HSG80 ACS Solution Software Version 8.7 for Compaq Tru64 UNIX Installation and Configuration Guide

Planning a Subsystem
1–5HSG80 ACS Solution Software Version 8.7 for Compaq Tru64 UNIX Installation and
Configuration Guide
1–5
What is Failover Mode?
Failover is a way to keep the storage array available to the host if one of the controllers
becomes unresponsive. A controller can become unresponsive because of a controller
hardware failure or, in multiple-bus mode only, due to a failure of the link between
host and controller or host-bus adapter. Failover keeps the storage array available to
the hosts by allowing the surviving controller to take over total control of the
subsystem.
There are two failover modes:
• Transparent, which is handled by the surviving controller and is invisible
(transparent) to the hosts.
• Multiple-bus, which is handled by the surviving controller and which is handled
by the hosts through either additional software or as a feature of the operating
system.
Either mode of failover can work with loop or fabric topology.
Transparent Failover Mode
Transparent failover mode has the following characteristics:
• Hosts do not know failover has taken place
• Units are divided between host ports 1 and 2
A unit or storageset is a physical or virtual device of the subsystem. It is typically
assigned a logical unit number (LUN) and is managed by the HSG80 controller and
presented to a server through the Fibre Channel bus and the server’s host bus adapter.
Disks that are set up as independent disks (JBODs) or RAIDsets are referred to as
storagesets.
In transparent failover mode, host port 1 of controller A and host port 1 of controller B
must be on the same Fibre Channel link. Host port 2 of controller A and host port 2 of
controller B must also be on the same Fibre Channel link. Depending on operating
system restrictions and requirements, the port 1 link and the port 2 link can be separate
links, or they can be the same link.
At any time, host port 1 is active on only one controller, and host port 2 is active on
only one controller. The other ports are in standby mode. In normal operation, both
host port 1 on controller A and host port 2 on controller B are active. A representative
configuration is shown in Figure 1–5. The active and standby ports share port identity,