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Setup a swap file in the stateful partition

Run these commands as root to create a 512 megabyte swap.

fallocate -l 512m /media/state/512MiB.swap
chmod 600 /media/state/512MiB.swap
mkswap /media/state/512MiB.swap

Create a unit to initialize it on boot

Create a unit file at /media/state/units/swapon.service with these contents:

[Unit]
Description=Turn on swap

[Service]
Type=oneshot
ExecStart=/sbin/swapon /media/state/512MiB.swap

[Install]
WantedBy=local.target

Enable the unit and start using swap

systemctl enable --runtime /media/state/units/swapon.service
systemctl start swapon

On your next reboot the swap will be turned on automatically.