:compat-mode: legacy
= Configure Fencing =
== What is Fencing? ==
Fencing protects your data from being corrupted, and your application from
becoming unavailable, due to unintended concurrent access by rogue nodes.
Just because a node is unresponsive doesn't mean it has stopped
accessing your data. The only way to be 100% sure that your data is
safe, is to use fencing to ensure that the node is truly
offline before allowing the data to be accessed from another node.
Fencing also has a role to play in the event that a clustered service
cannot be stopped. In this case, the cluster uses fencing to force the
whole node offline, thereby making it safe to start the service
elsewhere.
Fencing is also known as STONITH, an acronym for "Shoot The Other Node In The
Head", since the most popular form of fencing is cutting a host's power.
In order to guarantee the safety of your data,
footnote:[If the data is corrupt, there is little point in continuing to make it available]
fencing is enabled by default.
[NOTE]
====
It is possible to tell the cluster not to use fencing, by setting the
*stonith-enabled* cluster option to false:
----
[root@pcmk-1 ~]# pcs property set stonith-enabled=false
[root@pcmk-1 ~]# crm_verify -L
----
However, this is completely inappropriate for a production cluster. It tells
the cluster to simply pretend that failed nodes are safely powered off. Some
vendors will refuse to support clusters that have fencing disabled. Even
disabling it for a test cluster means you won't be able to test real failure
scenarios.
====
== Choose a Fence Device ==
The two broad categories of fence device are power fencing, which cuts off
power to the target, and fabric fencing, which cuts off the target's access to
some critical resource, such as a shared disk or access to the local network.
Power fencing devices include:
* Intelligent power switches
* IPMI
* Hardware watchdog device (alone, or in combination with shared storage used
as a "poison pill" mechanism)
Fabric fencing devices include:
* Shared storage that can be cut off for a target host by another host (for
example, an external storage device that supports SCSI-3 persistent
reservations)
* Intelligent network switches
Using IPMI as a power fencing device may seem like a good choice. However,
if the IPMI shares power and/or network access with the host (such as most
onboard IPMI controllers), a power or network failure will cause both the
host and its fencing device to fail. The cluster will be unable to recover,
and must stop all resources to avoid a possible split-brain situation.
Likewise, any device that relies on the machine being active (such as
SSH-based "devices" sometimes used during testing) is inappropriate,
because fencing will be required when the node is completely unresponsive.
== Configure the Cluster for Fencing ==
. Install the fence agent(s). To see what packages are available, run `yum
search fence-`. Be sure to install the package(s) on all cluster nodes.
. Configure the fence device itself to be able to fence your nodes and accept
fencing requests. This includes any necessary configuration on the device and
on the nodes, and any firewall or SELinux changes needed. Test the
communication between the device and your nodes.
. Find the name of the correct fence agent: `pcs stonith list`
. Find the parameters associated with the device:
+pcs stonith describe pass:[<replaceable>agent_name</replaceable>]+
. Create a local copy of the CIB: `pcs cluster cib stonith_cfg`
. Create the fencing resource: +pcs -f stonith_cfg stonith create pass:[<replaceable>stonith_id
stonith_device_type [stonith_device_options]</replaceable>]+
+
Any flags that do not take arguments, such as +--ssl+, should be passed as +ssl=1+.
. Enable fencing in the cluster: `pcs -f stonith_cfg property set stonith-enabled=true`
. If the device does not know how to fence nodes based on their cluster node
name, you may also need to set the special *pcmk_host_map* parameter. See
`man pacemaker-fenced` for details.
. If the device does not support the *list* command, you may also need
to set the special *pcmk_host_list* and/or *pcmk_host_check*
parameters. See `man pacemaker-fenced` for details.
. If the device does not expect the victim to be specified with the
*port* parameter, you may also need to set the special
*pcmk_host_argument* parameter. See `man pacemaker-fenced` for details.
. Commit the new configuration: `pcs cluster cib-push stonith_cfg`
. Once the fence device resource is running, test it (you might want to stop
the cluster on that machine first):
+stonith_admin --reboot pass:[<replaceable>nodename</replaceable>]+
== Example ==
For this example, assume we have a chassis containing four nodes
and a separately powered IPMI device active on 10.0.0.1. Following the steps
above would go something like this:
Step 1: Install the *fence-agents-ipmilan* package on both nodes.
Step 2: Configure the IP address, authentication credentials, etc. in the IPMI device itself.
Step 3: Choose the *fence_ipmilan* STONITH agent.
Step 4: Obtain the agent's possible parameters:
----
[root@pcmk-1 ~]# pcs stonith describe fence_ipmilan
fence_ipmilan - Fence agent for IPMI
fence_ipmilan is an I/O Fencing agentwhich can be used with machines controlled by IPMI.This agent calls support software ipmitool (http://ipmitool.sf.net/). WARNING! This fence agent might report success before the node is powered off. You should use -m/method onoff if your fence device works correctly with that option.
Stonith options:
ipport: TCP/UDP port to use for connection with device
hexadecimal_kg: Hexadecimal-encoded Kg key for IPMIv2 authentication
port: IP address or hostname of fencing device (together with --port-as-ip)
inet6_only: Forces agent to use IPv6 addresses only
ipaddr: IP Address or Hostname
passwd_script: Script to retrieve password
method: Method to fence (onoff|cycle)
inet4_only: Forces agent to use IPv4 addresses only
passwd: Login password or passphrase
lanplus: Use Lanplus to improve security of connection
auth: IPMI Lan Auth type.
cipher: Ciphersuite to use (same as ipmitool -C parameter)
target: Bridge IPMI requests to the remote target address
privlvl: Privilege level on IPMI device
timeout: Timeout (sec) for IPMI operation
login: Login Name
verbose: Verbose mode
debug: Write debug information to given file
power_wait: Wait X seconds after issuing ON/OFF
login_timeout: Wait X seconds for cmd prompt after login
delay: Wait X seconds before fencing is started
power_timeout: Test X seconds for status change after ON/OFF
ipmitool_path: Path to ipmitool binary
shell_timeout: Wait X seconds for cmd prompt after issuing command
port_as_ip: Make "port/plug" to be an alias to IP address
retry_on: Count of attempts to retry power on
sudo: Use sudo (without password) when calling 3rd party sotfware.
priority: The priority of the stonith resource. Devices are tried in order of highest priority to lowest.
pcmk_host_map: A mapping of host names to ports numbers for devices that do not support host names. Eg. node1:1;node2:2,3 would tell the cluster to use port 1 for node1 and ports 2 and
3 for node2
pcmk_host_list: A list of machines controlled by this device (Optional unless pcmk_host_check=static-list).
pcmk_host_check: How to determine which machines are controlled by the device. Allowed values: dynamic-list (query the device), static-list (check the pcmk_host_list attribute), none
(assume every device can fence every machine)
pcmk_delay_max: Enable a random delay for stonith actions and specify the maximum of random delay. This prevents double fencing when using slow devices such as sbd. Use this to enable a
random delay for stonith actions. The overall delay is derived from this random delay value adding a static delay so that the sum is kept below the maximum delay.
pcmk_delay_base: Enable a base delay for stonith actions and specify base delay value. This prevents double fencing when different delays are configured on the nodes. Use this to enable
a static delay for stonith actions. The overall delay is derived from a random delay value adding this static delay so that the sum is kept below the maximum delay.
pcmk_action_limit: The maximum number of actions can be performed in parallel on this device Pengine property concurrent-fencing=true needs to be configured first. Then use this to
specify the maximum number of actions can be performed in parallel on this device. -1 is unlimited.
Default operations:
monitor: interval=60s
----
Step 5: `pcs cluster cib stonith_cfg`
Step 6: Here are example parameters for creating our fence device resource:
----
[root@pcmk-1 ~]# pcs -f stonith_cfg stonith create ipmi-fencing fence_ipmilan \
pcmk_host_list="pcmk-1 pcmk-2" ipaddr=10.0.0.1 login=testuser \
passwd=acd123 op monitor interval=60s
[root@pcmk-1 ~]# pcs -f stonith_cfg stonith
ipmi-fencing (stonith:fence_ipmilan): Stopped
----
Steps 7-10: Enable fencing in the cluster:
----
[root@pcmk-1 ~]# pcs -f stonith_cfg property set stonith-enabled=true
[root@pcmk-1 ~]# pcs -f stonith_cfg property
Cluster Properties:
cluster-infrastructure: corosync
cluster-name: mycluster
dc-version: 1.1.18-11.el7_5.3-2b07d5c5a9
have-watchdog: false
stonith-enabled: true
----
Step 11: `pcs cluster cib-push stonith_cfg --config`
Step 12: Test:
----
[root@pcmk-1 ~]# pcs cluster stop pcmk-2
[root@pcmk-1 ~]# stonith_admin --reboot pcmk-2
----
After a successful test, login to any rebooted nodes, and start the cluster
(with `pcs cluster start`).