Libvirt Fencing on a Physical KVM Host

The Red Hat High Availability Add-On ships with a number of different fencing agents for different hypervisors. Cluster nodes that are VMs running on a KVM/libvirt host require the software fencing device fence-virtd configured.

Our goal is to to configure STONITH agent fence_xvm in a RHEL cluster. For this we first need to configure libvirt fencing on our physical KVM host.

Installation

Our KVM server runs CentOS 7.

On the hypervisor, install the following packages:

[kvm]# yum install fence-virtd fence-virtd-libvirt fence-virtd-multicast

Create a shared secret key:

[kvm]# mkdir /etc/cluster
[kvm]# dd if=/dev/urandom of=/etc/cluster/fence_xvm.key bs=4k count=1

Configuration

Configure the fence_virtd daemon. The important things are to select the libvirt back end and the multicast listener.

Also, make sure to select the correct interface used for communication between the cluster nodes (it’s br0 in our case).

[kvm]# fence_virtd -c
Module search path [/usr/lib64/fence-virt]: 

Available backends:
    libvirt 0.3
Available listeners:
    multicast 1.2

Listener modules are responsible for accepting requests
from fencing clients.

Listener module [multicast]: 

The multicast listener module is designed for use environments
where the guests and hosts may communicate over a network using
multicast.

The multicast address is the address that a client will use to
send fencing requests to fence_virtd.

Multicast IP Address [225.0.0.12]: 

Using ipv4 as family.

Multicast IP Port [1229]: 

Setting a preferred interface causes fence_virtd to listen only
on that interface.  Normally, it listens on all interfaces.
In environments where the virtual machines are using the host
machine as a gateway, this *must* be set (typically to virbr0).
Set to 'none' for no interface.

Interface [br0]: 

The key file is the shared key information which is used to
authenticate fencing requests.  The contents of this file must
be distributed to each physical host and virtual machine within
a cluster.

Key File [/etc/cluster/fence_xvm.key]: 

Backend modules are responsible for routing requests to
the appropriate hypervisor or management layer.

Backend module [libvirt]: 

Configuration complete.

=== Begin Configuration ===
fence_virtd {
	listener = "multicast";
	backend = "libvirt";
	module_path = "/usr/lib64/fence-virt";
}

listeners {
	multicast {
		key_file = "/etc/cluster/fence_xvm.key";
		address = "225.0.0.12";
		interface = "br0";
		family = "ipv4";
		port = "1229";
	}

}

backends {
	libvirt {
		uri = "qemu:///system";
	}

}

=== End Configuration ===
Replace /etc/fence_virt.conf with the above [y/N]? y

Enable and start the service on the hypervisor:

[kvm]# systemctl enable fence_virtd
[kvm]# systemctl start fence_virtd

This is important, do not forget to open a firewall port UDP 1229 on the hypervisor:

[kvm]# firewall-cmd --permanent --add-port=1229/udp
[kvm]# firewall-cmd --reload

Copy the fence secret key /etc/cluster/fence_xvm.key to all cluster nodes. Make sure that the file name and the path are the same as on the hypervisor.

[kvm]# for i in $(seq 1 3);do \
  ssh node$i mkdir /etc/cluster; \
  scp /etc/cluster/fence_xvm.key node$i:/etc/cluster/;\
  done

Configure Fence Agent fence_xvm

This configuration applies to cluster nodes and not the hypervisor.

Our cluster nodes have RHEL High Availability Add-On configured. See here for more info.

Install fence-virt package on every cluster node.

[nodex]# yum install fence-virt
[nodex]# ls -Z /etc/cluster/fence_xvm.key
-rw-r--r--. root root unconfined_u:object_r:cluster_conf_t:s0 /etc/cluster/fence_xvm.key

Open a firewall port TCP 1229 on all cluster nodes:

[nodex]# firewall-cmd --permanent --add-port=1229/tcp
[nodex]# firewall-cmd --reload

Check fencing:

[nodex]# fence_xvm -o list
nfs        66bc6e9e-73dd-41af-85f0-e50b34e1fc07 on
node1      c6220f3a-f937-4470-bfae-d3a3f49e2500 on
node2      2711db33-da71-4119-85da-ae7b294d9d4a on
node3      632121f5-6e40-4910-b863-f4f16d7abcaf on

Try fencing one of the cluster nodes:

[node1]# fence_xvm -o off -H node2

Add a stonith resource to the pacemaker cluster:

[node1]# pcs stonith create fence_node1 fence_xvm \
  key_file="/etc/cluster/fence_xvm.key" \
  action="reboot" \
  port="node1" \
  pcmk_host_list="node1.hl.local"

The port is the name of the VM as seen by libvirt (virsh list), and pcmk_host_list contains the name of the cluster node.

This would create a stonith resource per cluster node.

Alternatively, a host map can be used:

[node1]# pcs stonith create fence_all fence_xvm \
  key_file="/etc/cluster/fence_xvm.key" \
  action="reboot" \
  pcmk_host_map="node1.hl.local:node1,node2.hl.local:node2,node3.hl.local:node3" \
  pcmk_host_list="node1,node2,node3" \
  pcmk_host_check=static-list

The command above would create a single stonith resource that can fence all cluster nodes.

References

https://access.redhat.com/solutions/917833

2 thoughts on “Libvirt Fencing on a Physical KVM Host

  1. It has worked. Great!!! Thanks a lot.
    I have got an error msg at my environment as below. I have omitted the action parameter and it has worked. I think there is an default action which will work.
    Another thing at my environment the selinux file context is as below. I haven’t modified the context.
    [[email protected] cluster]# ls -Z /etc/cluster/fence_xvm.key
    -rw-r–r–. root root unconfined_u:object_r:etc_t:s0 /etc/cluster/fence_xvm.key

    [[email protected] cluster]# pcs stonith create fence_all fence_xvm key_file=”/etc/cluster/fence_xvm.key” action=reboot pcmk_host_map=”node1.public.example.com:node1,node2.public.example.com:node2,node3.public.example.com:node3″ pcmk_host_list=”node1,node2,node3″ pcmk_host_check=static-list
    Error: stonith option ‘action’ is deprecated and should not be used, use pcmk_off_action, pcmk_reboot_action instead, use –force to override

    [[email protected] cluster]# pcs stonith create fence_all fence_xvm key_file=”/etc/cluster/fence_xvm.key” pcmk_host_map=”node1.public.example.com:node1,node2.public.example.com:node2,node3.public.example.com:node3″ pcmk_host_list=”node1,node2,node3″ pcmk_host_check=static-list

    [[email protected] cluster]# pcs stonith show
    fence_all (stonith:fence_xvm): Started node1.public.example.com
    [[email protected] cluster]#

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