If your server ever runs out of physical memory during heavy load, some critical services such as MySQL can crash (as I discovered the hard way!). In this guide we will set up some swap space on Ubuntu 20.04 & 18.04.

Just as a disclaimer, it is not recommend that you use swap space as a replacement to physical memory, especially on cloud hosting. If you are continuously eating into swap space, you should upgrade your physical memory. Swap should really only be a fall back in case memory usage peaks abnormally.

First check if the system has any swap configured.

swapon --show

If the output is blank, there is no swap configured so we can continue with this guide.

1. Create a Swap File

We will use the fallocate program to create a swap file. Best practice is to create a swap file double the amount of your RAM. If you have 1024MB of RAM, create a 2GB swap file.

sudo fallocate -l 2G /swapfile

Now check if the file was created.

ls -lh /swapfile

If it was created correctly, you should see something like:

-rw-r--r-- 1 root root 2.0G Aug 3 18:59 /swapfile

2. Configure Swap File

Make the swap file only accessible to root.

sudo chmod 600 /swapfile

Mark the file as a swap file.

sudo mkswap /swapfile

If it was successful, you should see something like

Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
no label, UUID=00aafaee-51c9-46b3-a0fc-8240c134048e

Finally we will tell the system to start using our new swap file,

sudo swapon /swapfile

To verify that the swap is now available type:

sudo swapon --show


/swapfile file  2G   0B   -2

We can also run the following to see our new swap file alongside physical memory

free -h


              total        used        free      shared  buff/cache   available
Mem:           985M        418M        104M         26M        462M        392M
Swap:          2.0G          0B        2.0G

3. Make it Persistent

This swap will only last until next reboot. In order to make it permanent, we will add it to the /etc/fstab file.

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

4. Some Final Tweaks

For a server, you should change the swappiness value to 10.

sudo sysctl vm.swappiness=10

Now change the vfs_cache_pressure value to 50.

sudo sysctl vm.vfs_cache_pressure=50

To make these two settings persist after next reboot, edit the following file:

sudo nano /etc/sysctl.conf

Add this to the bottom.


Save file and exit. (Press CTRL + X, press Y and then press ENTER).

If you can, reboot the server with sudo reboot and run sudo swapon --show just to make sure the swap space was created automatically on startup.

A useful way to keep an eye on your swap usage and system resources is to run htop