Early Kdump HOWTO

Introduction
------------

Kdump service starts too late, so early crashes will have no chance to get
kdump kernel booting, this will cause crash information to be lost. It is
necessary to add a dracut module in order to load crash kernel and initramfs
as early as possible. You can provide "rd.earlykdump" in grub commandline
to enable, then the early kdump will load those files like the normal kdump,
which 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 --add earlykdump

3. add rd.earlykdump in grub kernel command line.

Note:
[1]. Early kdump initramfs size will be large because it includes vmlinuz and
kdump initramfs. And for step 2 if you are sure to overwrite system initramfs
you can backup the original initramfs and use "--force" option.

[2]. 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.

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 -x|grep early-kdump

Then, you will see some useful logs, for exapmle:

1. if early kdump is successful.
Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: early-kdump is enabled.
Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: kexec: loaded early-
kdump kernel

2. if early kdump is disabled.
Mar 09 10:02:47 localhost.localdomain dracut-cmdline[189]: early-kdump is disabled.

Limitation
----------

At present, early kdump doesn't support fadump.