BTRFS on kernel5.x supports swapfiles located on the partition.
The current version is not able to use a swapfile
commands to create:
cd /mnt/cache
truncate -s 0 swapfile
chattr +C swapfile
fallocate -l 16G swapfile
chmod 0600 swapfile
mkswap swapfile
swapon swapfile
cat /proc/swaps
actual: