HP Hitachi Dynamic Link Manager Software User Manual

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2. HDLM Functions

2.8 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 executed the HDLM command's

offline

operation.

For details on the

offline

operation, see 6.4 offline (Places Paths Offline).

For details on path statuses and the transitions of those statuses, see 2.8.3 Path Status

Transition.
Notes

Switching a reserved path might take several seconds. A reserved path is switched

in the following cases:

• The reserved path is placed offline.
• An owner path is placed online when a path has been reserved while only

non-owner paths are online.

2.8.1 Automatic Path Switching

This section describes the automatic failover and automatic failback functions that

automatically switch paths.

(1) Automatic Failovers

If you detect an error in a path being used, you can keep operating the system by

changing the path state to offline, and using other online paths. This function is called

automatic failover. Automatic failovers can be used for the following levels of errors:
Critical