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32.2.3. Configuring kdump on the Command Line

Configuring the Memory Usage

Memory reserved for the kdump kernel is always reserved during system boot, which means that the amount of memory is specified in the system's boot loader configuration. This section will explain how to change the amount of reserved memory on AMD64 and Intel 64 systems and IBM Power Systems servers using the GRUB boot loader, and on IBM System z using zipl. To configure the amount of memory to be reserved for the kdump kernel, edit the /boot/grub/grub.conf file and add crashkernel=<size>M or crashkernel=auto to the list of kernel options as shown in Example 32.1, “A sample /boot/grub/grub.conf file”. Note that the crashkernel=auto option only reserves the memory if the physical memory of the system is equal to or greater than:
  • 2 GB on 32-bit and 64-bit x86 architectures;
  • 2 GB on PowerPC if the page size is 4 KB, or 8 GB otherwise;
  • 4 GB on IBM S/390.

Example 32.1. 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
          title Red Hat Enterprise Linux Server (2.6.32-220.el6.x86_64)
          root (hd0,0)
          kernel /vmlinuz-2.6.32-220.el6.x86_64 ro root=/dev/sda3 crashkernel=128M
          initrd /initramfs-2.6.32-220.el6.x86_64.img


This section is available only if the system has enough memory. To learn about minimum memory requirements of the Red Hat Enterprise Linux 6 system, read the Required minimums section of the Red Hat Enterprise Linux Technology Capabilities and Limits comparison chart. 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 the user, and defaults to 128 MB plus 64 MB for each TB of physical memory (that is, a total of 192 MB for a system with 1 TB of physical memory). The memory can be attempted up to the maximum of 896 MB if required. This is recommended especially in large environments, for example in systems with a large number of Logical Unit Numbers (LUNs).

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. 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 want 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 line, and replace the value with a valid host name and directory path. For example:
To store the dump to a remote machine using the SSH protocol, remove the hash sign (#) from the beginning of the #net line, and replace the value with a valid user name and host name. For example:
See Chapter 14, 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 32.1, “Supported kdump targets”.


When using Direct-Access Storage Devices (DASDs) as the kdump target, the devices must be specified in the /etc/dasd.conf file with other DASDs, for example:
Where 0.0.2298 and 0.0.2398 are the DASDs used as the kdump target.
Similarly, when using FCP-attached Small Computer System Interface (SCSI) disks as the kdump target, the disks must be specified in the /etc/zfcp.conf file with other FCP-Attached SCSI disks, for example:
0.0.3d0c 0x500507630508c1ae 0x402424aa00000000
0.0.3d0c 0x500507630508c1ae 0x402424ab00000000
0.0.3d0c 0x500507630508c1ae 0x402424ac00000000
Where 0.0.3d0c 0x500507630508c1ae 0x402424ab00000000 and 0.0.3d0c 0x500507630508c1ae 0x402424ac00000000 are the FCP-attached SCSI disks used as the kdump target.
See the Adding DASDs and Adding FCP-Attached Logical Units (LUNs) chapters in the Installation Guide for Red Hat Enterprise Linux 6 for detailed information about configuring DASDs and FCP-attached SCSI disks.


When transferring a core file to a remote target over SSH, the core file needs to be serialized for the transfer. This creates a vmcore.flat file in the /var/crash/ directory on the target system, which is unreadable by the crash utility. To convert vmcore.flat to a dump file that is readable by crash, run the following command as root on the target system:
~]# /usr/sbin/makedumpfile -R */tmp/vmcore-rearranged* < *vmcore.flat*

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, 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 32.2, “Supported filtering levels”. For example, to remove both zero and free pages, use the following:
core_collector makedumpfile -d 17 -c
See the manual page for makedumpfile for a complete list of available options.

Table 32.2. Supported filtering levels

Option Description
1 Zero pages
2 Cache pages
4 Cache private
8 User pages
16 Free pages

Changing the Default Action

With Red Hat Enterprise Linux 6.0, up to, and including version 6.2, the default action when kdump fails to create a core dump, the root file system is mounted and /sbin/init is run.
From Red Hat Enterprise Linux 6.3 onwards, the default behavior is to reboot the machine. This change was necessary to ensure that kdump could operate reliably using less reserved memory. To allow the previous behavior, the mount_root_run_init option has been added to Table 32.3, “Supported actions”.
To change the default 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 32.3, “Supported actions”.

Table 32.3. Supported actions

Option Description
reboot Reboot the system, losing the core in the process.
halt Halt the system.
poweroff Power off the system.
shell Run the msh session from within the initramfs, allowing a user to record the core manually.
mount_root_run_init Enable the default failback behavior from Red Hat Enterprise Linux 6.2 and earlier.
For example:
default halt

Enabling the Service

To start the kdump daemon at boot time, type the following at a shell prompt as root:
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 as root:
service kdump start
For more information on runlevels and configuring services in general, see Chapter 12, Services and Daemons.