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Applies to SUSE Linux Enterprise High Availability Extension 15 SP2

10 Fencing and STONITH Edit source

Fencing is a very important concept in computer clusters for HA (High Availability). A cluster sometimes detects that one of the nodes is behaving strangely and needs to remove it. This is called fencing and is commonly done with a STONITH resource. Fencing may be defined as a method to bring an HA cluster to a known state.

Every resource in a cluster has a state attached. For example: resource r1 is started on alice. In an HA cluster, such a state implies that resource r1 is stopped on all nodes except alice, because the cluster must make sure that every resource may be started on only one node. Every node must report every change that happens to a resource. The cluster state is thus a collection of resource states and node states.

When the state of a node or resource cannot be established with certainty, fencing comes in. Even when the cluster is not aware of what is happening on a given node, fencing can ensure that the node does not run any important resources.

10.1 Classes of Fencing Edit source

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

Resource Level Fencing

Resource level fencing ensures exclusive access to a given resource. Common examples of this are changing the zoning of the node from a SAN fiber channel switch (thus locking the node out of access to its disks) or methods like SCSI reserve. For examples, refer to Section 11.10, “Additional Mechanisms for Storage Protection”.

Node Level Fencing

Node level fencing prevents a failed node from accessing shared resources entirely. This is usually done in a simple and abrupt way: reset or power off the node.

10.2 Node Level Fencing Edit source

In a Pacemaker cluster, the implementation of node level fencing is STONITH (Shoot The Other Node in the Head). The High Availability Extension 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.

10.2.1 STONITH Devices Edit source

To use node level fencing, you first need to have a fencing device. To get a list of STONITH devices which are supported by the High Availability Extension, run one of the following commands on any of the nodes:

root # stonith -L

or

root # crm ra list stonith

STONITH devices may be classified into the following categories:

Power Distribution Units (PDU)

Power Distribution Units are an essential element in managing power capacity and functionality for critical network, server and data center equipment. They can provide remote load monitoring of connected equipment and individual outlet power control for remote power recycling.

Uninterruptible Power Supplies (UPS)

A stable power supply provides emergency power to connected equipment by supplying power from a separate source if a utility power failure occurs.

Blade Power Control Devices

If you are running a cluster on a set of blades, then the power control device in the blade enclosure is the only candidate for fencing. Of course, this device must be capable of managing single blade computers.

Lights-out Devices

Lights-out devices (IBM RSA, HP iLO, Dell DRAC) are becoming increasingly popular and may even become standard in off-the-shelf computers. However, they are inferior to UPS devices, because they share a power supply with their host (a cluster node). If a node stays without power, the device supposed to control it would be useless. In that case, the CRM would continue its attempts to fence the node indefinitely while all other resource operations would wait for the fencing/STONITH operation to complete.

Testing Devices

Testing devices are used exclusively for testing purposes. They are usually more gentle on the hardware. Before the cluster goes into production, they must be replaced with real fencing devices.

The choice of the STONITH device depends mainly on your budget and the kind of hardware you use.

10.2.2 STONITH Implementation Edit source

The STONITH implementation of SUSE® Linux Enterprise High Availability Extension consists of two components:

pacemaker-fenced

pacemaker-fenced is a daemon which can be accessed by local processes or over the network. It accepts the commands which correspond to fencing operations: reset, power-off, and power-on. It can also check the status of the fencing device.

The pacemaker-fenced daemon runs on every node in the High Availability cluster. The pacemaker-fenced instance running on the DC node receives a fencing request from the pacemaker-controld. It is up to this and other pacemaker-fenced programs to carry out the desired fencing operation.

STONITH Plug-ins

For every supported fencing device there is a STONITH plug-in which is capable of controlling said device. A STONITH plug-in is the interface to the fencing device. The STONITH plug-ins contained in the cluster-glue package reside in /usr/lib64/stonith/plugins on each node. (If you installed the fence-agents package, too, the plug-ins contained there are installed in /usr/sbin/fence_*.) All STONITH plug-ins look the same to pacemaker-fenced, but are quite different on the other side, reflecting the nature of the fencing device.

Some plug-ins support more than one device. A typical example is ipmilan (or external/ipmi) which implements the IPMI protocol and can control any device which supports this protocol.

10.3 STONITH Resources and Configuration Edit source

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 6.3.2, “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 it 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

10.3.1 Example STONITH Resource Configurations Edit source

In the following, find some example configurations written in the syntax of the crm command line tool. To apply them, put the sample in a text file (for example, sample.txt) and run:

root # crm < sample.txt

For more information about configuring resources with the crm command line tool, refer to Chapter 8, Configuring and Managing Cluster Resources (Command Line).

Example 10.1: Configuration of an IBM RSA Lights-out Device

An IBM RSA lights-out device might be configured like this:

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 -Mandatory: alice
location l-st-bob st-ibmrsa-2 -Mandatory: bob
commit

In this example, location constraints are used for the following reason: There is always a certain probability that the STONITH operation is going to fail. Therefore, a STONITH operation on the node which is the executioner as well is not reliable. If the node is reset, it cannot send the notification about the fencing operation outcome. The only way to do that is to assume that the operation is going to succeed and send the notification beforehand. But if the operation fails, problems could arise. Therefore, by convention, pacemaker-fenced refuses to terminate its host.

Example 10.2: Configuration of a UPS Fencing Device

The configuration of a UPS type fencing device is similar to the examples above. The details are not covered here. All UPS devices employ the same mechanics for fencing. How the device is accessed varies. Old UPS devices only had a serial port, usually connected at 1200baud using a special serial cable. Many new ones still have a serial port, but often they also use a USB or Ethernet interface. The kind of connection you can use depends on what the plug-in supports.

For example, compare the apcmaster with the apcsmart device by using the stonith -t stonith-device-type -n command:

stonith -t apcmaster -h

returns the following information:

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

With

stonith -t apcsmart -h

you get the following output:

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

The first plug-in supports APC UPS with a network port and telnet protocol. The second plug-in uses the APC SMART protocol over the serial line, which is supported by many APC UPS product lines.

Example 10.3: Configuration of a Kdump Device

Kdump belongs to the Special Fencing Devices and is in fact the opposite of a fencing device. The plug-in checks if a Kernel dump is in progress on a node. If so, it returns true, and acts as if the node has been fenced.

The Kdump plug-in must be used in concert with another, real STONITH device, for example, external/ipmi. For the fencing mechanism to work properly, you must specify that Kdump is checked before a real STONITH device is triggered. Use crm configure fencing_topology to specify the order of the fencing devices as shown in the following procedure.

  1. Use the stonith:fence_kdump resource agent (provided by the package fence-agents) to monitor all nodes with the Kdump function enabled. Find a configuration example for the resource below:

    configure
      primitive st-kdump stonith:fence_kdump \
        params nodename="alice "\ 1
        pcmk_host_check="static-list" \
        pcmk_reboot_action="off" \
        pcmk_monitor_action="metadata" \
        pcmk_reboot_retries="1" \
        timeout="60"
    commit

    1

    Name of the node to be monitored. If you need to monitor more than one node, configure more STONITH resources. To prevent a specific node from using a fencing device, add location constraints.

    The fencing action will be started after the timeout of the resource.

  2. In /etc/sysconfig/kdump on each node, configure KDUMP_POSTSCRIPT to send a notification to all nodes when the Kdump process is finished. For example:

    /usr/lib/fence_kdump_send -i INTERVAL -p PORT -c 1 alice bob charlie [...]

    The node that does a Kdump will restart automatically after Kdump has finished.

  3. Write a new initrd to include the library fence_kdump_send with network enabled. Use the -f option to overwrite the existing file, so the new file will be used for the next boot process:

    root # dracut -f -a kdump
  4. Open a port in the firewall for the fence_kdump resource. The default port is 7410.

  5. To achieve that Kdump is checked before triggering a real fencing mechanism (like external/ipmi), use a configuration similar to the following:

    fencing_topology \
      alice: kdump-node1 ipmi-node1 \
      bob: kdump-node2 ipmi-node2

    For more details on fencing_topology:

    crm configure help fencing_topology

10.4 Monitoring Fencing Devices Edit source

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

Note
Note: Monitoring STONITH Resources

Monitor STONITH resources regularly, yet sparingly. For most devices a monitoring interval of at least 1800 seconds (30 minutes) should suffice.

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.

Checking the status of fencing devices once every few hours should usually be enough. The probability that a fencing operation needs to be performed and the power switch fails is low.

For detailed information on how to configure monitor operations, refer to Section 8.3.9, “Configuring Resource Monitoring” for the command line approach.

10.5 Special Fencing Devices Edit source

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

Warning
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

fence_kdump

This plug-in checks if a Kernel dump is in progress on a node. If so, it returns true, and acts as if the node has been fenced. The node cannot run any resources during the dump anyway. This avoids fencing a node that is already down but doing a dump, which takes some time. The plug-in must be used in concert with another, real STONITH device.

For configuration details, see Example 10.3, “Configuration of a Kdump Device”.

external/sbd

This is a self-fencing device. It reacts to a so-called poison pill which can be inserted into a shared disk. On shared-storage connection loss, it stops the node from operating. Learn how to use this STONITH agent to implement storage-based fencing in Chapter 11, Procedure 11.7, “Configuring the Cluster to Use SBD”. See also http://www.linux-ha.org/wiki/SBD_Fencing for more details.

Important
Important: external/sbd and DRBD

The external/sbd fencing mechanism requires that the SBD partition is readable directly from each node. Thus, a DRBD* device must not be used for an SBD partition.

However, you can use the fencing mechanism for a DRBD cluster, provided the SBD partition is located on a shared disk that is not mirrored or replicated.

external/ssh

Another software-based fencing mechanism. The nodes must be able to log in to each other as root without passwords. It takes a single parameter, hostlist, specifying the nodes that it will target. As it is not able to reset a truly failed node, it must not be used for real-life clusters—for testing and demonstration purposes only. Using it for shared storage would result in data corruption.

meatware

meatware requires help from the user to operate. Whenever invoked, meatware logs a CRIT severity message which shows up on the node's console. The operator then confirms that the node is down and issues a meatclient(8) command. This tells meatware to inform the cluster that the node should be considered dead. See /usr/share/doc/packages/cluster-glue/README.meatware for more information.

suicide

This is a software-only device, which can reboot a node it is running on, using the reboot command. This requires action by the node's operating system and can fail under certain circumstances. Therefore avoid using this device whenever possible. However, it is safe to use on one-node clusters.

Diskless SBD

This configuration is useful if you want a fencing mechanism without shared storage. In this diskless mode, SBD fences nodes by using the hardware watchdog without relying on any shared device. However, diskless SBD cannot handle a split brain scenario for a two-node cluster. Use this option only for clusters with more than two nodes.

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

10.6 Basic Recommendations Edit source

Check the following list of recommendations to avoid common mistakes:

  • Do not configure several power switches in parallel.

  • To test your STONITH devices and their configuration, pull the plug once from each node and verify that fencing the node does takes place.

  • Test your resources under load and verify the timeout values are appropriate. Setting timeout values too low can trigger (unnecessary) fencing operations. For details, refer to Section 6.3.9, “Timeout Values”.

  • Use appropriate fencing devices for your setup. For details, also refer to Section 10.5, “Special Fencing Devices”.

  • Configure one or more STONITH resources. By default, the global cluster option stonith-enabled is set to true. If no STONITH resources have been defined, the cluster will refuse to start any resources.

  • Do not set the global cluster option stonith-enabled to false for the following reasons:

    • Clusters without STONITH enabled are not supported.

    • DLM/OCFS2 will block forever waiting for a fencing operation that will never happen.

  • Do not set the global cluster option startup-fencing to false. By default, it is set to true for the following reason: If a node is in an unknown state during cluster start-up, the node will be fenced once to clarify its status.

10.7 For More Information Edit source

/usr/share/doc/packages/cluster-glue

In your installed system, this directory contains README files for many STONITH plug-ins and devices.

http://www.linux-ha.org/wiki/STONITH

Information about STONITH on the home page of The High Availability Linux Project.

http://www.clusterlabs.org/pacemaker/doc/
  • Pacemaker Explained: Explains the concepts used to configure Pacemaker. Contains comprehensive and very detailed information for reference.

http://techthoughts.typepad.com/managing_computers/2007/10/split-brain-quo.html

Article explaining the concepts of split brain, quorum and fencing in HA clusters.

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