Recovering from a quorum failure

There are various reason why a Proxmox cluster can lose a quorum. For the cluster to operate correctly, a quorum must exist within the nodes. A quorum is established when the majority of the nodes are online. If 51% of the nodes go offline for whatever reason, a quorum will be lost, resulting in a cluster error. A Proxmox quorum relies on multicast. So if multicast gets disabled in the switch, the cluster can also lose a quorum. A manual misconfiguration in the cluster file can also cause loss of a quorum. When a quorum is lost, the following error messages will appear in log files under /var/log/corosync:

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corosync[9999]:  [QUORUM] Quorum provider: corosync_votequorum failed to initialize.
corosync[9999]:  [SERV  ] Service engine 'corosync_quorum' failed to load for reason
    'configuration error: nodelist...