HiCommand Dynamic Link Manager (HDLM) for Windows Systems User's Guide (HIT5201-96001, January 2008)

HiCommand
®
Dynamic Link Manager (HDLM) for Windows
®
Systems User’s Guide 27
2.7 Failover and Failback Using Path Switching
When the system contains multiple paths to an LU and an error occurs in the path being
used, HDLM can switch to another normal path to allow the system to continue to operate.
This functionality is called failover.
When the path in which an error occurred recovers from the error, HDLM can switch the
paths so that the recovered path is used. This functionality is called failback.
Two types of failover and failback are available:
 Automatic path switching
 Manual path switching
Failover and failback change the path statuses and switch the paths. Path statuses are
classified into online statuses and offline statuses. Online statuses allow the path to
normally receive I/Os. Offline statuses prevent the path from receiving I/Os for the
following reasons:
 An error occurred in the path.
 A user placed the path offline using the Path Management window of the HDLM GUI.
 A user executed the HDLM command's offline operation.
For details on the offline operation, see section
7.4.
 Hardware such as cables or HBAs is removed.
For details on path statuses and the status transitions, see section
2.7.3.
Note: When using the TagmaStore AMS/WMS series, Thunder 9500V Series, or Thunder 9200,
make sure that you set the data share mode to ON (Used). For details on the data share
mode, see the manual for the TagmaStore AMS/WMS series, Thunder 9500V Series, or
Thunder 9200.
2.7.1 Automatic Path Switching
The following describes the automatic failover and automatic failback functionalities that
automatically switch a path.
2.7.1.1 Automatic Failover
If you detect an error for a path in use, you can keep operating the system by changing the
state to offline, and using other online paths. This functionality is called automatic failover.
Automatic failover is applicable to the following levels of errors that occur on a path:
 Critical: A fatal error that may stop the system.
 Error: A high-risk error that can be avoided by using failover or other countermeasures.
For details on error levels, see section
2.11.2.