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early-kdump-howto.txt
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Early Kdump HOWTO Introduction ------------ Early kdump is a mechanism to make kdump operational earlier than normal kdump service. The kdump service starts early enough for general crash cases, but there are some cases where it has no chance to make kdump operational in boot sequence, such as detecting devices and starting early services. If you hit such a case, early kdump may allow you to get more information of it. Early kdump is implemented as a dracut module. It adds a kernel (vmlinuz) and initramfs for kdump to your system's initramfs in order to load them as early as possible. After that, if you provide "rd.earlykdump" in kernel command line, then in the initramfs, early kdump will load those files like the normal kdump service. This is disabled by default. For the normal kdump service, it can check whether the early kdump has loaded the crash kernel and initramfs. It has no conflict with the early kdump. How to configure early kdump ---------------------------- We assume if you're reading this document, you should already have kexec-tools installed. You can rebuild the initramfs with earlykdump support with below steps: 1. start kdump service to make sure kdump initramfs is created. # systemctl start kdump NOTE: If a crash occurs during boot process, early kdump captures a vmcore and reboot the system by default, so the system might go into crash loop. You can avoid such a crash loop by adding the following settings, which power off the system after dump capturing, to kdump.conf in advance: final_action poweroff failure_action poweroff For the failure_action, you can choose anything other than "reboot". 2. rebuild system initramfs with earlykdump support. # dracut --force --add earlykdump NOTE: Recommend to backup the original system initramfs before performing this step to put it back if something happens during boot-up. 3. add rd.earlykdump in grub kernel command line. After making said changes, reboot your system to take effect. Of course, if you want to disable early kdump, you can simply remove "rd.earlykdump" from kernel boot parameters in grub, and reboot system like above. Once the boot is completed, you can check the status of the early kdump support on the command prompt: # journalctl -b | grep early-kdump Then, you will see some useful logs, for example: - if early kdump is successful. Mar 09 09:57:56 localhost dracut-cmdline[190]: early-kdump is enabled. Mar 09 09:57:56 localhost dracut-cmdline[190]: kexec: loaded early-kdump kernel - if early kdump is disabled. Mar 09 10:02:47 localhost dracut-cmdline[189]: early-kdump is disabled. Notes ----- - The size of early kdump initramfs will be large because it includes vmlinuz and kdump initramfs. - Early kdump inherits the settings of normal kdump, so any changes that caused normal kdump rebuilding also require rebuilding the system initramfs to make sure that the changes take effect for early kdump. Therefore, after the rebuilding of kdump initramfs is completed, provide a prompt message to tell the fact. - If you install an updated kernel and reboot the system with it, the early kdump will be disabled by default. To enable it with the new kernel, you need to take the above steps again. Limitation ---------- - At present, early kdump doesn't support fadump. - Early kdump loads a crash kernel and initramfs at the beginning of the process in system's initramfs, so a crash at earlier than that (e.g. in kernel initialization) cannot be captured even with the early kdump.