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8.3.2 Constraints Versus Clones
In Section 8.3.1, “Example STONITH Resource Configurations” (page 85) you learned
that there are several ways to configure a STONITH resource: using constraints clones
or both. The choice of which construct to use for configuration depends on several
factors (nature of the fencing device, number of hosts managed by the device, number
of cluster nodes, or personal preference).
In short: if clones are safe to use with your configuration and if they reduce the confi-
guration, then use cloned STONITH resources.
8.4 Monitoring Fencing Devices
Just like any other resource, the STONITH class agents also support the monitoring
operation which is used for checking status.
NOTE: Monitoring STONITH Resources
Monitoring STONITH resources is strongly recommended. Monitor them regu-
larly, yet sparingly.
Fencing devices are an indispensable part of an HA cluster, but the less you need to
utilize them, the better. Power management equipment is known to be rather fragile on
the communication side. Some devices give up if there is too much broadcast traffic.
Some cannot handle more than ten or so connections per minute. Some get confused
if two clients try to connect at the same time. Most cannot handle more than one session
at a time.
Checking the fencing devices once every couple of hours should be enough in most
cases. The probability that within those few hours there will be a need for a fencing
operation and that the power switch would fail is usually low.
For detailed information on how to configure monitor operations, refer to Procedure 4.2,
“Adding or Modifying Meta and Instance Attributes” (page 36) for the GUI approach
or to Section 5.8, “Configuring Resource Monitoring” (page 69) for the command line
approach.
88 High Availability Guide