How to Add Swap Space on CentOS 8

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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. Typically, when running CentOS on a virtual machine, a swap partition is not present, so the only option is to create a swap file.

This article covers the steps for adding a swap file on CentOS 8 systems.

Creating and Activating a Swap File

Perform the following steps as root or user with sudo privileges to add swap space on a CentOS 8 system.

  1. Start by creating a file that will serve as swap space:

    sudo fallocate -l 1G /swapfile

    In this example, we are creating a swap file with a size of 1G. If you need more swap, replace 1G with the desired size.

    If the fallocate utility is not available on your system or you get an error message saying fallocate failed: Operation not supported, use the dd command to create the swap file:

    sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576
  2. Set the file permissions so that only the root user can read and write the swap file:

    sudo chmod 600 /swapfile
  3. Next, set up a Linux swap area on the file:

    sudo mkswap /swapfile
    Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
    no label, UUID=0abdb8ba-57d6-4435-8fd8-5db9fc705045
  4. Activate the swap by executing the following command:

    sudo swapon /swapfile
  5. Verify that the swap is active by using either the swapon or the 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
  6. Make the change permanent by adding a swap entry in the /etc/fstab file:

    sudo nano /etc/fstab

    Paste the following line:

    /etc/fstab
    /swapfile swap swap defaults 0 0

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 on CentOS 8 is 30. You can check the current swappiness value by typing the following command:

cat /proc/sys/vm/swappiness
30

While the swappiness value of 30 is OK for desktop and development machines, 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.

Removing a Swap File

To deactivate and remove the swap file, follow these steps:

  1. Deactivate the swap space by typing:

    sudo swapoff -v /swapfile
  2. Remove the swap entry /swapfile swap swap defaults 0 0 from the /etc/fstab file.

  3. Delete the actual swapfile file with rm :

    sudo rm /swapfile

Conclusion

We have shown you how to create a swap file and activate and configure swap space on your CentOS 8 system.

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