Managing swap space Flatcar Container Linux

    Swap is the process of moving pages of memory to a designated part of the hard disk, freeing up space when needed. Swap can be used to alleviate problems with low-memory environments. An alternative is to use RAM compression with zram.

    By default Flatcar Container Linux does not include a partition for swap, however one can configure their system to have swap, either by including a dedicated partition for it or creating a swapfile.

    Managing swap with systemd

    systemd provides a specialized .swap unit file type which may be used to activate swap. The below example shows how to add a swapfile and activate it using systemd.

    Creating a swapfile

    The following commands, run as root, will make a 1GiB file suitable for use as swap.

    mkdir -p /var/vm
    fallocate -l 1024m /var/vm/swapfile1
    chmod 600 /var/vm/swapfile1
    mkswap /var/vm/swapfile1

    Creating the systemd unit file

    The following systemd unit activates the swapfile we created. It should be written to /etc/systemd/system/var-vm-swapfile1.swap.

    Description=Turn on swap

    Enable the unit and start using swap

    Use systemctl to enable the unit once created. The swappiness value may be modified if desired.

    $ systemctl enable --now var-vm-swapfile1.swap
    # Optionally
    $ echo 'vm.swappiness=10' | sudo tee /etc/sysctl.d/80-swappiness.conf
    $ systemctl restart systemd-sysctl

    Swap has been enabled and will be started automatically on subsequent reboots. We can verify that the swap is activated by running swapon:

    $ swapon
    NAME              TYPE       SIZE USED PRIO
    /var/vm/swapfile1 file      1024M   0B   -1

    Problems and Considerations

    Btrfs and xfs

    Please check the btrfs instructions on how to create swapfiles on btrfs. In summary, you must use a single device filesystem, make sure you create the file on a non-snapshotted subvolume (e.g., to make sure this is the case you can create a new subvolume for the file), create the file with truncate -s 0 ./swapfile1 and then disable CoW and compression (chattr +C ./swapfile1, btrfs property set ./swapfile1 compression none).

    Swapfiles should not be created on xfs volumes. For systems using xfs, it is recommended to create a dedicated swap partition.

    Partition size

    The swapfile cannot be larger than the partition on which it is stored.

    Checking if a system can use a swapfile

    Use the df(1) command to verify that a partition has the right format and enough available space:

    $ df -Th
    Filesystem     Type      Size  Used Avail Use% Mounted on
    /dev/sdXN      ext4      2.0G  3.0M  1.8G   1% /var

    The block device mounted at /var/, /dev/sdXN, is the correct filesystem type and has enough space for a 1GiB swapfile.

    Adding swap with a Butane Config

    The following config sets up a 1GiB swapfile located at /var/vm/swapfile1.

    variant: flatcar
    version: 1.0.0
      - path: /etc/sysctl.d/80-swappiness.conf
          inline: "vm.swappiness=10"
        - name: var-vm-swapfile1.swap
          enabled: true
          contents: |
            Description=Turn on swap
        - name: create-swapfile.service
          contents: |
            Description=Create a swapfile
            ExecStart=/usr/bin/mkdir -p /var/vm
            ExecStart=/usr/bin/fallocate -l 1024m /var/vm/swapfile1
            ExecStart=/usr/bin/chmod 600 /var/vm/swapfile1
            ExecStart=/usr/sbin/mkswap /var/vm/swapfile1

    Using a dedicated swap disk

    The following Butane config sets up /dev/sdb to be used as swap:

    variant: flatcar
    version: 1.0.0
        - device: /dev/sdb 
          wipe_table: true 
            - label: swap
              type_guid: 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
        - device: /dev/disk/by-partlabel/swap
          format: swap
          wipe_filesystem: true
          label: swap
          with_mount_unit: true

    NB the systemd unit name is created by systemd-escape -p /dev/disk/by-partlabel/swap as systemd uses - as the path separator meaning that paths containing - have to be escaped. This leads to a file 'dev-disk-by\x2dpartlabel-swap.swap' being created in /etc/systemd/system.

    Using zram

    With zram a virtual /dev/zram0 device acts as swap space which lives compressed in memory. At the moment there is no zram generator and instead, a manual setup needs to be done, similar to the creation of a swap file.

    $ sudo modprobe zram
    $ sudo zramctl -f -s 1G
    $ sudo mkswap /dev/zram0
    $ sudo swapon /dev/zram0
    $ zramctl
    /dev/zram0 lzo-rle         1G   4K   74B   12K       8 [SWAP]