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/

The code “btrfs_map_physical.c” for finding out the swap file offset for hibernation resume doesn't work on 32bits.

Change:

	fd = open(argv[optind], O_RDONLY);

For:

fd = open(argv[optind], O_RDONLY |O_LARGEFILE );

and compile using the following command line:

gcc -O2 -o btrfs_map_physical -D_FILE_OFFSET_BITS=64 btrfs_map_physical.c