How to create swap file on linux

Step-by-step guide about how to create a swap file on Linux.

Before using the guide, login as the Root user. Or use sudo.

Step 1. Create storage file

Type the following command to create 512MB swap file (1024 * 512MB = 524288 block size):

dd if=/dev/zero of=/swapfile bs=1024 count=524288

Where

• 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 /swapfile
• of=/swapfile : read from /dev/zero write stoage file to /swapfile
• bs=1024 : read and write 1024 BYTES bytes at a time.
• count=524288 : copy only 523288 BLOCKS input blocks.

Step 2. Set up a Linux swap area

Type the following command to set up a Linux swap area in a file:

# mkswap /swapfile

Setup correct file permission for security reasons, enter:

# chown root:root /swapfile
# chmod 0600 /swapfile

A world-readable swap file is a huge local vulnerability. The above command makes sure only root user can read/write to the file. Finally, activate /swapfile swap space immediately, enter:

# swapon /swapfile

To activate /swapfile after Linux system reboot, add entry to /etc/fstab file. Open this file using a text editor such as nano:

# nano /etc/fstab

Append the following line:

/swapfile swap swap defaults 0 0

Step 3. Verify swap is activated

Simply use the free command:

$ free -m