FENCED(8) cluster FENCED(8)NAMEfenced - the I/O Fencing daemon
SYNOPSISfenced [OPTIONS]
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.
Once started, fenced waits for the fence_tool(8) join command to be
run, telling it to join the fence domain: a group of nodes that will
fence group members that fail. When the cluster does not have quorum,
fencing operations are postponed until quorum is restored. If a failed
fence domain member is reset and rejoins the cluster before the remain‐
ing domain members have fenced it, the fencing is no longer needed and
will be skipped.
fenced uses the corosync cluster membership system, it's closed process
group library (libcpg), and the cman quorum and configuration libraries
(libcman, libccs).
The cman init script usually starts the fenced daemon and runs
fence_tool join and leave.
Node failure
When a fence domain member fails, fenced runs an agent to fence it.
The specific agent to run and the agent parameters are all read from
the cluster.conf file (using libccs) at the time of fencing. The fenc‐
ing 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 configu‐
ration) until fencing succeeds. Other systems such as DLM and GFS wait
for fencing to complete before starting their own recovery for a failed
node. 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.
Domain startup
When the fence domain is first created in the cluster (by the first
node to join it) and subsequently enabled (by the cluster gaining quo‐
rum) any nodes listed in cluster.conf that are not presently members of
the corosync cluster are fenced. The status of these nodes is unknown,
and to be safe they are assumed to need 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 fencing.
The following example illustrates why startup fencing is important.
Take a three node cluster with nodes A, B and C; all three have a GFS
file system 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 reboot, 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 cluster.conf
clean_start setting (or -c option) can be used to declare that all
nodes are in a clean or safe state to start. This setting/option
should not generally be used since it risks not fencing a node that
needs it, which can lead to corruption in other applications (like GFS)
that depend on fencing.
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 manu‐
ally reset the failed node, and then use fence_ack_manual(8) to tell
fenced to continue without fencing the node.
OPTIONS
Command line options override a corresponding setting in cluster.conf.
-D Enable debugging to stderr and don't fork.
See also fence_tool dump in fence_tool(8).
-L Enable debugging to log file.
See also logging in cluster.conf(5).
-g num groupd compatibility mode, 0 off, 1 on. Default 0.
-r path
Register a directory that needs to be empty for the daemon to
start. Use a dash (-) to skip default directories /sys/fs/gfs,
/sys/fs/gfs2, /sys/kernel/dlm.
-c All nodes are in a clean state to start. Do no startup fencing.
-s Skip startup fencing of nodes with no defined fence methods.
-q Disable dbus signals.
-j secs
Post-join fencing delay. Default 6.
-f secs
Post-fail fencing delay. Default 0.
-R secs
Number of seconds to wait for a manual override after a failed
fencing attempt before the next attempt. Default 3.
-O path
Location of a FIFO used for communication between fenced and
fence_ack_manual.
-h Print a help message describing available options, then exit.
-V Print program version information, then exit.
FILEScluster.conf(5) is usually located at /etc/cluster/cluster.conf. It is
not read directly. Other cluster components load the contents into
memory, and the values are accessed through the libccs library.
Configuration options for fenced are added to the <fence_daemon /> sec‐
tion of cluster.conf, within the top level <cluster> section.
post_join_delay
is the number of seconds the daemon will wait before fencing any
victims after a node joins the domain. Default 6.
<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. Default 0.
<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. Default 0.
<fence_daemon clean_start="0"/>
override_path
is the location of a FIFO used for communication between fenced
and fence_ack_manual. Default shown.
<fence_daemon override_path="/var/run/cluster/fenced_override"/>
override_time
is the number of seconds to wait for administrator intervention
between fencing attempts following fence agent failures. Default
3.
<fence_daemon override_time="3"/>
Per-node fencing settings
The per-node fencing configuration is partly dependant on the specific
agent/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 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="1">
<device name="myswitch" foo="x"/>
</method>
</fence>
</clusternode>
<clusternode name="node2" nodeid="2">
<fence>
<method name="1">
<device name="myswitch" foo="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" agent="..." something="..."/>
</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="1">
<device name="myswitch" foo="x"/>
</method>
<method name="2">
<device name="another" bar="123"/>
</method>
</fence>
</clusternode>
<fencedevices>
<fencedevice name="myswitch" agent="..." something="..."/>
<fencedevice name="another" agent="..."/>
</fencedevices>
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.
fenced will run the agent for the device twice, once for each device
line, and both must succeed for fencing to be considered successful.
<clusternode name="node1" nodeid="1">
<fence>
<method name="1">
<device name="sanswitch1" port="11"/>
<device name="sanswitch2" port="11"/>
</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="1">
<device name="nps1" port="11" action="off"/>
<device name="nps2" port="11" action="off"/>
<device name="nps1" port="11" action="on"/>
<device name="nps2" port="11" action="on"/>
</method>
</fence>
</clusternode>
NOTES
Due to a limitation in XML/DTD validation, the name="" value within the
device or fencedevice section cannot start with a number.
Hardware-specific settings
Find documentation for configuring specific devices from the device
agent's man page.
SEE ALSOfence_tool(8), fence_ack_manual(8), fence_node(8), cluster.conf(5)cluster 2009-12-21 FENCED(8)
