When a UPS runs out of battery, every node in an HA cluster should go down in a controlled order. Getting that right on Proxmox with NUT (Network UPS Tools) is harder than it looks.
The Problem
If every node in an HA cluster is connected to upsmon, every node receives the shutdown signal at the same time. Each one then tries to migrate its HA guests to the other nodes before powering off. Those migrations never finish, and the shutdown stalls.
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