Tutorial
How To Add Swap on CentOS 6
Not using CentOS 6?Choose a different version or distribution.
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 For this reason, this guide is no longer maintained.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.
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>
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chown and chmod need sudo, as dd also used sudo to create them. Actually, chown can be skipped as the file is already owned by root:root.
thank you for this
Thank you! Nice!
Thank you!
very useful on basic droplets with 512MB ram. i was getting clamav error due and adding swap space fixed it.
thank you
This may seem like a silly question, but which parameter should I change in the “dd” command if I want to make a 1024 Mb swap file? Thank you!
@Nathan: Edit the count=512k argument with the size of your choice, in your case it would be count=1024k
I have read that, as a rule, swap space should be 2x ram. I will be using a 512 ram or 1g ram droplet to host 1 or 2 drupal sites, plus a vpn with a few users. Should I follow the 2x rule?
A couple more quetions:
Does this tutorial create a partition, or just a swap file?
Is it important to do this upon droplet creation, or is it also ok to do it at any time?
Thanks for the excellent tutorial and info!
@pensivedog: 512MB should be enough to prevent a droplet from crashing. We don’t recommend creating swap files larger than 512MB.
<strong>Does this tutorial create a partition, or just a swap file?</strong> Swap file.
<strong>Is it important to do this upon droplet creation, or is it also ok to do it at any time?</strong> It doesn’t matter – you can add swap whenever you want.
Very nice :> i’ve made myself a little swamp…