Red Hat Training

A Red Hat training course is available for RHEL 8

Chapter 30. Configuring kernel parameters permanently by using the kernel_settings RHEL System Role

As an experienced user with good knowledge of Red Hat Ansible, you can use the kernel_settings role to configure kernel parameters on multiple clients at once. This solution:

  • Provides a friendly interface with efficient input setting.
  • Keeps all intended kernel parameters in one place.

After you run the kernel_settings role from the control machine, the kernel parameters are applied to the managed systems immediately and persist across reboots.


Note that RHEL System Role delivered over RHEL channels are available to RHEL customers as an RPM package in the default AppStream repository. RHEL System Role are also available as a collection to customers with Ansible subscriptions over Ansible Automation Hub.

30.1. Introduction to the kernel_settings role

RHEL System Roles is a set of roles that provide a consistent configuration interface to remotely manage multiple systems.

RHEL System Roles were introduced for automated configurations of the kernel using the kernel_settings System Role. The rhel-system-roles package contains this system role, and also the reference documentation.

To apply the kernel parameters on one or more systems in an automated fashion, use the kernel_settings role with one or more of its role variables of your choice in a playbook. A playbook is a list of one or more plays that are human-readable, and are written in the YAML format.

You can use an inventory file to define a set of systems that you want Ansible to configure according to the playbook.

With the kernel_settings role you can configure:

  • The kernel parameters using the kernel_settings_sysctl role variable
  • Various kernel subsystems, hardware devices, and device drivers using the kernel_settings_sysfs role variable
  • The CPU affinity for the systemd service manager and processes it forks using the kernel_settings_systemd_cpu_affinity role variable
  • The kernel memory subsystem transparent hugepages using the kernel_settings_transparent_hugepages and kernel_settings_transparent_hugepages_defrag role variables

Additional resources

30.2. Applying selected kernel parameters using the kernel_settings role

Follow these steps to prepare and apply an Ansible playbook to remotely configure kernel parameters with persisting effect on multiple managed operating systems.


  • You have root permissions.
  • Entitled by your RHEL subscription, you installed the ansible-core and rhel-system-roles packages on the control machine.
  • An inventory of managed hosts is present on the control machine and Ansible is able to connect to them.

RHEL 8.0 - 8.5 provided access to a separate Ansible repository that contains Ansible Engine 2.9 for automation based on Ansible. Ansible Engine contains command-line utilities such as ansible, ansible-playbook; connectors such as docker and podman; and the entire world of plugins and modules. For information about how to obtain and install Ansible Engine, refer to How do I Download and Install Red Hat Ansible Engine?.

RHEL 8.6 and 9.0 has introduced Ansible Core (provided as ansible-core RPM), which contains the Ansible command-line utilities, commands, and a small set of built-in Ansible plugins. The AppStream repository provides ansible-core, which has a limited scope of support. You can learn more by reviewing Scope of support for the ansible-core package included in the RHEL 9 AppStream.


  1. Optionally, review the inventory file for illustration purposes:

    #  cat /home/jdoe/<ansible_project_name>/inventory

    The file defines the [testingservers] group and other groups. It allows you to run Ansible more effectively against a specific set of systems.

  2. Create a configuration file to set defaults and privilege escalation for Ansible operations.

    1. Create a new YAML file and open it in a text editor, for example:

      #  vi /home/jdoe/<ansible_project_name>/ansible.cfg
    2. Insert the following content into the file:

      inventory = ./inventory
      become = true
      become_method = sudo
      become_user = root
      become_ask_pass = true

      The [defaults] section specifies a path to the inventory file of managed hosts. The [privilege_escalation] section defines that user privileges be shifted to root on the specified managed hosts. This is necessary for successful configuration of kernel parameters. When Ansible playbook is run, you will be prompted for user password. The user automatically switches to root by means of sudo after connecting to a managed host.

  3. Create an Ansible playbook that uses the kernel_settings role.

    1. Create a new YAML file and open it in a text editor, for example:

      #  vi /home/jdoe/<ansible_project_name>/kernel-roles.yml

      This file represents a playbook and usually contains an ordered list of tasks, also called plays, that are run against specific managed hosts selected from your inventory file.

    2. Insert the following content into the file:

        hosts: testingservers
        name: "Configure kernel settings"
          - rhel-system-roles.kernel_settings
            - name: fs.file-max
              value: 400000
            - name: kernel.threads-max
              value: 65536
            - name: /sys/class/net/lo/mtu
              value: 65000
          kernel_settings_transparent_hugepages: madvise

      The name key is optional. It associates an arbitrary string with the play as a label and identifies what the play is for. The hosts key in the play specifies the hosts against which the play is run. The value or values for this key can be provided as individual names of managed hosts or as groups of hosts as defined in the inventory file.

      The vars section represents a list of variables containing selected kernel parameter names and values to which they have to be set.

      The roles key specifies what system role is going to configure the parameters and values mentioned in the vars section.


      You can modify the kernel parameters and their values in the playbook to fit your needs.

  4. Optionally, verify that the syntax in your play is correct.

    #  ansible-playbook --syntax-check kernel-roles.yml
    playbook: kernel-roles.yml

    This example shows the successful verification of a playbook.

  5. Execute your playbook.

    #  ansible-playbook kernel-roles.yml
    BECOME password:
    PLAY [Configure kernel settings] **********************************************************************************
    PLAY RECAP ********************************************************************************************************
    fdoe@       : ok=10   changed=4    unreachable=0    failed=0    skipped=6    rescued=0    ignored=0
    pdoe@        : ok=10   changed=4    unreachable=0    failed=0    skipped=6    rescued=0    ignored=0

    Before Ansible runs your playbook, you are going to be prompted for your password and so that a user on managed hosts can be switched to root, which is necessary for configuring kernel parameters.

    The recap section shows that the play finished successfully (failed=0) for all managed hosts, and that 4 kernel parameters have been applied (changed=4).

  6. Restart your managed hosts and check the affected kernel parameters to verify that the changes have been applied and persist across reboots.

Additional resources