Tuesday, 26 August 2014

RHEV and RHEL Clustering - Fencing without RHEVM - The script



Following up on the blog post from a few weeks ago (here) I finally got around to creating a fence script that allows for fencing a VM without an available RHEV manager.


I've placed the script in my GitHub HERE, you need to copy it to /usr/sbin and give it execution permission.


How does it work?

I replicated the fence_virtsh script and changed the code to add the necessary commands.
The list of hosts where the script checks for the VM is passed in the "ipaddr" field.

Since this is a custom fencing script you cannot configure it directly in Luci, you need to edit the cluster.conf file manually.

The fence device can be added to a node like this:

<clusternode name="server1" nodeid="1">
    <fence>
        <method name="RHEV-NOMGT">
            <device name="rhev-nomgt" port="Linux-Serv1"/>
        </method>
    </fence>
</clusternode>

The fence device itself is define like this:

<fencedevice agent="fence_rhev_nomgt" ipaddr="192.168.1.1,192.168.1.2" login="root" name="rhev-nomgt" passwd="password"/>

In the clusternode fence method definition, the "port" is the name of VM in the RHEV system.

The "ipaddr" parameter, in the fence device, is a list of the hostnames (or ip addresses) of the hypervisors where the VM can run separated by a comma. The login and password refer to the login and password of the root user on the hypervisors. I know, this is not very safe but the hypervisors don't allow the creation of other users and for my scenario this won't be an issue.

Example cluster.conf for a two-node cluster:


<?xml version="1.0"?>
<cluster config_version="1" name="RHEVCLUS">
        <clusternodes>
                <clusternode name="server1" nodeid="1">
                        <fence>
                                <method name="RHEV">
                                        <device name="rhev-nomgt" port="Linux-Serv1"/>
                                </method>
                        </fence>
                </clusternode>
                <clusternode name="server2" nodeid="2">
                        <fence>
                                <method name="RHEV">
                                        <device name="rhev-nomgt" port="Linux-Serv2"/>
                                </method>
                        </fence>
                </clusternode>
        </clusternodes>
        <cman expected_votes="1" two_node="1"/>
        <fencedevices>
                <fencedevice agent="fence_rhev_nomgt" ipaddr="172.18.56.251,172.18.56.252" login="root" name="rhev-nomgt" passwd="password"/>
              
        </fencedevices>
</cluster>


PLEASE TAKE NOTE:
This fencing method is intended as a fail-safe option when there is no other possible fencing option available and to ensure the cluster doesn't halt in specific situations where there RHEV Manager isn't available. This should no be used as a primary fencing method! And please note that it can cause issues in databases such as loss of data and file corruption. To use this method you should be fully aware of the risks.


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