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editing: early-kdump-howto.txt

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 kdump-utils
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.

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