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

1–4 HSG60 ACS Solution Software Version 8.6 for Compaq Tru64 UNIX Installation and Configuration Guide
Selecting a 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 hardware
failure, such as a controller or, in multiple-bus 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 enabling the surviving controller to take over total control of the subsystem.
There are two failover modes:
■ Transparent is handled by the surviving controller and is invisible to the hosts.
■ Multiple-bus is handled by the hosts.
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 storage unit is a physical or virtual device of the subsystem. It is assigned a
logical unit number (LUN) and is managed by the HSG60 controller. The unit is presented
to a server through the Fibre Channel bus and servers host bus adapter. Disks that are set
up as independent disks (JBODs) or RAIDsets are referred to as storagesets. Storagesets
are units.
In transparent failover mode, the controller A host port 1 and the controller B host port 1
must be on the same Fibre Channel link. Controller A host port 2 and controller B host
port 2 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 one 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, controller A
port 1 is active, and controller B port 2 is active. A representative configuration is shown
in Figure 1–3. The active and standby ports share port identity, enabling the standby port
to take over for the active one. If one controller fails, its companion controller (known as
the surviving controller) takes control by making both its host ports active, as shown in
Figure 1–4.