13.2. About Kdump

13.2. About Kdump
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13.2.1. Configuring and Using Kdump
13.2.2. Files Used by Kdump

Kdump is the Linux kernel crash-dump mechanism. Oracle recommends that you enable the Kdump feature. In the event of a system crash, Kdump creates a memory image (vmcore) that can help in determining the cause of the crash. Enabling Kdump requires you to reserve a portion of system memory for exclusive use by Kdump. This memory is unavailable for other uses.

Kdump uses kexec to boot into a second kernel whenever the system crashes. kexec is a fast-boot mechanism which allows a Linux kernel to boot from inside the context of a kernel that is already running without passing through the bootloader stage.

13.2.1. Configuring and Using Kdump

During installation, you are given the option of enabling Kdump and specifying the amount of memory to reserve for it. If you prefer, you can enable kdump at a later time as described in this section.

If the kexec-tools package is not already installed on your system, use yum to install it.

To enable Kdump by using the Kernel Dump Configuration GUI.

Enter the following command.
```
# system-config-kdump
```
The Kernel Dump Configuration GUI starts. If Kdump is currently disabled, the green Enable button is selectable and the Disable button is greyed out.
Click Enable to enable Kdump.
You can select the following settings tags to adjust the configuration of Kdump.
Basic Settings
Allows you to specify the amount of memory to reserve for Kdump. The default setting is 128 MB.
Target Settings
Allows you to specify the target location for the vmcore dump file on a locally accessible file system, to a raw disk device, or to a remote directory using NFS or SSH over IPv4. The default location is /var/crash.
You cannot save a dump file on an eCryptfs file system, on remote directories that are NFS mounted on the rootfs file system, or on remote directories that access require the use of IPv6, SMB, CIFS, FCoE, wireless NICs, multipathed storage, or iSCSI over software initiators to access them.
Filtering Settings
Allows to select which type of data to include in or exclude from the dump file. Selecting or deselecting the options alters the value of the argument that Kdump specifies to the -d option of the core collector program, makedumpfile.
Expert Settings
Allows you to choose which kernel to use, edit the command line options that are passed to the kernel and the core collector program, choose the default action if the dump fails, and modify the options to the core collector program, makedumpfile.
For example, if Kdump fails to start, and the following error appears in /var/log/messages:
```
kdump: No crashkernel parameter specified for running kernel
```
set the offset for the reserved memory to 48 MB or greater in the command line options, for example: crashkernel=128M@48M.
For systems with more than 128 GB of memory, the recommended setting is crashkernel=512M@64M.
Click Help for more information on these settings.
Click Apply to save your changes. The GUI displays a popup message to remind you that you must reboot the system for the changes to take effect.
Click OK to dismiss the popup messages.
Select File > Quit.
Reboot the system at a suitable time.

13.2.2. Files Used by Kdump

The Kernel Dump Configuration GUI modifies the following files:

File	Description
`/boot/grub/grub.conf`	Appends the `crashkernel` option to the kernel line to specify the amount of reserved memory and any offset value. Note The Unbreakable Enterprise Kernel does not support the use of the setting `crashkernel=auto`.
`/etc/kdump.conf`	Sets the location where the dump file can be written, the filtering level for the makedumpfile command, and the default behavior to take if the dump fails. See the comments in the file for information about the supported parameters.

If you edit these files, you must reboot the system to have the changes take effect.

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