A Proxmox cluster's honest requirements fit in one sentence: three nodes, a low-latency private link between them, and storage the surviving nodes can reach. Everything else (quorum, fencing, Ceph versus replication) is the reasoning behind those three. This guide covers what clustering gives you before HA even enters the picture, why quorum makes three the magic number, the corosync network rules people violate first, the three storage strategies, and what a failover actually looks like when a node dies at 3 a.m.