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-2G:192M,2G-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]`
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. 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.
