Edit: Solved!
Unfortunately I'm not sure exactly what fixed it, because I was running btrfs commands like a madman.
Some combination of the following caused my 100GB labelled as UNREACHABLE to turn into UNUSED, which allowed that space to be written to as normal:
sudo btrfs balance start -v /
sudo btrfs filesystem defrag -v /
sudo btrfs filesystem defrag -v -r /
Also the tool btdu was incredibly helpful!
One of my linux boxes ran out of disk space, which surprised me, because it definitely didn't have that much stuff on it. When I check with df it says I have used 212GB on my / path:
$ df -h /
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 227G 212G 5.2G 98% /
So, I tried to use du to see if maybe a runaway log file was the cause, but this says I have only used 101GB on my / path (this is also more in-line with how much space I expected to be used):
$ du -h | sort -h
...
101G /
Using those commands with sudo outputs the same sizes.
My filesystem is Btrfs, I've tried the suggestion to use btrfs balance start ... but this actually INCREASED my disk usage to 99% lol
So my question is... what on earth is using the remaining 111GB?? Why can I not see it in du?