Red Hat Enterprise Linux 5 Administration Unleashed

440CHAPTER 21Monitoring and Tuning the KernelFIGURE 21.1Kdump Graphical ConfigurationClick OK to save the changes to the Kdump configuration file and the GRUB boot loaderconfiguration file. If you just enabled Kdump or changed the amount of reservedmemory, a message appears reminding you that you must reboot the system for thechanges to take effect. This allows the set amount of memory to be reserved for thesecondary kernel should a crash occur.Testing KdumpBecause it is difficult to know whether Kdump is working before you have a kernel crash,there is a way to force a kernel crash.echo c > /proc/sysrq-triggerAs soon as the command is execute, there should be a panic, and the system should beunresponsive. Then, the system should restart into the second kernel using Kexec.Because Kexec is used, you will not see the BIOS or GRUB boot screen. After the dump fileis created and saved, the system is rebooted into the normal kernel. This time you will seethe BIOS and GRUB screens. Depending on how big your system memory is, this entireprocess might take a while.Refer to the next section “Analyzing the Crash” to learn how to gather information fromthis dump file.Analyzing the CrashThe location of the dump file depends on the location you selected in /etc/kdump.conf.If no location is set in kdump.conf, the vmcore file is written to the /var/crash//