To elaborate a bit further - when things are fine, they're, well, fine. Unfortunately, the assumptions of how computers actually function go out of the window when enough things go wrong, like lightning striking the wrong thing. Things go weird sometimes, like the accepting TCP connections, but not sending responses. can't ping but can SSH in, or vice versa. In a cluster with automatic failover, you want to be sure the bad node is dead dead dead because a bad machine can cause harm to all the packets in the rack. (Eg HA with floating IPs and ARP takeover.) Thus the requirement is to forcefully disable the bad node, which is where the acronym STONITH comes from - Shoot The Other Node In The Head. (HA clusters with more than two nodes existed, but were rarer than 2-node clusters simply due to cost.) Before cloud computing was so pervasive, high availability was implemented with serial cables between in physical hardware and when one machine stopped responding, the equipment basically yanked its power cable out.