I've just realized that this VM has a strange storage configuration.
It's using LVM thin pools, which I don't use anywhere else.
Someone else set this up, and I think I've literally never used pools before.
Some time ago, the pool ran out of space, and I ran LVM repair on it.
It seems very possible that's the issue.
A latent problem might've been tickled by pg_upgrade --link.
That said, the relevant table is the active "alarms" table, and it would've
gotten plenty of DML with no issue for months running v13.
Feel free to dismiss this report if it seems dubious.
--
Justin
