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.
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
.
[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
Swapfiles should not be created on btrfs or xfs volumes. For systems using btrfs or 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 Container Linux Config
The following config sets up a 1GiB swapfile located at /var/vm/swapfile1
.
storage:
files:
- path: /etc/sysctl.d/80-swappiness.conf
filesystem: root
contents:
inline: "vm.swappiness=10"
systemd:
units:
- name: var-vm-swapfile1.swap
enabled: true
contents: |
[Unit]
Description=Turn on swap
Requires=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
ConditionPathExists=!/var/vm/swapfile1
[Service]
Type=oneshot
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
RemainAfterExit=true