Swap is a space on a disk that is used when the amount of physical RAM memory is full. When a Linux system runs out of RAM, inactive pages are moved from the RAM to the swap space.

Swap space can take the form of either a dedicated swap partition or a swap file. Usually, when running a Debian virtual machine a swap partition is not present so the only option is to create a swap file.

This tutorial outlines the steps necessary to add a swap file on Debian 9 systems.

Before You Begin

Before continuing with this tutorial, check if your Debian installation already has swap enabled by typing:

sudo swapon --show

If the output is empty, it means that the system doesn’t have swap space.

Otherwise, if you get something like below, you already have swap enabled on your machine.

NAME      TYPE      SIZE USED PRIO
/dev/sda2 partition   4G   0B   -1

Although possible, it is not common to have multiple swap spaces on a single machine.

Creating a Swap File

The user you are logged in as must have sudo privileges to be able to activate swap. In this guide, we will add 1G of swap, if you want to create a bigger swap, replace 1G with the size of the swap space you need.

The steps below show how to add swap space on Debian 9.

  1. Start by creating a file which will be used for swap: 
    sudo fallocate -l 1G /swapfile

    If fallocate is not installed or you get an error message saying fallocate failed: Operation not supported then use the following command to create the swap file:

    sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576
  2. Only the root user should be able to read and write to the swap file. Issue the command below to set the correct permissions: 
    sudo chmod 600 /swapfile
  3. Use the mkswap tool to set up a Linux swap area on the file: 
    sudo mkswap /swapfile
  4. Activate the swap file by typing: 
    sudo swapon /swapfile

    Make the change permanent by opening the /etc/fstab file:

    sudo nano /etc/fstab

    and pasting the following line:

    /etc/fstab

    /swapfile swap swap defaults 0 0
  5. To verify the swap is active use either the swapon or free command as shown below: 
    sudo swapon --show
    NAME      TYPE  SIZE   USED PRIO
/swapfile file 1024M 507.4M   -1
    sudo free -h
                  total        used        free      shared  buff/cache   available
Mem:           488M        158M         83M        2.3M        246M        217M
Swap:          1.0G        506M        517M

Adjusting the Swappiness Value

Swappiness is a Linux kernel property that defines how often the system will use the swap space. Swappiness can have a value between 0 and 100. A low value will make the kernel to try to avoid swapping whenever possible while a higher value will make the kernel to use the swap space more aggressively.

The default swappiness value is 60. You can check the current swappiness value by typing the following command:

cat /proc/sys/vm/swappiness
60

While the swappiness value of 60 is OK for most Linux systems, for production servers you may need to set a lower value.

For example, to set the swappiness value to 10, type:

sudo sysctl vm.swappiness=10

To make this parameter persistent across reboots append the following line to the /etc/sysctl.conf file:

/etc/sysctl.conf

The optimal swappiness value depends on your system workload and how the memory is being used. You should adjust this parameter in small increments to find an optimal value.

Removing a Swap File

To deactivate and remove the swap file, perform the steps below:

  1. First deactivate the swap space by typing: 
    sudo swapoff -v /swapfile
  2. Next, remove the swap file entry /swapfile swap swap defaults 0 0 from the /etc/fstab file.
  3. Finally, delete the actual swapfile file: 
    sudo rm /swapfile

Conclusion

You have learned how to create a swap file and activate and configure swap space on your Debian 9 machine.

If you hit a problem or have feedback, leave a comment below.

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