[steve_rambo]

btrfs does reserve some space for exactly this issue, although it might not always be enough.

https://btrfs.readthedocs.io/en/latest/btrfs-filesystem.html

> GlobalReserve is an artificial and internal emergency space. It is used e.g. when the filesystem is full. Its total size is dynamic based on the filesystem size, usually not larger than 512MiB, used may fluctuate.