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

2. rebuild system initramfs with earlykdump support.
    # dracut --add earlykdump

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

Note: earlykdump 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.

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.