How to add a Swap File in Linux

There might be situations when you need extra swap space to improve your Linux server/desktop system performance. How do you add a swap file to Linux system using command line options without creating a new partitions? Or is it even possible. Well in answer for the second question..Yes, its possible. and as for the question on how it is done, come lets find out 🙂

In Linux, as in most other Unix-like operating systems, it is common to use a whole partition of a hard disk for swapping. You can add swap file as a dedicated partition which is the usual way we do it or use following instructions below to create a swap file.

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 - (or sudo -s) and entering the root password, when prompted:

OR

Step #2: Create Storage File

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

Sample outputs:

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 storage 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: Secure swap file

Setup correct file permission for security reasons, enter

The reason we set the permissions 600 is self explanatory. A world-readable swap file is a huge local vulnerability which should not be the permissions to be given for a swap partition. The above commands make sure only root user can read and write to the file.

Step #4: Set up a Linux swap area

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

Sample outputs:

Step #5: Enabling the swap file

Finally, activate /swapfile1 swap space immediately, enter:

Step #6: Update /etc/fstab file

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

Append the following line:

Save and close the file. Next time Linux comes up after reboot, it enables the new swap file for you automatically.

How do I verify swap is activated or not?

Simply use the free command:

How can I display swap usage summary on Linux?

Type the following swapon command:

How do I set swappiness on a Linux server?

The syntax is:

OR

The value which is specified in /proc/sys/vm/swappiness file controls how aggressively the kernel will swap memory pages. Specifying a heigher value increase its agressiveness. The default value is 60. To make changes permanent add the following line to/etc/sysctl.conf: