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Chapter 16. Dumping a crashed kernel for later analysis

To analyze why a system crashed, you can use the kdump service to save the contents of the system’s memory for later analysis. This section provides a brief introduction to kdump, and information about configuring kdump using the RHEL web console or using the corresponding RHEL system role.

16.1. What is kdump

kdump is a service that provides a crash dumping mechanism and generates a crash dump or a vmcore dump file. vmcore includes the contents of the system memory for analysis and troubleshooting. kdump uses the kexec system call to boot into the second kernel, capture kernel, without a reboot. This kernel captures the contents of the crashed kernel’s memory and saves it into a file. The second kernel is available in a reserved part of the system memory.

Important

A kernel crash dump can be the only information available if a system failure occur. Therefore, operational kdump is important in mission-critical environments. Red Hat advises to regularly update and test kexec-tools in your normal kernel update cycle. This is important when you install new kernel features.

If you have multiple kernels on a machine, you can enable kdump for all installed kernels or for specified kernels only. When you install kdump, the system creates a default /etc/kdump.conf file. /etc/kdump.conf includes the default minimum kdump configuration, which you can edit to customize the kdump configuration.

16.2. Configuring kdump memory usage and target location in web console

You can configure the memory reserve for the kdump kernel and also specify the target location to capture the vmcore dump file with the RHEL web console interface.

Prerequisites

Procedure

  1. In the web console, open the Kernel dump tab and start the kdump service by setting the Kernel crash dump switch to on.

  2. Configure the kdump memory usage in the terminal, for example: 

    $ sudo grubby --update-kernel ALL --args crashkernel=512M
    Copy to Clipboard

    Restart the system to apply the changes.

  3. In the Kernel dump tab, click Edit at the end of the Crash dump location field.

  4. Specify the target directory for saving the vmcore dump file:

    • For a local filesystem, select Local Filesystem from the drop-down menu.

    • For a remote system by using the SSH protocol, select Remote over SSH from the drop-down menu and specify the following fields:

      • In the Server field, enter the remote server address.
      • In the SSH key field, enter the SSH key location.
      • In the Directory field, enter the target directory.

    • For a remote system by using the NFS protocol, select Remote over NFS from the drop-down menu and specify the following fields:

      • In the Server field, enter the remote server address.
      • In the Export field, enter the location of the shared folder of an NFS server.

      • In the Directory field, enter the target directory.

        Note

        You can reduce the size of the vmcore file by selecting the Compression checkbox.

  5. Optional: Display the automation script by clicking View automation script.

    A window with the generated script opens. You can browse a shell script and an Ansible playbook generation options tab.

  6. Optional: Copy the script by clicking Copy to clipboard.

    You can use this script to apply the same configuration on multiple machines.

Verification

  1. Click Test configuration.

  2. Click Crash system under Test kdump settings.

    Warning

    When you start the system crash, the kernel operation stops and results in a system crash with data loss.

16.3. kdump using RHEL system roles

RHEL system roles is a collection of Ansible roles and modules that provide a consistent configuration interface to remotely manage multiple RHEL systems. With the kdump role, you can set the basic kernel dump parameters on multiple systems.

Warning

The kdump role replaces the kdump configuration of the managed hosts entirely by replacing the /etc/kdump.conf file. Additionally, if the kdump role is applied, all previous kdump settings are also replaced, even if they are not specified by the role variables, by replacing the /etc/sysconfig/kdump file.

The following example playbook shows how to apply the kdump system role to set the location of the crash dump files: 

---
- hosts: kdump-test
  vars:
    kdump_path: /var/crash
  roles:
    - rhel-system-roles.kdump
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For a detailed reference on kdump role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/kdump directory.

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