Type following command to create 512MB swap file (1024 * 512MB = 524288 block size):
# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288

chmod 600 /swapfile1

c) Set up a Linux swap area:
# mkswap /swapfile1

d) Activate /swapfile1 swap space immediately:
# swapon /swapfile1

e) To activate /swapfile1 after Linux system reboot, add entry to /etc/fstab file. Open this file using text editor such as vi:
# vi /etc/fstab

Append following line:
/swapfile1 none swap defaults 0 0


f) activate swap

#swapon /swapfile1

or to check /etc/fstab entry:

#swapon -a



g) How do I verify swap is activated or not?
Simply use free command:
$ free -m

------------

This does not work when root file system is on a "unionfs"

eg:

root@vpn:/# mkswap /swapfile1
Setting up swapspace version 1, size = 536866 kB
no label, UUID=792c61bb-0425-4441-a7c9-1f103b84c296
root@vpn:/# swapon /swapfile1
swapon: /swapfile1: Invalid argument
root@vpn:/# mount
unionfs on / type unionfs (rw)