[scottlamb]

> it had an extremely strange behavior (possibly relating to snapshots/CoW?) where once it was full, it stayed full forever until you rebooted.

Do you have Time Machine enabled? I think it uses snapshots, which explains why the filesystem stays full. I've hit this myself and was initially surprised to see rm not improving matters (possibly even making it worse) but it makes sense with snapshots. The working on reboot was a surprise. I'd put off fixing the machine for at least a week, and when I went to actually fix it, it was quite anticlimatic to just reboot and have it work. Maybe it checks for this condition on reboot and dumps Time Machine snapshots if so.

That was the less scary part of my macOS filesystem integrity worries. My full disk started when it was staging a full Time Machine backup after I got a dialog saying:

> Time Machine completed a verification of your backups on "my.nas.address". To improve reliability, Time Machine must create a new backup for you.

...for the Nth time. I don't know for certain if the problem is with Apple's software or with my NAS's (Synology) but these backups are clearly not as reliable as one would hope...