This is an old revision of the document!
https://www.cyberciti.biz/faq/linux-btrfs-fstab-entry-to-mount-filesystem-at-startup/
How to balance a btrfs (when running out of free space even when there is really free space)
https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-space-emergency-response/