How to create a 2-nodes HA cluster in Proxmox using Qdevice

Luca Dell'Oca,
Proxmox uses corosync for its clustering technology. The solution requires an odd number of cluster nodes. In production environments, this is easy to achieve by deploying at least 3 nodes, and any following expansion will add 2 nodes each time.
But there are situations like a home lab, where people would like to use the absolute minimum numbers of nodes, that is 2. But then, in a failure like a network segmentation each node would believe it’s the one alive and that it was the other node that went offline. This situation is called split brain: there is no majority vote between the nodes, and so there’s no way to promote any of them to be the primary one.
For home labs, we can create a machine running the Qdevice, a witness that contributes to the cluster vote but without running the whole PVE solution.
 
First, I create a Linux machine. Depending on the situation, this can be a virtual machine (please don’t put it inside the cluster for which it has to be a witness!), a raspberry PI, or a proper physical server. In a lab, this is usually a virtual machine.
In my lab I create a Ubuntu machine, and after installing and configuring it, I install the Qdevice component:
sudo apt install corosync-qnetd corosync-qdevice
Then, because in my case it’s Ubuntu, I need to enable root login and set a password:
sudo sed -i 's/#PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config
sudo systemctl restart ssh
sudo passwd
Then, on the two Proxmox nodes I also install the qdevice components:
sudo apt install corosync-qdevice
This is not enough. If I check the status of the cluster using the pvecm status command I see this:
As you can notice, there are only the two PVE nodes.
To add the qdevice to the cluster, in one of the proxmox nodes I run this command:

pvecm qdevice setup 172.27.217.181
Where .181 is the ip address of the qdevice. If I then check again the status of the cluster, I see this:

We now have 3 nodes, each bringing one vote, so that we can have a quorum of 2 out of 3, avoiding split brain.
Interesting fact, if at some point a cluster has an odd number of PVE nodes plus the qdevice, this one has two votes, to maintain an odd amount of votes.
Tech