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Filename : crashkernel-howto.txt
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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.