Can I have a swapfile on btrfs?

It is possible to use a swap file on btrfs, but there are some considerations that need taking care of.

btrfs filesystem doesn't let to create snapshots if there is a working swap file on the subvolume. That means that it is highly recommended to place a swap file on a separate subvolume.

Lets assume that the current swap is already off, the / is on /dev/sda1 and Ubuntu is installed with / on @ subvolume and /home is on @home subvolume.

1. Mount /dev/sda1 to /mnt.

    sudo mount /dev/sda1 /mnt
   

If you run ls /mnt, you'll see @, @home and other subvolumes that may be there.

2. Create a new @swap subvolume.

    sudo btrfs sub create /mnt/@swap
   

3. Unmount /dev/sda1 from /mnt

    sudo umount /mnt
   

4. Create /swap directory where we plan to mount the @swap subvolume.

    sudo mkdir /swap
   

5. Mount the @swap subvolume to /swap.

    sudo mount -o subvol=@swap /dev/sda1 /swap
   

6. Create the swap file.

    sudo touch /swap/swapfile
   

7. Set 600 permissions to the file.

    sudo chmod 600 /swap/swapfile
   

8. Disable COW for this file.

    sudo chattr +C /swap/swapfile
   

9. Set size of the swap file to 4G as an example.

    sudo fallocate /swap/swapfile -l4g
   

10. Format the swapfile

    sudo mkswap /swap/swapfile
    

11. Turn the swap file on.

    sudo swapon /swap/swapfile
    

Now the new swap should be working.

You also need to update /etc/fstab to mount all this on boot. Add there two lines:

UUID=XXXXXXXXXXXXXXX /swap btrfs subvol=@swap 0 0
/swap/swapfile none swap sw 0 0

The UUID is the one of your /dev/sda1.

Comments and sugestions are welcome.