Red Hat Enterprise Linux 5 Administration Unleashed

434CHAPTER 21Monitoring and Tuning the Kernelfor Kdump to function properly. The kexec-tools package (and the kernel-kdumppackage for ppc64) should already be installed. If it is not, refer to Chapter 3 for instructionson installing RPM packages from Red Hat Network.This section describes how to capture a kernel dump when the system crashes and thenhow to analyze its contents.NOTEKdump is only available for the i686, x86_64, ia64, and ppc64 kernels in Red HatEnterprise Linux.The Kdump program cannot be used with the Virtualization kernels. If the output of theuname -r command ends in xen, you are running a Virtualization kernel. Refer to the“Installing a New Kernel” section of Chapter 5 for details on booting a different kernel.Booting with KexecKexec is usually used in conjunction with Kdump to boot into a secondary kernel so thatthe memory from the initial kernel is preserved. However, it can be used alone to performa warm reboot. A warm reboot uses the context of the running kernel to reboot the systemwithout going through the BIOS, resulting in a faster boot time.To perform a warm reboot with Kexec, use the following steps:1. Load the kernel to boot in to the currently running kernel (must be executed as theroot user):kexec -l /boot/vmlinuz-`uname -r` --initrd=/boot/initrd-`uname -r`.img \--command-line=”`cat /proc/cmdline`”Notice that the command includes three commands in back quotes (uname -r twiceand cat /proc/cmdline). Because these commands are in back quotes, the results ofthe commands replace the back quotes and the commands when the entirecommand is executed. Because uname -r returns the currently running kernel, thiskexec command will reboot into the same kernel version. To boot in to a differentkernel, use the different kernel version instead.The cat /proc/cmdline command embedded in the kexec command sets the kernelparameters to be used for the warm reboot to the parameters used to boot thecurrently running kernel. To use different parameters, list them instead.2. Reboot the system and watch the warm reboot. This can be performed by executingthe reboot command as root from the command line or selecting System, ShutDown from the desktop menus.As the system reboots, you will notice that after going through the normal shutdownprocess, you see the Linux startup messages immediately. You do not see the system BIOSor the GRUB boot loader.