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Fencing In the Cluster.

Fencing  is a device. It will allow the cluster software to power on, power off, or restart the node remotely.
Why Fencing –maintain Data Integrity.
Without Fencing – Data Duplication (or) Data Corruption may occur.
HOW Fencing -- When any one of the node is not responding (or) time exceed to cman fencing will happened. Fencing is the process of isolating a node of a computer cluster or disconnects the node from the cluster Shared Storage.

it is like hardware come under /dev/ devices from the base machine.
Like /dev/kvm ,Console Devices /dev/ipmi0, or HP ILO 
Referece Link: https://access.redhat.com/site/articles/28603

KVM Fencing
fence_virt  >  Fencing Agent for Virtual machine using VM Channel 
fence_xvm > Fencing Agent for Virtual machine using Multicast 

Method To Add Fence Device 
1)Fence Device > Add > Fence Virt (Multicast Mode) > Give Name : KVM Fencing 
2)choose node > Add Fence Method > Give Name: Method > Add Fence Instance > KVM_Fencing(xvm Virtual Machine Fencing)
For Cluster.conf file it will reflect in
Refer Link : http://www.studentcpu.com/2014/06/redhat-suite-high-availability-overview.html

            <method name="Method">
                    <device domain="node10" name="KVM_Fencing"/>

                <fencedevice agent="fence_xvm" name="KVM_Fencing"/>
3)Configure fence Daemon From Bash Machine.
bash-machine#fence_virt -c
#fence_xvm -H node10 (Command will trigger node10 to reboot)

# ccs_tool lsfence
Name             Agent
KVM_Fencing      fence_xvm
 #ccs_tool lsnode
Cluster name: MY_CLUSTER2, config_version: 22

Nodename                        Votes Nodeid Fencetype
node10                             1    1    KVM_Fencing
node11                             1    2    KVM_Fencing

Fencing Method Vs Fencing Agent : https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/GFS_6.0_Administration_Guide/s1-fence-methods.html

Fencing Reference : http://clusterlabs.org/wiki/Guest_Fencing

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