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Chapter 17. Enabling kdump

For your RHEL 8 systems, you can configure enabling or disabling the kdump functionality on a specific kernel or on all installed kernels. However, you must routinely test the kdump functionality and validate its working status.

17.1. Enabling kdump for all installed kernels

The kdump service starts by enabling kdump.service after the kexec tool is installed. You can enable and start the kdump service for all kernels installed on the machine.

Prerequisites

  • You have administrator privileges.

Procedure

  1. Add the crashkernel= command-line parameter to all installed kernels: 

    # grubby --update-kernel=ALL --args="crashkernel=xxM"
    Copy to Clipboard

    xxM is the required memory in megabytes.

  2. Reboot the system: 

    # reboot
    Copy to Clipboard

  3. Enable the kdump service: 

    # systemctl enable --now kdump.service
    Copy to Clipboard

Verification

  • Check that the kdump service is running: 

    # systemctl status kdump.service

○ kdump.service - Crash recovery kernel arming
     Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: disabled)
     Active: active (live)
    Copy to Clipboard

17.2. Enabling kdump for a specific installed kernel

You can enable the kdump service for a specific kernel on the machine.

Prerequisites

  • You have administrator privileges.

Procedure

  1. List the kernels installed on the machine. 

    # ls -a /boot/vmlinuz-*
/boot/vmlinuz-0-rescue-2930657cd0dc43c2b75db480e5e5b4a9
/boot/vmlinuz-4.18.0-330.el8.x86_64
/boot/vmlinuz-4.18.0-330.rt7.111.el8.x86_64
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  2. Add a specific kdump kernel to the system’s Grand Unified Bootloader (GRUB) configuration.

    For example: 

    # grubby --update-kernel=vmlinuz-4.18.0-330.el8.x86_64 --args="crashkernel=xxM"
    Copy to Clipboard

    xxM is the required memory reserve in megabytes.

  3. Enable the kdump service. 

    # systemctl enable --now kdump.service
    Copy to Clipboard

Verification

  • Check that the kdump service is running. 

    # systemctl status kdump.service

○ kdump.service - Crash recovery kernel arming
     Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: disabled)
     Active: active (live)
    Copy to Clipboard

17.3. Disabling the kdump service

You can stop the kdump.service and disable the service from starting on your RHEL 8 systems.

Prerequisites

Procedure

  1. To stop the kdump service in the current session: 

    # systemctl stop kdump.service
    Copy to Clipboard

  2. To disable the kdump service: 

    # systemctl disable kdump.service
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Warning

It is recommended to set kptr_restrict=1 as default. When kptr_restrict is set to (1) as default, the kdumpctl service loads the crash kernel regardless of whether the Kernel Address Space Layout (KASLR) is enabled.

If kptr_restrict is not set to 1 and KASLR is enabled, the contents of /proc/kore file are generated as all zeros. The kdumpctl service fails to access the /proc/kcore file and load the crash kernel. The kexec-kdump-howto.txt file displays a warning message, which recommends you to set kptr_restrict=1. Verify for the following in the sysctl.conf file to ensure that kdumpctl service loads the crash kernel:

  • Kernel kptr_restrict=1 in the sysctl.conf file.
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