Btrfs Swapfile

Tutorial

This tutorial contains the necessary steps to create a Btrfs swapfile. The reference system is running elementary OS 5.1 and a flat Btrfs layout. As a matter of preference, the commands here use the fish shell's syntax.

I have not been able to get hibernation working when using a Btrfs swapfile. You should consider a dedicated, encrypted swap partition if you desire this feature.

Mount the root Btrfs filesystem to create a subvolume.

sudo mount (df --output=source / | tail -n 1) /mnt

Create a dedicated Btrfs subvolume for swap in order to exclude the swapfile from snapshots.
```
sudo btrfs subvolume create /mnt/swap
Create subvolume '/mnt/swap'
```
Set the appropriate permissions on the swap subvolume so that only the owner, the root user in this case, has access to the subvolume.
```
sudo chmod 700 /mnt/swap
```
Create a directory at where the swap subvolume will be mounted.
```
sudo mkdir /swap
```

Add the subvolume to /etc/fstab.

echo (df --output=source / \
  | tail -n 1)" /swap btrfs defaults,noatime,subvol=swap 0 0" \
  | sudo tee -a /etc/fstab
/dev/mapper/sda2_crypt /swap btrfs defaults,noatime,subvol=swap 0 0

Now mount the swap subvolume according to the rules just add in fstab.
```
sudo mount /swap
```
Create an empty swapfile within the swap subvolume.
```
sudo truncate -s 0 /swap/swapfile
```
Disable Copy-on-Write for the swapfile.
```
sudo chattr +C /swap/swapfile
```

Make sure to disable compression on the swapfile.

sudo btrfs property set /swap/swapfile compression none

Allocate the file with as much space as there is RAM on the system.^[1]

sudo fallocate -l (free -h | awk 'NR == 2 {print $2}') /swap/swapfile

Only allow access to the swapfile by its owner, the root user, to prevent snooping.
```
sudo chmod 600 /swap/swapfile
```

Initialize the swapfile.

sudo mkswap /swap/swapfile
Setting up swapspace version 1, size = 7.8 GiB (8355049472 bytes)
no label, UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

Enable the swapfile!
```
sudo swapon /swap/swapfile
```
Add the swapfile to /etc/fstab so that systemd will initialize it automatically when booting the system.^[2]
```
echo "/swap/swapfile none swap defaults 0 0" | sudo tee -a /etc/fstab
```
Verify there are no errors in /etc/fstab.
```
sudo findmnt --verify --verbose
```
Set a lower swappiness in an attempt to improve performance.
This is described in the ArchWiki’s page on Swap in the Swappiness section. A lower setting as used here advises the kernel to avoid swapping.
echo vm.swappiness=10 | sudo tee /etc/sysctl.d/99-swappiness.conf vm.swappiness=10
This setting will be applied at the next reboot.

References

ArchWiki Btrfs - Swap file ArchWiki Swap - Swap file creation Debian Wiki Swap

Conclusion

You should now know all you need to know to make a Btrfs swapfile on Linux.