Man Linux: Main Page and Category List


       fenced - the I/O Fencing daemon


       fenced [OPTION]...


       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 also 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 truly safe to do so if
       an operator is present to verify that no cluster nodes are in  need  of

       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

       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

   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.


       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 unlimited 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"/>

       Override-time  is  the  amount  of  time  to  wait  for   administrator
       intervention after fencing has failed.  The default is 5 seconds.

         <fence_daemon override_time="10"/>

   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


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

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


       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.


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

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


   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.

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

   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">
                    <method name="first">
                       <device name="powerswitch" hw-specific-param="x"/>

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

   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">
                    <method name="single">
                       <device name="sanswitch1" hw-specific-param="x"/>
                       <device name="sanswitch2" hw-specific-param="x"/>

       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">
                    <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"/>

   Hardware-specific settings
       Find    documentation    for   configuring   specific   devices   at


       Command line options override corresponding 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.

       -T     Amount of time to  wait  for  administrator  intervention  after
              fencing has failed, in seconds.

       -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


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


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