Compaq StorageWorks HSG60 ACS Solution Software V8.6 for Compaq Tru64 UNIX Installation and Configuration Guide

1–18 HSG60 ACS Solution Software Version 8.6 for Compaq Tru64 UNIX Installation and Configuration Guide
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.
What 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 follows:
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)
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). A 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 controller of a dual-redundant pair that initially accesses the unit is determined by the
PREFERRED_PATH switch of the ADD UNIT (or SET unit) command. 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.
Assigning Unit Numbers Depending on SCSI_VERSION
The SCSI_VERSION switch of the SET THIS/OTHER CONTROLLER command
determines how the Command Console LUN (CCL) is presented. There are two choices:
SCSI-2 and SCSI-3. The choice for SCSI_VERSION affects how certain unit numbers and
certain host connection offsets interact.