Configuring the kdump Service

Configuring the kdump at First Boot

When the system boots for the first time, a firstboot application is launched allowing you to perform a basic configuration. This includes the kdump service.

Figure 44.1. The kdump configuration screen

Unless the system has enough memory, this option will not be available. For the information on minimum memory requirements, refer to the Required minimums section of the Community Enterprise Linux comparison chart. Note that when the kdump crash recovery is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by a user, and defaults to 128 MB.

Enabling the Service

To start the kdump daemon at boot time, select the Enable kdump? check box. This will enable the service for runlevels 2, 3, 4, and 5, and start it for the current session. Similarly, unselecting the check box will disable it for all runlevels and stop the service immediately.

Configuring the Memory Usage

To configure the amount of memory that is reserved for the kdump kernel, click the up and down arrow buttons next to the Kdump Memory field to increase or decrease the value. Notice that the Usable System Memory field changes accordingly showing you the remaining memory that will be available to the system.

Using the Kernel Dump Configuration Utility

To start the Kernel Dump Configuration utility, select Applications → System Tools → Kdump from the panel, or type system-config-kdump at a shell prompt (for example, xterm or GNOME Terminal). Unless you are already authenticated, you will be prompted to enter the superuser password.

Figure 44.2. The Kernel Dump Configuration utility

The utility allows you to configure kdump as well as to enable or disable starting the service at boot time. When you are done, click OK to save the changes. The system reboot will be requested.

Unless the system has enough memory, the utility will not start, and you will be presented with an error message. For the information on minimum memory requirements, refer to the Required minimums section of the Community Enterprise Linux comparison chart. Note that when the kdump crash recovery is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by a user, and defaults to 128 MB.

Enabling the Service

To start the kdump daemon at boot time, select the Enable kdump check box. This will enable the service for runlevels 2, 3, 4, and 5, and start it for the current session. Similarly, unselecting the check box will disable it for all runlevels and stop the service immediately.

Configuring the Memory Usage

To configure the amount of memory that is reserved for the kdump kernel, click the up and down arrow buttons next to the New kdump Memory field to increase or decrease the value. Notice that the Usable Memory field changes accordingly showing you the remaining memory that will be available to the system.

Configuring the Target Type

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. To change this, click the Edit Location button, and select a location type as described below.

Figure 44.3. The Edit Location dialog

To save the dump to the local file system, select file from the pulldown list. Optionally, if you wish to write the file to a different partition, select ext3 or ext2 from the pulldown list according to the file system you are using, and enter a valid device name to the Enter location field. Note that after clicking OK, you can then customize the destination directory by changing the value in the Path field at the bottom.

To write the dump directly to a device, select raw from the pulldown list, and enter a valid device name (for example, /dev/sdb1). When you are done, click OK to confirm your choice.

To store the dump to a remote machine using the NFS protocol, select nfs from the pulldown list, and enter a valid target in the hostname:directory form (for example, penguin.example.com:/export). Clicking the OK button will confirm your changes. Finally, edit the value of the Path field to customize the destination directory (for instance, cores).

To store the dump to a remote machine using the SSH protocol, select ssh from the pulldown list, and enter a valid username and hostname in the username@hostname form (for example, john@penguin.example.com). Clicking the OK button will confirm your changes. Finally, edit the value of the Path field to customize the destination directory (for instance, /export/cores).

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

Configuring the Core Collector

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 dump file compression, make sure the -c parameter is listed after the makedumpfile command in the Core Collector field (for example, makedumpfile -c).

To remove certain pages from the dump, add the -d value parameter after the makedumpfile command in the Core Collector field. The value is a sum of values of pages you want to omit as described in Table 44.1, "Supported filtering levels". For example, to remove both zero and free pages, use makedumpfile -d 17.

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

Changing the Default Action

To choose what action to perform when kdump fails to create a core dump, select the appropriate option from the Default Action pulldown list. Available options are mount rootfs and run /sbin/init (the default action), reboot (to reboot the system), shell (to present a user with an interactive shell prompt), and halt (to halt the system).

Configuring kdump on the Command Line

To perform actions described in this section, you have to be logged in as a superuser:

~]$ su -
Password:

Configuring the Memory Usage

To configure the amount of memory that is reserved for the kdump kernel, open the /boot/grub/grub.conf file in a text editor and add the crashkernel=<size>M@16M parameter to the list of kernel options as shown in Example 44.1, "A sample /boot/grub/grub.conf file".

Example 44.1. A sample /boot/grub/grub.conf file

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/sda3
#          initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Community Enterprise Linux Server (2.6.18-274.3.1.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-274.3.1.el5 ro root=/dev/sda3 crashkernel=128M@16M
        initrd /initrd-2.6.18-274.3.1.el5.img

When the kdump crash recovery is enabled, the minimum memory requirements increase by the amount of memory reserved for it. This value is determined by a user, and defaults to 128 MB, as lower values proved to be unreliable. For more information on minimum memory requirements for Community Enterprise Linux, refer to the Required minimums section of the Community Enterprise Linux comparison chart.

Configuring the Target Type

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. Note that only one of these options can be set at the moment. The default option is to store the vmcore file in the /var/crash/ directory of the local file system. To change this, 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 #ext3 /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 OpenSSH for information on how to configure an SSH server, and how to set up a key-based authentication.

Configuring the Core Collector

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, 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 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 44.1, "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.

Table 44.1. Supported filtering levels

Changing the Default Action

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, 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 44.2, "Supported actions". For example:

default halt

Table 44.2. Supported actions

Enabling the Service

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

~]# chkconfig kdump on

This will enable the service for runlevels 2, 3, 4, and 5. Similarly, typing chkconfig kdump off will disable it for all runlevels. To start the service in the current session, use the following command:

~]# service kdump start
No kdump initial ramdisk found.                            [WARNING]
Rebuilding /boot/initrd-2.6.18-194.8.1.el5kdump.img
Starting kdump:                                            [  OK  ]

For more information on runlevels and configuring services in general, refer to Controlling Access to Services.

Testing the Configuration

The commands below will cause the kernel to crash. Use caution when following these steps, and by no means use them on a production machine.

To test the configuration, reboot the system with kdump enabled, and make sure that the service is running:

~]# service kdump status
Kdump is operational

Then type the following commands at a shell prompt:

~]# echo 1 > /proc/sys/kernel/sysrq
~]# echo c > /proc/sysrq-trigger

This will force the Linux kernel to crash, and the YYYY-MM-DD-HH:MM/vmcore file will be copied to the location you have selected in the configuration (that is, to /var/crash/ by default).