Swap file is more flexible than swap partition.

• Use file as swap space.

  # create a 4G swap image
  dd if=/dev/zero of=swap.img bs=1024M count=4
  mkswap swap.img
  
  # see the currently used swap space
  swapon -s
  
  # mount this file as swap space
  swapon swap.img
  

  Optionally you can use swapoff to disable some swap space.

• Persistent

  Add this entry into /etc/fstab

  /opt/swap.img  none            swap    defaults        0       0