// Tutorial //

How To Add Swap on CentOS 6

Published on August 17, 2012
Default avatar
By Etel Sverdlov
Developer and author at DigitalOcean.
How To Add Swap on CentOS 6
Not using CentOS 6?Choose a different version or distribution.
CentOS 6

Status: Deprecated

This article covers a version of CentOS that is no longer supported. If you are currently operating a server running CentOS 6, we highly recommend upgrading or migrating to a supported version of CentOS.

Reason: CentOS 6 reached end of life (EOL) on November 30th, 2020 and no longer receives security patches or updates. For this reason, this guide is no longer maintained.

See Instead:
This guide might still be useful as a reference, but may not work on other CentOS releases. If available, we strongly recommend using a guide written for the version of CentOS you are using.

The following DigitalOcean tutorial may be of immediate interest, as it outlines adding swap space on a CentOS 7 server:


About Linux Swapping

Linux RAM is composed of chunks of memory called pages. To free up pages of RAM, a “linux swap” can occur and a page of memory is copied from the RAM to preconfigured space on the hard disk. Linux swaps allow a system to harness more memory than was originally physically available.

However, swapping does have disadvantages. Because hard disks have a much slower memory than RAM, server performance may slow down considerably. Additionally, swap thrashing can begin to take place if the system gets swamped from too many files being swapped in and out.

Note

Although swap is generally recommended for systems utilizing traditional spinning hard drives, using swap with SSDs can cause issues with hardware degradation over time. Due to this consideration, we do not recommend enabling swap on DigitalOcean or any other provider that utilizes SSD storage. Doing so can impact the reliability of the underlying hardware for you and your neighbors.

If you need to improve the performance of your server, we recommend upgrading your Droplet. This will lead to better results in general and will decrease the likelihood of contributing to hardware issues that can affect your service.

Check for Swap Space

Before we proceed to set up a swap file, we need to check if any swap files have been enabled by looking at the summary of swap usage.

swapon -s

If nothing is returned, the summary is empty and no swap file exists.

Check the File System

After we know that we do not have a swap file enabled, we can check how much space we have on the server with the df command. The swap file will take 512MB— since we are only using up about 7% of the /dev/hda, we can proceed.

df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda              20642428   1347968  18245884   7% /

Create and Enable the Swap File

Now it’s time to create the swap file itself using the dd command :

sudo dd if=/dev/zero of=/swapfile bs=1024 count=512k

“of=/swapfile” designates the file’s name. In this case the name is swapfile.

Subsequently we are going to prepare the swap file by creating a linux swap area:

sudo mkswap /swapfile

The results display:

Setting up swapspace version 1, size = 536866 kB

Finish up by activating the swap file:

sudo swapon /swapfile

You will then be able to see the new swap file when you view the swap summary.

 swapon -s
Filename				Type		Size	Used	Priority
/swapfile                               file		524280	0	-1

This file will last on the server until the machine reboots. You can ensure that the swap is permanent by adding it to the fstab file.

Open up the file:

sudo nano /etc/fstab

Paste in the following line:

/swapfile          swap            swap    defaults        0 0

To prevent the file from being world-readable, you should set up the correct permissions on the swap file:

chown root:root /swapfile 
chmod 0600 /swapfile

How To Configure Swappiness

The operating system kernel can adjust how often it relies on swap through a configuration parameter known as swappiness.

To find the current swappiness settings, type:

<pre>cat /proc/sys/vm/swappiness</pre>
<pre>60</pre>

Swapiness can be a value from 0 to 100. Swappiness near 100 means that the operating system will swap often and usually, too soon. Although swap provides extra resources, RAM is much faster than swap space. Any time something is moved from RAM to swap, it slows down.

A swappiness value of 0 means that the operating will only rely on swap when it absolutely needs to. We can adjust the swappiness with the sysctl command:

<pre>sysctl vm.swappiness=10</pre>
<pre>vm.swappiness=10</pre>

If we check the system swappiness again, we can confirm that the setting was applied:

<pre>cat /proc/sys/vm/swappiness</pre>
<pre>10</pre>

To make your VPS automatically apply this setting every time it boots up, you can add the setting to the /etc/sysctl.conf file:

<pre>sudo nano /etc/sysctl.conf</pre>
<pre># Search for the vm.swappiness setting.  Uncomment and change it as necessary.
vm.swappiness=10</pre>
By Etel Sverdlov

If you’ve enjoyed this tutorial and our broader community, consider checking out our DigitalOcean products which can also help you achieve your development goals.

Learn more here


About the authors
Default avatar
Developer and author at DigitalOcean.

Still looking for an answer?

Was this helpful?
10 Comments

This textbox defaults to using Markdown to format your answer.

You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!

This is very useful for the $5 DO plan as the droplet only gets 512MB RAM. My wordpress works again because I use this to release enough memory for mariadb.

thank you, its working on centos 6.7 32bit

Great Tutorial. Recently I migrated from $20 plan to $40 Plan. In $20 plan swap memory was used. However, after migration to $40 plan, i see 0% use of swap memory. Current server status shows. Server Load 0.43 (2 CPUs) Memory Used 16.35% (662,808 of 4,054,852) Swap Used 0% (0 of 524,280)

Is it normal or I need to do something?

Thank you Etel for your time. Excellent article!

Me to having the same question as l33.adrian

Do I have to paste the one you mentioned

/swapfile swap swap defaults 0 0

or should it be in this format?

swap /swapfile swap defaults 0 0

Please check the article and reply…

@Manuel: Did you add the line to your fstab file?

I have same problem like l33.adrian, swap disappear after reboot

Agree with Adrian. Creating swap without label is a wrong way for beginners.

It would be better to do this way: $ sudo mkswap -L myswap /swapfile

And string for fstab file will looks more “traditional”: myswap swap swap defaults 0 0

Just my 5 cents.

Your fstab file looks normal – just add the swap file line as mentioned in the article.

Mine looks similar