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Create a Linux Swap File

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. In most cases when running Linux on a virtual machine a swap partition is not present so our only option is to create a swap file.

This tutorial was tested on a Linux systems with Ubuntu 18.04 and CentOS 7 but it should work with any other Linux distribution.

How can I add Swap ?

Follow this steps to add 1GB of swap to your server. In case you want to add 2GB instead of 1 GB, in the first step replace 1G with 2G.

  1. Create a file which will be used for swap.

    sudo fallocate -l 1G /swapfile

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

    sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576
  2. Set the correct permissions.

    Only the root user should be able to write and read the swap file. To set the right permissions type:

    sudo chmod 600 /swapfile
  3. Format the file to swap:

    Use the mkswap utility for format the file to swap:

    sudo mkswap /swapfile
  4. Enable the swap.

    Activate the swap with the following command:

    sudo swapon /swapfile

    To make the change permanent open the /etc/fstab file and append the following line:

    /etc/fstab
    /swapfile swap swap defaults 0 0
    
  5. Verify the swap status.

    To verify that the swap is active we can use either the swapon or the free command as shown bellow:

    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

How to adjust 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 Desktops, 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
vm.swappiness=10

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.

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How can I remove the Swap ?

If for any reason you want to deactivate and remove the swap file, follow these steps:

  1. First deactivate the swap using the following command:

    sudo swapoff -v /swapfile
  2. 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 Linux system. If you hit a problem or have a feedback, leave a comment below.