HSG80 ACS Solution Software V8.6 for Windows NT and Windows 2000 Installation and Configuration Guide

1–20 HSG80 ACS Solution Software Version 8.6 for Windows NT and Windows 2000 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.
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.
Assigning Unit Numbers Depending on SCSI_VERSION
The SCSI_VERSION switch of the SET THIS_CONTROLLER/OTHER_CONTROLLER command
determines how the 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.