HN uses two—one live and one backup, so we can fail over if there's a hardware issue or we need to upgrade something.
It's a nice pattern. Just don't make them clones of each other, or they might go BLAM at the same time!
https://news.ycombinator.com/item?id=32049205
https://news.ycombinator.com/item?id=32032235
https://news.ycombinator.com/item?id=32028511 (<-- this is where it got figured out)
---
Edit: both these points are mentioned in the OP.
replies(2):