update crashkernel-howto
Update crashkernel-howto since crashkernel.default has been removed. The documentation is also simplified as a result. Reviewed-by: Pingfan Liu <piliu@redhat.com> Reviewed-by: Philipp Rudo <prudo@redhat.com> Signed-off-by: Coiby Xu <coxu@redhat.com>
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Coiby Xu
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@ -13,13 +13,14 @@ 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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Latest kernel packages include a `crashkernel.default` file installed in kernel
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modules folder, available as:
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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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/usr/lib/modules/<kernel>/crashkernel.default
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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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The content of the file will be taken as the default value of 'crashkernel=', or
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take this file as a reference for setting crashkernel value manually.
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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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New installed system
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@ -27,7 +28,7 @@ New installed system
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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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will use the `crashkernel.default` file as the default `crashkernel=` value on
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will use the default crashkernel value as the default `crashkernel=` value on
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the newly installed system.
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Users can override the value during Anaconda installation manually.
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@ -36,20 +37,11 @@ Users can override the value during Anaconda installation manually.
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Auto update of crashkernel boot parameter
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=========================================
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Following context in this section assumes all kernel packages have a
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`crashkernel.default` file bundled, which is true for the latest official kernel
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packages. For kexec-tools behavior with a kernel that doesn't have a
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`crashkernel.default` file, please refer to the “Custom Kernel” section of this
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doc.
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When `crashkernel=` is using the default value, kexec-tools will need to update
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the `crashkernel=` value of new installed kernels, since the default value may
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change in new kernel packages.
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kexec-tools does so by adding a kernel installation hook, which gets triggered
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every time a new kernel is installed, so kexec-tools can do necessary checks and
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updates.
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A new release of kexec-tools could update the default crashkernel value.
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By default, kexec-tools would reset crashkernel to the new default value if it
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detects old default crashkernel value is used by installed kernels. If you don't
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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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@ -59,92 +51,13 @@ 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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Updating kernel package
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-----------------------
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When a new version of package kernel is released in the official repository, the
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package will always come with a `crashkernel.default` file bundled. Kexec-tools
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will act with following rules:
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If current boot kernel is using the default `crashkernel=` boot param value from
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its `crashkernel.default` file, then kexec-tools will update new installed
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kernel’s `crashkernel=` boot param using the value from the new installed
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kernel’s `crashkernel.default` file. This ensures `crashkernel=` is always using
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the latest default value.
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If current boot kernel's `crashkernel=` value is set to a non-default value, the
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new installed kernel simply inherits this value.
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On systems using GRUB2 as the bootloader, each kernel has its own boot entry,
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making it possible to set different `crashkernel=` boot param values for
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different kernels. So kexec-tools won’t touch any already installed kernel's
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boot param, only new installed kernel's `crashkernel=` boot param value will be
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updated.
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But some utilities like `grub2-mkconfig` and `grubby` can override all boot
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entry's boot params with the boot params value from the GRUB config file
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`/etc/defaults/grub`, so kexec-tools will also update the GRUB config file in
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case old `crashkernel=` value overrides new installed kernel’s boot param.
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Downgrading kernel package
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--------------------------
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When upgrading a kernel package, kexec-tools may update the `crashkernel=` value
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in GRUB2 config file to the new value. So when downgrading the kernel package,
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kexec-tools will also try to revert that update by setting GRUB2 config file’s
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`crashkernel=` value back to the default value in the older kernel package. This
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will only occur when the GRUB2 config file is using the default `crashkernel=`
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value.
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Custom kernel
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=============
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To make auto crashkernel update more robust, kexec-tools will try to keep
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tracking the default 'crashkernel=` value with kernels that don’t have a
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`crashkernel.default` file, such kernels are referred to as “custom kernel” in
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this doc. This is only a best-effort support to make it easier debugging and
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testing the system.
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When installing a custom kernel that doesn’t have a `crashkernel.default` file,
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the `crashkernel=` value will be simply inherited from the current boot kernel.
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When installing a new official kernel package and current boot kernel is a
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custom kernel, since the boot kernel doesn’t have a `crashkernel.default` file,
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kexec-tools will iterate installed kernels and check if the boot kernel
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inherited the default value from any other existing kernels’
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`crashkernel.default` file. If a matching `crashkernel.default` file is found,
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kexec-tools will update the new installed kernel `crashkernel=` boot param using
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the value from the new installed kernel’s `crashkernel.default` file, ensures
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the auto crashkernel value update won’t break over one or two custom kernel
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installations.
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It is possible that the auto crashkernel value update will fail when custom
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kernels are used. One example is a custom kernel inheriting the default
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`crashkernel=` value from an older official kernel package, but later that
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kernel package is uninstalled. So when booted with the custom kernel,
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kexec-tools can't determine if the boot kernel is inheriting a default
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`crashkernel=` value from any official build. In such a case, please refer to
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the "Reset crashkernel to default value" section of this doc.
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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 or inherited from any installed kernel.
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kexec-tools may fail to determine if the boot kernel is using default
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crashkernel value in some use cases:
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- kexec-tools package is absent during a kernel package upgrade, and the new
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kernel package’s `crashkernel.default` value has changed.
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- Custom kernel is used and the kernel it inherits `crashkernel=` value from is
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uninstalled.
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So it's recommended to reset the crashkernel value if users have uninstalled
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kexec-tools or using a custom kernel.
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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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@ -152,21 +65,23 @@ Reset using kdumpctl
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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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`kdumpctl reset-crashkernel [<kernel version>]`
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`kdumpctl reset-crashkernel [--kernel=path_to_kernel] [--reboot]`
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This command will read from the `crashkernel.default` file and reset
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bootloader's kernel cmdline to the default value. It will also update bootloader
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config if the bootloader has a standalone config file. User will have to reboot
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the machine after this command to make it take effect.
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the machine after this command to make it take effect if --reboot is not specified.
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For ppc64le, an optional "[--fadump=[on|off|nocma]]" can also be specified to toggle
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FADump on/off.
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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 current boot kernel's crashkernel.default` is:
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kernels to current boot kernel's crashkernel.default` is:
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grubby --update-kernel ALL --args "$(cat /usr/lib/modules/$(uname -r)/crashkernel.default)"
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grubby --update-kernel ALL --args "crashkernel=$(kdumpctl get-default-crashkernel)"
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Estimate crashkernel
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====================
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