Chapter 15. Configuring kdump on the command line

Procedure

• Execute the following command to generate a memory usage report:

  # makedumpfile --mem-usage /proc/kcore
  
  
  TYPE        PAGES    EXCLUDABLE    DESCRIPTION
  -------------------------------------------------------------
  ZERO          501635      yes        Pages filled with zero
  CACHE         51657       yes        Cache pages
  CACHE_PRIVATE 5442        yes        Cache pages + private
  USER          16301       yes        User process pages
  FREE          77738211    yes        Free pages
  KERN_DATA     1333192     no         Dumpable kernel data

Prerequisites

• You have root permissions on the system.
• Fulfilled requirements for kdump configurations and targets. For details, see Supported kdump configurations and targets.

Prerequisites

• You have root permissions on the system.
• Fulfilled requirements for kdump configurations and targets. For details, see Supported kdump configurations and targets.

Procedure

• To store the crash dump file in /var/crash/ directory of the local file system, edit the /etc/kdump.conf file and specify the path:

  path /var/crash

  The option path /var/crash represents the path to the file system in which kdump saves the crash dump file.

  Note
  • When you specify a dump target in the /etc/kdump.conf file, then the path is relative to the specified dump target.
  • When you do not specify a dump target in the /etc/kdump.conf file, then the path represents the absolute path from the root directory.

  Depending on what is mounted in the current system, the dump target and the adjusted dump path are taken automatically.

  To secure the crash dump file and the accompanying files produced by kdump, you should set up proper attributes for the target destination directory, such as user permissions and SELinux contexts. Additionally, you can define a script, for example kdump_post.sh in the kdump.conf file as follows:

  kdump_post <path_to_kdump_post.sh>

  The kdump_post directive specifies a shell script or a command that is executed after kdump has completed capturing and saving a crash dump to the specified destination. You can use this mechanism to extend the functionality of kdump to perform actions including the adjustment of file permissions.

• To change the local directory in which the crash dump is to be saved, as root, edit the /etc/kdump.conf configuration file:

  1. Remove the hash sign (#) from the beginning of the #path /var/crash line.

  2. Replace the value with the intended directory path. For example:

     path /usr/local/cores

     Important

     In RHEL 8, the directory defined as the kdump target using the path directive must exist when the kdump systemd service starts to avoid failures. This behavior is different from versions earlier than RHEL, where the directory is created automatically if it did not exist when the service starts.

• To write the file to a different partition, edit the /etc/kdump.conf configuration file:

  1. Remove the hash sign (#) from the beginning of the #ext4 line, depending on your choice.

     • device name (the #ext4 /dev/vg/lv_kdump line)
     • file system label (the #ext4 LABEL=/boot line)
     • UUID (the #ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937 line)

  2. Change the file system type and the device name, label or UUID, to the required values. The correct syntax for specifying UUID values is both UUID="correct-uuid" and UUID=correct-uuid. For example:

     ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937

     Important

     It is recommended to specify storage devices using a LABEL= or UUID=. Disk device names such as /dev/sda3 are not guaranteed to be consistent across reboot.

     When you use Direct Access Storage Device (DASD) on IBM Z hardware, ensure the dump devices are correctly specified in /etc/dasd.conf before you proceed with kdump.

• To write the crash dump directly to a device, edit the /etc/kdump.conf configuration file:

  1. Remove the hash sign (#) from the beginning of the #raw /dev/vg/lv_kdump line.

  2. Replace the value with the intended device name. For example:

     raw /dev/sdb1

• To store the crash dump to a remote machine using the NFS protocol:

  1. Remove the hash sign (#) from the beginning of the #nfs my.server.com:/export/tmp line.

  2. Replace the value with a valid hostname and directory path. For example:

     nfs penguin.example.com:/export/cores

  3. Restart the kdump service for the changes to take effect:

     sudo systemctl restart kdump.service

     Note

     When using the NFS directive to specify the NFS target, kdump.service automatically attempts to mount the NFS target to check the disk space. There is no need to mount the NFS target beforehand. To prevent kdump.service from mounting the target, use the dracut_args --mount directive in kdump.conf so that kdump.service calls the dracut utility with the --mount argument to specify the NFS target.

• To store the crash dump to a remote machine using the SSH protocol:

  1. Remove the hash sign (#) from the beginning of the #ssh user@my.server.com line.
  2. Replace the value with a valid username and hostname.

  3. Include your SSH key in the configuration.

     1. Remove the hash sign from the beginning of the #sshkey /root/.ssh/kdump_id_rsa line.

     2. Change the value to the location of a key valid on the server you are trying to dump to. For example:

        ssh john@penguin.example.com
        sshkey /root/.ssh/mykey

• Compressing the size of a crash dump file and copying only necessary pages using various dump levels.
• Excluding unnecessary crash dump pages.
• Filtering the page types to be included in the crash dump.

Options

• -c, -l or -p: specify compress dump file format by each page using either, zlib for -c option, lzo for -l option or snappy for -p option.
• -d (dump_level): excludes pages so that they are not copied to the dump file.
• --message-level : specify the message types. You can restrict outputs printed by specifying message_level with this option. For example, specifying 7 as message_level prints common messages and error messages. The maximum value of message_level is 31

Prerequisites

• You have root permissions on the system.
• Fulfilled requirements for kdump configurations and targets. For details, see Supported kdump configurations and targets.

• To compress a crash dump file using -c:

• To compress a crash dump file using -p:

Prerequisites

• Root permissions.
• Fulfilled requirements for kdump configurations and targets. For details, see Supported kdump configurations and targets.

Prerequisites

• You have root permissions on the system.
• You have saved all important data. The commands to test kdump cause the kernel to crash with loss of data.
• You have scheduled significant machine maintenance time depending on the system architecture.

• vmcore - main kernel memory dump file containing system memory at the time of the crash. It includes data as per the configuration of the core_collector program specified in kdump configuration. By default the kernel data structures, process information, stack traces, and other diagnostic information.
• vmcore-dmesg.txt - contents of the kernel ring buffer log (dmesg) from the primary kernel that panicked.
• kexec-dmesg.log - contains kernel and system log messages from the execution of the secondary kexec kernel that collects the vmcore data.

Prerequisites

• You have root permissions on the system.
• You have completed kdump requirements for configurations and targets. See Supported kdump configurations and targets.
• All configurations for installing kdump are set up as required.

Procedure

• Enable the kdump service for multi-user.target:

  # systemctl enable kdump.service

• Start the service in the current session:

  # systemctl start kdump.service

• Stop the kdump service:

  # systemctl stop kdump.service

• Disable the kdump service:

  # systemctl disable kdump.service

• rd.driver.blacklist=<modules>
• modprobe.blacklist=<modules>

Prerequisites

• You have root permissions on the system.