// Tutorial //

How To Add Swap Space on Ubuntu 16.04

Published on April 25, 2016
Default avatar
By Justin Ellingwood
Developer and author at DigitalOcean.
How To Add Swap Space on Ubuntu 16.04
Not using Ubuntu 16.04?Choose a different version or distribution.
Ubuntu 16.04

Introduction

One of the easiest way of increasing the responsiveness of your server and guarding against out-of-memory errors in applications is to add some swap space. In this guide, we will cover how to add a swap file to an Ubuntu 16.04 server.

What is Swap?

Swap is an area on a hard drive that has been designated as a place where the operating system can temporarily store data that it can no longer hold in RAM. Basically, this gives you the ability to increase the amount of information that your server can keep in its working “memory”, with some caveats. The swap space on the hard drive will be used mainly when there is no longer sufficient space in RAM to hold in-use application data.

The information written to disk will be significantly slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fall back for when your system’s RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.

Check the System for Swap Information

Before we begin, we can check if the system already has some swap space available. It is possible to have multiple swap files or swap partitions, but generally one should be enough.

We can see if the system has any configured swap by typing:

  1. sudo swapon --show

If you don’t get back any output, this means your system does not have swap space available currently.

You can verify that there is no active swap using the free utility:

  1. free -h
Output
total used free shared buff/cache available Mem: 488M 36M 104M 652K 348M 426M Swap: 0B 0B 0B

As you can see in the “Swap” row of the output, no swap is active on the system.

Check Available Space on the Hard Drive Partition

The most common way of allocating space for swap is to use a separate partition devoted to the task. However, altering the partitioning scheme is not always possible. We can just as easily create a swap file that resides on an existing partition.

Before we do this, we should check the current disk usage by typing:

  1. df -h
Output
Filesystem Size Used Avail Use% Mounted on udev 238M 0 238M 0% /dev tmpfs 49M 624K 49M 2% /run /dev/vda1 20G 1.1G 18G 6% / tmpfs 245M 0 245M 0% /dev/shm tmpfs 5.0M 0 5.0M 0% /run/lock tmpfs 245M 0 245M 0% /sys/fs/cgroup tmpfs 49M 0 49M 0% /run/user/1001

The device under /dev is our disk in this case. We have plenty of space available in this example (only 1.1G used). Your usage will probably be different.

Although there are many opinions about the appropriate size of a swap space, it really depends on your personal preferences and your application requirements. Generally, an amount equal to or double the amount of RAM on your system is a good starting point. Another good rule of thumb is that anything over 4G of swap is probably unnecessary if you are just using it as a RAM fallback.

Create a Swap File

Now that we know our available hard drive space, we can go about creating a swap file within our filesystem. We will create a file of the swap size that we want called swapfile in our root (/) directory.

The best way of creating a swap file is with the fallocate program. This command creates a file of a preallocated size instantly.

Since the server in our example has 512MB of RAM, we will create a 1 Gigabyte file in this guide. Adjust this to meet the needs of your own server:

  1. sudo fallocate -l 1G /swapfile

We can verify that the correct amount of space was reserved by typing:

  1. ls -lh /swapfile
  1. -rw-r--r-- 1 root root 1.0G Apr 25 11:14 /swapfile

Our file has been created with the correct amount of space set aside.

Enabling the Swap File

Now that we have a file of the correct size available, we need to actually turn this into swap space.

First, we need to lock down the permissions of the file so that only the users with root privileges can read the contents. This prevents normal users from being able to access the file, which would have significant security implications.

Make the file only accessible to root by typing:

  1. sudo chmod 600 /swapfile

Verify the permissions change by typing:

  1. ls -lh /swapfile
Output
-rw------- 1 root root 1.0G Apr 25 11:14 /swapfile

As you can see, only the root user has the read and write flags enabled.

We can now mark the file as swap space by typing:

  1. sudo mkswap /swapfile
Output
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes) no label, UUID=6e965805-2ab9-450f-aed6-577e74089dbf

After marking the file, we can enable the swap file, allowing our system to start utilizing it:

  1. sudo swapon /swapfile

We can verify that the swap is available by typing:

  1. sudo swapon --show
Output
NAME TYPE SIZE USED PRIO /swapfile file 1024M 0B -1

We can check the output of the free utility again to corroborate our findings:

  1. free -h
Output
total used free shared buff/cache available Mem: 488M 37M 96M 652K 354M 425M Swap: 1.0G 0B 1.0G

Our swap has been set up successfully and our operating system will begin to use it as necessary.

Make the Swap File Permanent

Our recent changes have enabled the swap file for the current session. However, if we reboot, the server will not retain the swap settings automatically. We can change this by adding the swap file to our /etc/fstab file.

Back up the /etc/fstab file in case anything goes wrong:

  1. sudo cp /etc/fstab /etc/fstab.bak

You can add the swap file information to the end of your /etc/fstab file by typing:

  1. echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Tweak your Swap Settings

There are a few options that you can configure that will have an impact on your system’s performance when dealing with swap.

Adjusting the Swappiness Property

The swappiness parameter configures how often your system swaps data out of RAM to the swap space. This is a value between 0 and 100 that represents a percentage.

With values close to zero, the kernel will not swap data to the disk unless absolutely necessary. Remember, interactions with the swap file are “expensive” in that they take a lot longer than interactions with RAM and they can cause a significant reduction in performance. Telling the system not to rely on the swap much will generally make your system faster.

Values that are closer to 100 will try to put more data into swap in an effort to keep more RAM space free. Depending on your applications’ memory profile or what you are using your server for, this might be better in some cases.

We can see the current swappiness value by typing:

  1. cat /proc/sys/vm/swappiness
Output
60

For a Desktop, a swappiness setting of 60 is not a bad value. For a server, you might want to move it closer to 0.

We can set the swappiness to a different value by using the sysctl command.

For instance, to set the swappiness to 10, we could type:

  1. sudo sysctl vm.swappiness=10
Output
vm.swappiness = 10

This setting will persist until the next reboot. We can set this value automatically at restart by adding the line to our /etc/sysctl.conf file:

  1. sudo nano /etc/sysctl.conf

At the bottom, you can add:

/etc/sysctl.conf
vm.swappiness=10

Save and close the file when you are finished.

Adjusting the Cache Pressure Setting

Another related value that you might want to modify is the vfs_cache_pressure. This setting configures how much the system will choose to cache inode and dentry information over other data.

Basically, this is access data about the filesystem. This is generally very costly to look up and very frequently requested, so it’s an excellent thing for your system to cache. You can see the current value by querying the proc filesystem again:

  1. cat /proc/sys/vm/vfs_cache_pressure
Output
100

As it is currently configured, our system removes inode information from the cache too quickly. We can set this to a more conservative setting like 50 by typing:

  1. sudo sysctl vm.vfs_cache_pressure=50
Output
vm.vfs_cache_pressure = 50

Again, this is only valid for our current session. We can change that by adding it to our configuration file like we did with our swappiness setting:

  1. sudo nano /etc/sysctl.conf

At the bottom, add the line that specifies your new value:

/etc/sysctl.conf
vm.vfs_cache_pressure=50

Save and close the file when you are finished.

Conclusion

Following the steps in this guide will give you some breathing room in cases that would otherwise lead to out-of-memory exceptions. Swap space can be incredibly useful in avoiding some of these common problems.

If you are running into OOM (out of memory) errors, or if you find that your system is unable to use the applications you need, the best solution is to optimize your application configurations or upgrade your server.


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Developer and author at DigitalOcean.

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I’ve made this into a quicky .sh script, with a one line copy and paste into the terminal to download it, run it, and then cleanup. Figured I’d share it with you guys since this is my 7th time doing this.

Kinda seems silly that this isn’t done when a droplet is created.

https://github.com/spikeon/digitalocean_swapfile

Thanks. Reply to your WARNING - generally, swap on SSD might be ‘evil’ but it’s necessary to run some apps or to be able to upgrade some apps on basic 512 MB RAM droplet (in my case, I’ve tried to upgrade MariaDB; without swap it just failed and I was unable to run it again ever, with swap it upgraded like charm and runs smoothly).

I’ll leave this here as it can help someone in the future: Heads up on “Make the Swap File Permanent” step.

I had to destoy one droplet because it was entering on “linux emergency mode” after finishing up this tutorial and I couldn’t find the reason. As I created a brand new droplet and finished this tutorial again, it was entering on emergency mode just like the last one.

After some minutes thinking what would be the problem, I realized that I got wrong the step “You can add the swap file information to the end of your /etc/fstab file by typing:echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

The command above should be inserted and commited on the bash line command, and I have inserted it as is on the last line of the file /etc/fstab.

A very simple mistake, but I’m a begginer here and I got used to sudo nano files and insert the codes by hand on the other 10 previous tutorials from DO that I’ve consumed. I hope this save someone droplet one day…

I’m not too sure I understand the warning about having a swap file being “dangerous” for your hardware. If that is true, wouldn’t it be a problem if my application writes several MB of logs every day, enlarge it’s database by many MB every day… etc.

Is there is a difference between a swap file or other files that make them so dangerous?

If u are newbie and bought 512mb set swapfile 4GB

thx very clear and useful and this solve my problem restoring huge mysql backup file with hundreds of thousand rows (none of online suggestions to increase max_allowed_packets and other time setting works)

I am not able to create the swap file even though I have enough space in the root directory. This is what my df -h returns:

root@droplet1:/var/log# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            234M  4.0K  234M   1% /dev
tmpfs            49M  340K   49M   1% /run
/dev/vda1        20G   11G  8.4G  55% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
none            5.0M     0  5.0M   0% /run/lock
none            245M     0  245M   0% /run/shm
none            100M     0  100M   0% /run/user

Can anyone please help? My MySQL db isn’t starting and giving an error of low memory so I wanted to add swap space until I figure out what’s eating up my RAM.

Very clear and useful tutorial. My mongorestore command was crashing on my staging server when trying to set it up with production data. After setting up a swap of 2GB (droplet has only 1GB or ram) the import went well. It’s a one time process, with no time pressure, so upgrading the ram just for the import felt like overkill.

Very detailed step by step tutorial… perfect!!

Hi, I’m new here. I use ubuntu 16.04 I made a swap space, looks everything ok to me. But when I open Gparted, I only see /dev/sda1. From used space I understand, that I created a swap space, but I don’t see seperate line with swap? Thax in advance.