Linux Add a Swap File / space

You need to use the dd command to create swap file. The mkswap command is used to set up a Linux swap area on a device or in a file.

Step #1: Login as the Root User

Open a terminal window (select Applications > Accessories > Terminal) or login to remote server using the ssh client. Switch to the root user by typing su - and entering the root password, when prompted

Step #2: Create Storage File

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

1. if=/dev/zero : Read from /dev/zero file. /dev/zero is a special file in that provides as many null characters to build storage file called /swapfile1.
2. of=/swapfile1 : Read from /dev/zero write stoage file to /swapfile1.
3. bs=1024 : Read and write 1024 BYTES bytes at a time.
4. count=524288 : Copy only 523288 BLOCKS input blocks.

Step #3: Set Up a Linux Swap Area

Type the following command to set up a Linux swap area in a file:
# mkswap /swapfile1
Setup correct file permission for security reasons, enter:
# chown root:root /swapfile1
# chmod 0600 /swapfile1
A world-readable swap file is a huge local vulnerability. The above command make sure only root user can read/write to the file. Finally, activate /swapfile1 swap space immediately, enter:
# swapon /swapfile1
To activate /swapfile1 after Linux system reboot, add entry to /etc/fstab file. Open this file using a text editor such as vi:
# vi /etc/fstab
Append the following line:
/swapfile1 swap swap defaults 0 0
Save and close the file. Next time Linux comes up after reboot, it enables the new swap file for you automatically.

taken from http://www.cyberciti.biz/faq/linux-add-a-swap-file-howto/