Early Kdump HOWTO

Introduction

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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

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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

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- 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

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- 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.