From Proxmox 4.0 and later, the High Availability feature has been completely redesigned from the ground up making it much simpler to configure and use. There are a few requirements that the environment must meet before configuring Proxmox HA. They are as follows:
Minimum three nodes
Shared storage
Fencing
HA must be configured in a cluster with a minimum of three nodes because with three nodes or more, achieving a Quorum is possible. A Quorum is the minimum number of votes required for a Proxmox cluster operation. This minimum number is the total vote by a majority of the nodes. For example, in a cluster of three Proxmox nodes, a minimum vote of two Proxmox nodes is required to form a Quorum. Or, in a cluster with eight nodes, a minimum vote of five Proxmox nodes is required to form a Quorum. With just two nodes, the ratio of vote remains at 1:1, so no Quorum is possible.
