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. A modern alternative to slow swap partitions is to use RAM compression with zram.

By default Flatcar Container Linux does not include a partition for swap nor a zram configuration, however one can configure their system to have swap, either by including a dedicated partition for it, creating a swapfile, or setting up zram.

Using zram

With zram a virtual /dev/zram0 device acts as swap space which lives compressed in memory. One can activate zram by creating a [zram0] section in /etc/systemd/zram-generator.conf. Here are Butane config (YAML) that creates the file:

variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/systemd/zram-generator.conf
      contents:
        inline: |
          [zram0]

You can tweak the size by using the minimum of either 8 GB if one has a lot of RAM or the size of the RAM, e.g., 1 GB, by setting zram-size:

variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/systemd/zram-generator.conf
      contents:
        inline: |
          [zram0]
          zram-size = min(ram, 8192)

Allocating a compressed RAM section that can hold an amount up to the size of the RAM itself works well because most RAM contents compress very well.

After a reboot you can check the results:

$ zramctl
NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lzo-rle         1G   4K   74B   12K       8 [SWAP]

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.

From Flatcar 4186, one can directly use mkswap to create a swap file as follow. For older Flatcar versions, you still need to create a swap file with dd if=/dev/zero of=/var/vm/swapfile1, chmod 600 /var/vm/swapfile1 and mkswap /var/vm/swapfile1

mkdir -p /var/vm
mkswap --size 1024m --file /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.

[Unit]
Description=Turn on swap

[Swap]
What=/var/vm/swapfile1

[Install]
WantedBy=multi-user.target

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
storage:
  files:
  - path: /etc/sysctl.d/80-swappiness.conf
    contents:
      inline: "vm.swappiness=10"

systemd:
  units:
    - name: var-vm-swapfile1.swap
      enabled: true
      contents: |
        [Unit]
        Description=Turn on swap
        BindsTo=create-swapfile.service
        After=create-swapfile.service

        [Swap]
        What=/var/vm/swapfile1

        [Install]
        WantedBy=multi-user.target
    - name: create-swapfile.service
      contents: |
        [Unit]
        Description=Create a swapfile
        RequiresMountsFor=/var
        DefaultDependencies=no

        [Service]
        Type=oneshot
        ExecStartPre=mkdir -p /var/vm
        ExecStartPre=rm -f /var/vm/swapfile1
        ExecStart=mkswap --size 1024m --file /var/vm/swapfile1
        RemainAfterExit=true

Using a dedicated swap disk

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

variant: flatcar
version: 1.0.0
storage:
  disks: 
    - device: /dev/sdb 
      wipe_table: true 
      partitions: 
        - label: swap
          type_guid: 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
  filesystems:
    - 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.