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Chapter 6. Configuring kernel parameters permanently by using RHEL system roles


You can use the kernel_settings RHEL system role to configure kernel parameters on multiple clients simultaneously. Simultaneous configuration has the following advantages:

  • 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.

Important

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.

6.1. Applying selected kernel parameters by using the kernel_settings RHEL system role

You can use the kernel_settings RHEL system role to remotely configure various kernel parameters across multiple managed operating systems with persistent effects. For example, you can configure:

  • Transparent hugepages to increase performance by reducing the overhead of managing smaller pages.
  • The largest packet sizes to be transmitted over the network with the loopback interface.
  • Limits on files to be opened simultaneously.

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: Configuring kernel settings
      hosts: managed-node-01.example.com
      tasks:
        - name: Configure hugepages, packet size for loopback device, and limits on simultaneously open files.
          ansible.builtin.include_role:
            name: rhel-system-roles.kernel_settings
          vars:
            kernel_settings_sysctl:
              - name: fs.file-max
                value: 400000
              - name: kernel.threads-max
                value: 65536
            kernel_settings_sysfs:
              - name: /sys/class/net/lo/mtu
                value: 65000
            kernel_settings_transparent_hugepages: madvise
            kernel_settings_reboot_ok: true

    The settings specified in the example playbook include the following:

    kernel_settings_sysfs: <list_of_sysctl_settings>
    A YAML list of sysctl settings and the values you want to assign to these settings.
    kernel_settings_transparent_hugepages: <value>
    Controls the memory subsystem Transparent Huge Pages (THP) setting. You can disable THP support (never), enable it system wide (always) or inside MAD_HUGEPAGE regions (madvise).
    kernel_settings_reboot_ok: <true|false>
    The default is false. If set to true, the system role will determine if a reboot of the managed host is necessary for the requested changes to take effect and reboot it. If set to false, the role will return the variable kernel_settings_reboot_required with a value of true, indicating that a reboot is required. In this case, a user must reboot the managed node manually.

For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.kdump/README.md file on the control node.

  1. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  2. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Verification

  • Verify the affected kernel parameters:

    # ansible managed-node-01.example.com -m command -a 'sysctl fs.file-max kernel.threads-max net.ipv6.conf.lo.mtu'
    # ansible managed-node-01.example.com -m command -a 'cat /sys/kernel/mm/transparent_hugepage/enabled'

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.kernel_settings/README.md file
  • /usr/share/doc/rhel-system-roles/kernel_settings/ directory
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