How a frozen `ls` turned into swap on bcachefs

I’ve run bcachefs as my desktop root filesystem for a couple of years now. Five devices (one NVMe as an SSD tier, four spinning disks as the cold tier) with data written to the fast tier and quietly migrated to the HDDs with compression in the background. It is a genuinely nice setup to live on: you get SSD latency for the working set and HDD capacity for everything else, and mostly you forget…