2021-06-24 20:01:45 +00:00
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Introduction
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============
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This document describes features the kexec-tools package provides for setting
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and estimating the crashkernel value.
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Kdump lives in a pre-reserved chunk of memory, and the size of the reserved
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memory is specified by the `crashkernel=` kernel parameter. It's hard to
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estimate an accurate `crashkernel=` value, so it's always recommended to test
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kdump after you updated the `crashkernel=` value or changed the dump target.
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Default crashkernel value
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=========================
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2022-01-05 03:53:22 +00:00
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Latest kexec-tools provides "kdumpctl get-default-crashkernel" to retrieve
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the default crashkernel value,
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2021-06-24 20:01:45 +00:00
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2022-01-05 03:53:22 +00:00
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$ echo $(kdumpctl get-default-crashkernel)
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1G-4G:192M,4G-64G:256M,64G-102400T:512M
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2022-01-05 03:53:22 +00:00
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It will be taken as the default value of 'crashkernel=', you can use
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this value as a reference for setting crashkernel value manually.
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2021-06-24 20:01:45 +00:00
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New installed system
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====================
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2021-12-07 12:41:28 +00:00
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Anaconda is the OS installer which sets all the kernel boot cmdline on a newly
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installed system. If kdump is enabled during Anaconda installation, Anaconda
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2022-01-05 03:53:22 +00:00
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will use the default crashkernel value as the default `crashkernel=` value on
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2021-12-07 12:41:28 +00:00
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the newly installed system.
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2021-06-24 20:01:45 +00:00
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2021-12-07 12:41:28 +00:00
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Users can override the value during Anaconda installation manually.
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2021-06-24 20:01:45 +00:00
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Auto update of crashkernel boot parameter
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=========================================
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2022-05-11 02:25:56 +00:00
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A new release of kexec-tools could update the default crashkernel value. By
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default, kexec-tools would reset crashkernel to the new default value if it
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detects the old default crashkernel value is used by installed kernels. If you
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don't want kexec-tools to update the old default crashkernel to the new default
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crashkernel, you can change auto_reset_crashkernel to no in kdump.conf.
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Supported Bootloaders
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---------------------
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This auto update only works with GRUB2 and ZIPL, as kexec-tools heavily depends
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on `grubby`. If other boot loaders are used, the user will have to update the
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`crashkernel=` value manually.
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Reset crashkernel to default value
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==================================
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kexec-tools only perform the auto update of crashkernel value when it can
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confirm the boot kernel's crashkernel value is using its corresponding default
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value and auto_reset_crashkernel=yes in kdump.conf. In other cases, the user
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can reset the crashkernel value by themselves.
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Reset using kdumpctl
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--------------------
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To make it easier to reset the `crashkernel=` kernel cmdline to this default
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value properly, `kdumpctl` also provides a sub-command:
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2022-01-05 03:53:22 +00:00
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`kdumpctl reset-crashkernel [--kernel=path_to_kernel] [--reboot]`
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This command will reset the bootloader's kernel cmdline to the default value.
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It will also update bootloader config if the bootloader has a standalone config
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file. User will have to reboot the machine after this command to make it take
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effect if --reboot is not specified. For more details, please refer to the
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reset-crashkernel command in `man kdumpctl`.
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Reset manually
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--------------
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To reset the crashkernel value manually, it's recommended to use utils like
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`grubby`. A one liner script for resetting `crashkernel=` value of all installed
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kernels to the default value is:
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2022-01-05 03:53:22 +00:00
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grubby --update-kernel ALL --args "crashkernel=$(kdumpctl get-default-crashkernel)"
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Estimate crashkernel
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====================
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The best way to estimate a usable crashkernel value is by testing kdump
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manually. And you can set crashkernel to a large value, then adjust the
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crashkernel value to an acceptable value gradually.
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`kdumpctl` also provides a sub-command for doing rough estimating without
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triggering kdump:
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`kdumpctl estimate`
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The output will be like this:
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```
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Encrypted kdump target requires extra memory, assuming using the keyslot with minimum memory requirement
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Reserved crashkernel: 256M
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Recommended crashkernel: 655M
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Kernel image size: 47M
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Kernel modules size: 12M
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Initramfs size: 19M
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Runtime reservation: 64M
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LUKS required size: 512M
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Large modules:
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xfs: 1892352
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nouveau: 2318336
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WARNING: Current crashkernel size is lower than recommended size 655M.
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```
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It will generate a summary report about the estimated memory consumption
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of each component of kdump. The value may not be accurate enough, but
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would be a good start for finding a suitable crashkernel value.
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