HP Hitachi Dynamic Link Manager Software User Manual

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2. HDLM Functions

2.7 Performing Failovers and Failbacks Using Path Switching

When the system contains multiple paths to an LU and an error occurs on the path that

is currently being used, HDLM can switch to another functional path, so that the

system can continue operating. This is called a failover.
If a path in which an error has occurred recovers from the error, HDLM can then switch

back to that path. This is called a failback.
Two types of failovers and failbacks are available:

• Automatic failovers and failbacks
• Manual failovers and failbacks

Failovers and failbacks switch which path is being used and also change the statuses

of the paths. A path status is either online or offline. An online status means that the

path can receive I/Os. On the other hand, an offline status means that the path cannot

receive I/Os. A path will go into the offline status for the following reasons:

• An error occurred on the path.
• A user intentionally placed the path offline by using the Path Management

window in the HDLM GUI.

• A user executed the HDLM command's

offline

operation.

For details on the

offline

operation, see 6.4 offline (Places Paths Offline).

• Hardware, such as cables or HBAs, has been removed.

For details on path statuses and the transitions of those statuses, see 2.7.3 Path Status

Transition.

2.7.1 Automatic Path Switching

The following describes the automatic failover and failback functions, which

automatically switch a path.

(1) Automatic Failovers

If you detect an error on the path that is currently being used, you can continue to use

the system by having the status of that path automatically changed to offline, and then

automatically have the system switch over to another online path. This functionality is

called automatic failover. Automatic failovers can be used for the following levels of

errors:
Critical

A fatal error that might stop the system.