Hitachi Dynamic Link Manager Software Users Guide for AIX (6.6) (T5208-96010, April 2011)

2. HDLM Functions
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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 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 2.7.3 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.
Error
A high-risk error, which can be avoided by performing a failover or some other
countermeasure.