To configure the amount of memory that is reserved for the kdump kernel, as root, edit the /etc/default/grub file and add the crashkernel=<size>M (or crashkernel=auto) parameter to the list of kernel options (the GRUB_CMDLINE_LINUX line). For example, to reserve 128 MB of memory, use:

GRUB_CMDLINE_LINUX="crashkernel=128M quiet rhgb"

Then update the configuration file for the GRUB 2 boot loader by typing the following at a shell prompt as root:

grub2-mkconfig -o /boot/grub2/grub.cfg

When a kernel crash is captured, the core dump can be either stored as a file in a local file system, written directly to a device, or sent over a network using the NFS (Network File System) or SSH (Secure Shell) protocol. Only one of these options can be set at the moment, and the default option is to store the vmcore file in the /var/crash/ directory of the local file system. To change this, as root, open the /etc/kdump.conf configuration file in a text editor and edit the options as described below.

To change the local directory in which the core dump is to be saved, remove the hash sign (“#”) from the beginning of the #path /var/crash line, and replace the value with a desired directory path. Optionally, if you wish to write the file to a different partition, follow the same procedure with the #ext4 /dev/sda3 line as well, and change both the file system type and the device (a device name, a file system label, and UUID are all supported) accordingly. For example:

ext3 /dev/sda4
path /usr/local/cores

To write the dump directly to a device, remove the hash sign (“#”) from the beginning of the #raw /dev/sda5 line, and replace the value with a desired device name. For example:

raw /dev/sdb1

To store the dump to a remote machine using the NFS protocol, remove the hash sign (“#”) from the beginning of the #net my.server.com:/export/tmp line, and replace the value with a valid hostname and directory path. For example:

net penguin.example.com:/export/cores

To store the dump to a remote machine using the SSH protocol, remove the hash sign (“#”) from the beginning of the #net user@my.server.com line, and replace the value with a valid username and hostname. For example:

net john@penguin.example.com

Refer to Chapter 10, OpenSSH for information on how to configure an SSH server, and how to set up a key-based authentication.

For a complete list of currently supported targets, see Table 23.1, “Supported kdump targets”.

To reduce the size of the vmcore dump file, kdump allows you to specify an external application (that is, a core collector) to compress the data, and optionally leave out all irrelevant information. Currently, the only fully supported core collector is makedumpfile.

To enable the core collector, as root, open the /etc/kdump.conf configuration file in a text editor, remove the hash sign (“#”) from the beginning of the #core_collector makedumpfile -c --message-level 1 -d 31 line, and edit the command line options as described below.

To enable the dump file compression, add the -c parameter. For example:

core_collector makedumpfile -c

To remove certain pages from the dump, add the -d value parameter, where value is a sum of values of pages you want to omit as described in Table 23.2, “Supported filtering levels”. For example, to remove both zero and free pages, use the following:

core_collector makedumpfile -d 17 -c

Refer to the manual page for makedumpfile for a complete list of available options.

By default, when kdump fails to create a core dump, the root file system is mounted and /sbin/init is run. To change this behavior, as root, open the /etc/kdump.conf configuration file in a text editor, remove the hash sign (“#”) from the beginning of the #default shell line, and replace the value with a desired action as described in Table 23.3, “Supported actions”.

For example:

default halt

To start the kdump daemon at boot time, type the following at a shell prompt as root:

systemctl enable kdump.service

Similarly, typing systemctl disable kdump.service will disable it. To start the service in the current session, use the following command as root:

systemctl start kdump.service

For more information on services and their configuration, refer to Chapter 8, Services and Daemons.