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fenced

NAME

fenced - the I/O Fencing daemon

SYNOPSIS

fenced [OPTION]...

DESCRIPTION

The fencing daemon, fenced, fences cluster nodes that have failed. Fencing a node generally means rebooting it or otherwise preventing it from writing to storage, e.g. disabling its port on a SAN switch. Fencing involves interacting with a hardware device, e.g. network power switch, SAN switch, storage array. Different "fencing agents" are run by fenced to interact with various hardware devices.

Software related to sharing storage among nodes in a cluster, e.g. GFS, usually requires fencing to be configured to prevent corruption of the storage in the presence of node failure and recovery. GFS will not allow a node to mount a GFS file system unless the node is running fenced. Fencing happens in the context of a cman/openais cluster. A node must be a cluster member before it can run fenced.

Once started, fenced waits for the 'fence_tool join' command to be run, telling it to join the fence domain: a group of nodes managed by the openais/cpg/groupd cluster infrastructure. In most cases, all nodes will join the fence domain after joining the cluster.

Fence domain members are aware of the membership of the group, and are notified when nodes join or leave. If a fence domain member fails, one of the remaining members will fence it. If the cluster has lost quorum, fencing won't occur until quorum has been regained. If a failed node is reset and rejoins the cluster before the remaining domain members have fenced it, the fencing will be bypassed.

Node failure

When a domain member fails, fenced runs an agent to fence it. The specific agent to run and the parameters the agent requires are all read from the cluster.conf file (using libccs) at the time of fencing. The fencing operation against a failed node is not considered complete until the exec'ed agent exits. The exit value of the agent indicates the success or failure of the operation. If the operation failed, fenced will retry (possibly with a different agent, depending on the configuration) until fencing succeeds. Other systems such as DLM and GFS will not begin their own recovery for a failed node until fenced has successfully completed fencing it. So, a delay or problem in fencing will result in other systems like DLM/GFS being blocked. Information about fencing operations will appear in syslog.

When a domain member fails, the actual fencing operation can be delayed by a configurable number of seconds (cluster.conf:post_fail_delay or -f). Within this time, the failed node could be reset and rejoin the cluster to avoid being fenced. This delay is 0 by default to minimize the time that other systems are blocked (see above).

Domain startup

When the domain is first created in the cluster (by the first node to join it) and subsequently enabled (by the cluster gaining quorum) any nodes listed in cluster.conf that are not presently members of the cman cluster are fenced. The status of these nodes is unknown, and to be on the side of safety they are assumed to be in need of fencing. This startup fencing can be disabled, but it's only truely safe to do so if an operator is present to verify that no cluster nodes are in need of fencing.

This example illustrates why startup fencing is important. Take a three node cluster with nodes A, B and C; all three have a GFS fs mounted. All three nodes experience a low-level kernel hang at about the same time. A watchdog triggers a reboot on nodes A and B, but not C. A and B boot back up, form the cluster again, gain quorum, join the fence domain, *don't* fence node C which is still hung and unresponsive, and mount the GFS fs again. If C were to come back to life, it could corrupt the fs. So, A and B need to fence C when they reform the fence domain since they don't know the state of C. If C *had* been reset by a watchdog like A and B, but was just slow in rebooting, then A and B might be fencing C unnecessarily when they do startup fencing.

The first way to avoid fencing nodes unnecessarily on startup is to ensure that all nodes have joined the cluster before any of the nodes start the fence daemon. This method is difficult to automate.

A second way to avoid fencing nodes unnecessarily on startup is using the cluster.conf:post_join_delay setting (or -j option). This is the number of seconds fenced will delay before actually fencing any victims after nodes join the domain. This delay gives nodes that have been tagged for fencing a chance to join the cluster and avoid being fenced. A delay of -1 here will cause the daemon to wait indefinitely for all nodes to join the cluster and no nodes will actually be fenced on startup.

To disable fencing at domain-creation time entirely, the -c option can be used to declare that all nodes are in a clean or safe state to start. The clean_start cluster.conf option can also be set to do this, but automatically disabling startup fencing in cluster.conf can risk file system corruption.

Avoiding unnecessary fencing at startup is primarily a concern when nodes are fenced by power cycling. If nodes are fenced by disabling their SAN access, then unnecessarily fencing a node is usually less disruptive.

Fencing override

If a fencing device fails, the agent may repeatedly return errors as fenced tries to fence a failed node. In this case, the admin can manually reset the failed node, and then use fence_ack_manual to tell fenced to continue without fencing the node.

CONFIGURATION FILE

Fencing daemon behavior can be controlled by setting options in the cluster.conf file under the section <fence_daemon> </fence_daemon>. See above for complete descriptions of these values. The delay values are in seconds; -1 secs means an unlimitted delay. The values shown are the defaults.

Post-join delay is the number of seconds the daemon will wait before fencing any victims after a node joins the domain.

  <fence_daemon post_join_delay="6"/>

Post-fail delay is the number of seconds the daemon will wait before fencing any victims after a domain member fails.

  <fence_daemon post_fail_delay="0"/>

Clean-start is used to prevent any startup fencing the daemon might do. It indicates that the daemon should assume all nodes are in a clean state to start.

  <fence_daemon clean_start="0"/>

Override-path is the location of a FIFO used for communication between fenced and fence_ack_manual.

  <fence_daemon override_path="/var/run/cluster/fenced_override"/>

Per-node fencing settings

The per-node fencing configuration can become complex and is largely specific to the hardware being used. The general framework begins like this:

  <clusternodes>

  <clusternode name="node1" nodeid="1">
          <fence>
          </fence>
  </clusternode>

  <clusternode name="node2" nodeid="2">
          <fence>
          </fence>
  </clusternode>

  ...
  </clusternodes>

The simple fragment above is a valid configuration: there is no way to fence these nodes. If one of these nodes is in the fence domain and fails, fenced will repeatedly fail in its attempts to fence it. The admin will need to manually reset the failed node and then use fence_ack_manual to tell fenced to continue on without fencing it (see override above).

There is typically a single method used to fence each node (the name given to the method is not significant). A method refers to a specific device listed in the separate <fencedevices> section, and then lists any node-specific parameters related to using the device.

  <clusternodes>

  <clusternode name="node1" nodeid="1">
          <fence>
             <method name="single">
                <device name="myswitch" hw-specific-param="x"/>
             </method>
          </fence>
  </clusternode>

  <clusternode name="node2" nodeid="2">
          <fence>
             <method name="single">
                <device name="myswitch" hw-specific-param="y"/>
             </method>
          </fence>
  </clusternode>

  ...
  </clusternodes>

Fence device settings

This section defines properties of the devices used to fence nodes. There may be one or more devices listed. The per-node fencing sections above reference one of these fence devices by name.

  <fencedevices>
          <fencedevice name="myswitch" ipaddr="1.2.3.4" .../>
  </fencedevices>

Multiple methods for a node

In more advanced configurations, multiple fencing methods can be defined for a node. If fencing fails using the first method, fenced will try the next method, and continue to cycle through methods until one succeeds.

  <clusternode name="node1" nodeid="1">
          <fence>
             <method name="first">
                <device name="powerswitch" hw-specific-param="x"/>
             </method>

             <method name="second">
                <device name="storageswitch" hw-specific-param="1"/>
             </method>
          </fence>
  </clusternode>

Dual path, redundant power

Sometimes fencing a node requires disabling two power ports or two i/o paths. This is done by specifying two or more devices within a method.

  <clusternode name="node1" nodeid="1">
          <fence>
             <method name="single">
                <device name="sanswitch1" hw-specific-param="x"/>
                <device name="sanswitch2" hw-specific-param="x"/>
             </method>
          </fence>
  </clusternode>

When using power switches to fence nodes with dual power supplies, the agents must be told to turn off both power ports before restoring power to either port. The default off-on behavior of the agent could result in the power never being fully disabled to the node.

  <clusternode name="node1" nodeid="1">
          <fence>
             <method name="single">
                <device name="nps1" hw-param="x" action="off"/>
                <device name="nps2" hw-param="x" action="off"/>
                <device name="nps1" hw-param="x" action="on"/>
                <device name="nps2" hw-param="x" action="on"/>
             </method>
          </fence>
  </clusternode>

Hardware-specific settings

Find documentation for configuring specific devices at http://sources.redhat.com/cluster/

OPTIONS

Command line options override corresonding values in cluster.conf.

-j secs

Post-join fencing delay

-f secs

Post-fail fencing delay

-c

All nodes are in a clean state to start.

-O

Path of the override fifo.

-D

Enable debugging code and don't fork into the background.

-V

Print the version information and exit.

-h

Print out a help message describing available options, then exit.

DEBUGGING

The fenced daemon keeps a circular buffer of debug messages that can be dumped with the 'fence_tool dump' command.

SEE ALSO

fence_tool(8), cman(8), groupd(8), group_tool(8)