Chapter 10. Configuring kdump on the command line

Prerequisites

• Root privileges
• Fulfilled kdump requirements for configurations and targets. For details, see Supported kdump configurations and targets
• On IBM Z systems, ensure the zipl utility is installed.

Prerequisites

• Root permissions.
• 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.

  Example 10.1. The kdump target configuration

  # grep -v ^# /etc/kdump.conf | grep -v ^$
  ext4 /dev/mapper/vg00-varcrashvol
  path /var/crash
  core_collector makedumpfile -c --message-level 1 -d 31

  Here, the dump target is specified (ext4 /dev/mapper/vg00-varcrashvol), and thus mounted at /var/crash. The path option is also set to /var/crash, so the kdump saves the vmcore file in the /var/crash/var/crash directory.

• 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 9, the directory defined as the kdump target using the path directive must exist when the kdump systemd service is started - otherwise the service fails.

• 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 as well as the device name, label or UUID to the desired values. For example:

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

     NOTE

     The correct syntax for specifying UUID values is both UUID="correct-uuid" and UUID=correct-uuid.

     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.

     Important

     When dumping to Direct Access Storage Device (DASD) on IBM Z hardware, it is essential that the dump devices are correctly specified in /etc/dasd.conf before proceeding.

• 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

• 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.

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

     • 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

Options

• -c, -l, -z, or -p: specifies the compress dump file format by each page when you use one of these options: -c for zlib, -l for lzo, -z for zstd, or -p for snappy.
• -d (dump_level): excludes pages so that they do not get copied to the dump file.

• --message-level : specifies the message types.

  Using --message-level, you can restrict the outputs to print. For example, specifying 7 as the message level prints common messages and error messages. The maximum value for --message_level is 31.

Prerequisites

• Fulfilled kdump requirements for configurations and targets.

• To compress a dump file using -l:

  core_collector makedumpfile -l -d 31 --message-level 1

Prerequisites

• Root permissions.
• Fulfilled requirements for kdump configurations and targets.

• KDUMP_COMMANDLINE_REMOVE

  This option removes arguments from the current kdump command line. It removes parameters that may cause kdump errors or kdump kernel boot failures. These parameters may have been parsed from the previous KDUMP_COMMANDLINE process or inherited from the /proc/cmdline file. When this variable is not configured, it inherits all values from the /proc/cmdline file. Configuring this option also provides information that is helpful in debugging an issue.

  Example

  To remove certain arguments, add them to KDUMP_COMMANDLINE_REMOVE as follows:

  # KDUMP_COMMANDLINE_REMOVE="hugepages hugepagesz slub_debug quiet log_buf_len swiotlb"

• KDUMP_COMMANDLINE_APPEND

  This option appends arguments to the current command line. These arguments may have been parsed by the previous KDUMP_COMMANDLINE_REMOVE variable.

  For the kdump kernel, disabling certain modules such as mce, cgroup, numa, hest_disable can help prevent kernel errors. These modules may consume a significant portion of the kernel memory reserved for kdump or cause kdump kernel boot failures.

  Example

  To disable memory cgroups on the kdump kernel command line, run the command as follows:

  # KDUMP_COMMANDLINE_APPEND="cgroup_disable=memory"

Prerequisites

• Fulfilled kdump requirements for configurations and targets.
• All configurations for installing kdump are set up according to your needs.

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

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