[Unit]
Description=Turn on swap
[Service]
Type=oneshot
RemainAfterExit=true
ExecStartPre=-/bin/bash -euxc ' \
  fallocate -l 8192m /swap &&\
  chmod 600 /swap &&\
  mkswap /swap'
ExecStart=/sbin/swapon /swap
ExecStop=/sbin/swapoff /swap
[Install]
WantedBy=local.target