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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-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] [--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.