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 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
systemctl enable --runtime /media/state/units/swapon.service
systemctl start swapon
On your next reboot the swap will be turned on automatically.
Same problem as @Kaijun. Also in DO.