Hitachi Dynamic Link Manager Software Users Guide for Linux

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
offline (Places Paths Offline) on
page 6-6.
For details on path statuses and the transitions of those statuses, see
Path
Status Transition on page 2-22.
Automatic Path Switching
The following describes the automatic failover and failback functions, which
automatically switch a path.
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.
For details on error levels, see Filtering of Error Information on page 2-35.
When the Thunder 9500V series, or Hitachi AMS/WMS series is being used,
HDLM will select the path to be used next from among the various paths that
access the same LU, starting with owner paths, and then non-owner paths.
For example, in
Figure 2-10 Path Switching on page 2-20, the owner
controller of an LU is CHA0, and access to the LU is made via only one path
(A). After that access path (A) is placed offline, the first choice for the
switching destination is the other path connected to CHA0 (B). If an error
also occurs on that path (B), then the next possibility for a path comes from
one of the two paths (C or D) connected to CHA1.
HDLM Functions
2-19
Hitachi Dynamic Link Manager (for Linux®) User Guide