Create Swapfile
To create a swapfile on btrfs filesystem.
Only works on kernel 5.0 or higher.
# truncate -s 0 /swapfile
# chattr +C /swapfile
# btrfs property set /swapfile compression none
# fallocate -l 512M /swapfile
# chmod 600 /swapfile
# mkswap /swapfile