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Pilot6
Asked: 2020-01-28 11:11:29 +0800 CST

Can I have a swapfile on btrfs?

  • 772

Modern Ubuntu versions are using a swap file instead of a swap partition by default.

Before the 5.0 Linux kernel it was not possible to place a swap file on a btrfs partition, btrfs file system could be damaged.

Now kernels support swap files on btrfs partitions. Can I use a swap file on Ubuntu installed on btrfs and what are possible problems?

btrfs swap

4 Answers

  1. Best Answer

    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.

    1. Create a new @swap subvolume.

       sudo btrfs sub create /mnt/@swap

    2. Unmount /dev/sda1 from /mnt

       sudo umount /mnt

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

       sudo mkdir /swap

    4. Mount the @swap subvolume to /swap.

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

    5. Create the swap file.

       sudo touch /swap/swapfile

    6. Set 600 permissions to the file.

       sudo chmod 600 /swap/swapfile

    7. Disable COW for this file.

       sudo chattr +C /swap/swapfile

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

       sudo dd if=/dev/zero of=/swap/swapfile bs=1M count=4096

    9. Format the swapfile

      sudo mkswap /swap/swapfile

    10. 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.

    Swap file can't be located on a btrfs raid of any sort.

    Comments and suggestions are welcome.

    • 18

  2. Note that for modern systems/laptops with NVME SSDs, you have /dev/nvme0n1p2 instead of /dev/sda1 and you should ignore /dev/nvme0n1p1 because that is your efi boot disk that your OS created when you installed it.

    In addition it should be recommended to add two mount options 'defaults' and 'noatime'. Defaults will automatically load the mount options for the drive (SSD, HDD). Noatime will prevent files being written if only opened:

    UUID=XXXXXXXXXXXXXXX /swap btrfs defaults,noatime,subvol=@swap 0 0
/swap/swapfile none swap sw 0 0
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  3. The popular answer is correct except that you should not use fallocate(1) to allocate space to the swapfile. It can create file-system 'holes' which are incompatible with swapfile usage (swap needs 100% contiguous space since a filesystem is not used within the swap space). You should use 'dd if=/dev/zero ...' instead. See the answer on this thread: fallocate vs dd for swapfile? The mkswap(8) and swapon(8) manpages both explicitly discourage the use of fallocate(1) to instantiate a swapfile on the filesystem.

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  4. If your partition is encrypted (LUKS), the mount point is on /dev/mapper for example:

    /dev/mapper/nvme0n1p5_crypt /swap btrfs defaults,noatime,subvol=@swap 0 0
/swap/swapfile none swap sw 0 0 here

    To get the uuids run the blkid command:

    /dev/mapper/nvme0n1p5_crypt: UUID="06c8c73c-1cc4-477b-a687-6c21697d645d"   UUID_SUB="7f884b26-d76e-49db-9959-311fa2a5dd20" TYPE="btrfs"
/dev/nvme0n1p1: UUID="8090a824-63fa-4087-a948-89cca1a369cd" TYPE="ext2" PARTUUID="887ff27b-01"
/dev/nvme0n1p5: UUID="46405308-2ed1-40f3-a86b-906f1118970b" TYPE="crypto_LUKS" PARTUUID="887ff27b-05"

    And then replace the mapper location with the specific uuid

    UUID=06c8c73c-1cc4-477b-a687-6c21697d645d /swap btrfs   defaults,noatime,subvol=@swap 0 0 
/swap/swapfile none swap sw 0 0
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