1FA-TE, March 2005)

Planning a Subsystem
50 HSG80 ACS Solution Software V8.8 for HP-UX Installation and Configuration Guide
An additional factor to consider when assigning unit numbers and offsets is SCSI
version. If the SCSI_VERSION switch of the
SET THIS_CONTROLLER/OTHER_CONTROLLER
command is set to SCSI-3, the CCL is presented as LUN 0 to every connection,
superseding any unit assignments. The interaction between SCSI version and unit
numbers is explained further in the next section.
In addition, the access path to the host connection must be enabled for the
connection to access the unit. See "Restricting Host Access in Transparent
Failover Mode", page 53.
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).
The unit would not be visible at all to a host connection with a unit offset of 18 or
greater, because that offset is not within the units range (unit number of 17 minus
offset of 18 is a negative number).
In addition, the access path to the host connection must be enabled for the
connection to access the unit. This is done through the
ENABLE_ACCESS_PATH switch of the
ADD UNIT (or SET unit) command.
The PREFERRED_PATH switch of the
ADD UNIT (or SET unit) command determines
which controller of a dual-redundant pair initially accesses the unit. Initially,
PREFERRED_PATH determines which controller presents the unit as Ready. The
other controller presents the unit as Not Ready. Hosts can issue a SCSI
Start Unit
command to move the unit from one controller to the other.