Introduction

============

This document describes features the kexec-tools package provides for setting

and estimating the crashkernel value.

Kdump lives in a pre-reserved chunk of memory, and the size of the reserved

memory is specified by the `crashkernel=` kernel parameter. It's hard to

estimate an accurate `crashkernel=` value, so it's always recommended to test

kdump after you updated the `crashkernel=` value or changed the dump target.

Default crashkernel value

=========================

Latest kexec-tools provides "kdumpctl get-default-crashkernel" to retrieve

the default crashkernel value,

	$ echo $(kdumpctl get-default-crashkernel)

	1G-4G:192M,4G-64G:256M,64G-:512M

It will be taken as the default value of 'crashkernel=', you can use

this value as a reference for setting crashkernel value manually.

New installed system

====================

Anaconda is the OS installer which sets all the kernel boot cmdline on a newly

installed system. If kdump is enabled during Anaconda installation, Anaconda

will use the default crashkernel value as the default `crashkernel=` value on

the newly installed system.

Users can override the value during Anaconda installation manually.

Auto update of crashkernel boot parameter

=========================================

A new release of kexec-tools could update the default crashkernel value.  By

default, kexec-tools would reset crashkernel to the new default value if it

detects the old default crashkernel value is used by installed kernels. If you

don't want kexec-tools to update the old default crashkernel to the new default

crashkernel, you can change auto_reset_crashkernel to no in kdump.conf.

Supported Bootloaders

---------------------

This auto update only works with GRUB2 and ZIPL, as kexec-tools heavily depends

on `grubby`. If other boot loaders are used, the user will have to update the

`crashkernel=` value manually.

Reset crashkernel to default value

==================================

kexec-tools only perform the auto update of crashkernel value when it can

confirm the boot kernel's crashkernel value is using its corresponding default

value and auto_reset_crashkernel=yes in kdump.conf. In other cases, the user

can reset the crashkernel value by themselves.

Reset using kdumpctl

--------------------

To make it easier to reset the `crashkernel=` kernel cmdline to this default

value properly, `kdumpctl` also provides a sub-command:

	`kdumpctl reset-crashkernel [--kernel=path_to_kernel] [--reboot]`

This command will reset the bootloader's kernel cmdline to the default value.

It will also update bootloader config if the bootloader has a standalone config

file. User will have to reboot the machine after this command to make it take

effect if --reboot is not specified. For more details, please refer to the

reset-crashkernel command in `man kdumpctl`.

Reset manually

--------------

To reset the crashkernel value manually, it's recommended to use utils like

`grubby`. A one liner script for resetting `crashkernel=` value of all installed

kernels to the default value is:

	grubby --update-kernel ALL --args "crashkernel=$(kdumpctl get-default-crashkernel)"

NOTE: On s390x you also need to run zipl for the change to take effect.

Estimate crashkernel

====================

The best way to estimate a usable crashkernel value is by testing kdump

manually.  And you can set crashkernel to a large value, then adjust the

crashkernel value to an acceptable value gradually.

`kdumpctl` also provides a sub-command for doing rough estimating without

triggering kdump:

	`kdumpctl estimate`

The output will be like this:

```

      Encrypted kdump target requires extra memory, assuming using the keyslot with minimum memory requirement

      Reserved crashkernel:    256M

      Recommended crashkernel: 655M

      Kernel image size:   47M

      Kernel modules size: 12M

      Initramfs size:      19M

      Runtime reservation: 64M

      LUKS required size:  512M

      Large modules:

          xfs: 1892352

          nouveau: 2318336

      WARNING: Current crashkernel size is lower than recommended size 655M.

```

It will generate a summary report about the estimated memory consumption

of each component of kdump. The value may not be accurate enough, but

would be a good start for finding a suitable crashkernel value.