12 Fencing and STONITH #

There are two classes of fencing: resource level and node level fencing. The latter is the primary subject of this chapter.

In a Pacemaker cluster, the implementation of node level fencing is STONITH (Shoot The Other Node in the Head). SUSE Linux Enterprise High Availability includes the stonith command line tool, an extensible interface for remotely powering down a node in the cluster. For an overview of the available options, run stonith --help or refer to the man page of stonith for more information.

# stonith -L

# crm ra list stonith

To set up fencing, you need to configure one or more STONITH resources—the pacemaker-fenced daemon requires no configuration. All configuration is stored in the CIB. A STONITH resource is a resource of class stonith (see Section 6.2, “Supported Resource Agent Classes”). STONITH resources are a representation of STONITH plug-ins in the CIB. Apart from the fencing operations, the STONITH resources can be started, stopped and monitored, like any other resource. Starting or stopping STONITH resources means loading and unloading the STONITH device driver on a node. Starting and stopping are thus only administrative operations and do not translate to any operation on the fencing device itself. However, monitoring does translate to logging in to the device (to verify that the device will work in case it is needed). When a STONITH resource fails over to another node it enables the current node to talk to the STONITH device by loading the respective driver.

STONITH resources can be configured like any other resource. For details how to do so with your preferred cluster management tool:

The list of parameters (attributes) depends on the respective STONITH type. To view a list of parameters for a specific device, use the stonith command:

stonith -t stonith-device-type -n

For example, to view the parameters for the ibmhmc device type, enter the following:

stonith -t ibmhmc -n

To get a short help text for the device, use the -h option:

stonith -t stonith-device-type -h

# crm < sample.txt

configure
primitive st-ibmrsa-1 stonith:external/ibmrsa-telnet \
params nodename=alice ip_address=192.168.0.101 \
username=USERNAME password=PASSW0RD
primitive st-ibmrsa-2 stonith:external/ibmrsa-telnet \
params nodename=bob ip_address=192.168.0.102 \
username=USERNAME password=PASSW0RD
location l-st-alice st-ibmrsa-1 -inf: alice
location l-st-bob st-ibmrsa-2 -inf: bob
commit

stonith -t apcmaster -h

STONITH Device: apcmaster - APC MasterSwitch (via telnet)
NOTE: The APC MasterSwitch accepts only one (telnet)
connection/session a time. When one session is active,
subsequent attempts to connect to the MasterSwitch will fail.
For more information see http://www.apc.com/
List of valid parameter names for apcmaster STONITH device:
        ipaddr
        login
        password
For Config info [-p] syntax, give each of the above parameters in order as
the -p value.
Arguments are separated by white space.
Config file [-F] syntax is the same as -p, except # at the start of a line
denotes a comment

stonith -t apcsmart -h

STONITH Device: apcsmart - APC Smart UPS
(via serial port - NOT USB!).
Works with higher-end APC UPSes, like
Back-UPS Pro, Smart-UPS, Matrix-UPS, etc.
(Smart-UPS may have to be >= Smart-UPS 700?).
See http://www.networkupstools.org/protocols/apcsmart.html
for protocol compatibility details.
For more information see http://www.apc.com/
List of valid parameter names for apcsmart STONITH device:
ttydev
hostlist

Like any other resource, the STONITH class agents also support the monitoring operation for checking status.

Fencing devices are an indispensable part of an HA cluster, but the less you need to use them, the better. Power management equipment is often affected by too much broadcast traffic. Some devices 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.

The probability that a fencing operation needs to be performed and the fencing device fails is low. For most devices, a monitoring interval of at least 1800 seconds (30 minutes) should suffice. The exact value depends on the device and infrastructure. STONITH SBD resources do not need a monitor at all. See Section 12.5, “Special Fencing Devices” and Chapter 13, Storage Protection and SBD.

For detailed information on how to configure monitor operations, refer to Section 6.10.2, “Configuring Resource Monitoring with crmsh” for the command line approach.

In addition to plug-ins which handle real STONITH devices, there are special purpose STONITH plug-ins.

Warning: For Testing Only

Some STONITH plug-ins mentioned below are for demonstration and testing purposes only. Do not use any of the following devices in real-life scenarios because this may lead to data corruption and unpredictable results:

• external/ssh

• ssh

suicide is the only exception to the “I do not shoot my host” rule.

Check the following list of recommendations to avoid common mistakes: