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Early Kdump HOWTO
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Introduction
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------------
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Kdump service starts too late, so early crashes will have no chance to get
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kdump kernel booting, this will cause crash information to be lost. It is
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necessary to add a dracut module in order to load crash kernel and initramfs
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as early as possible. You can provide "rd.earlykdump" in grub commandline
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to enable, then the early kdump will load those files like the normal kdump,
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which is disabled by default.
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For the normal kdump service, it can check whether the early kdump has loaded
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the crash kernel and initramfs. It has no conflict with the early kdump.
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How to configure early kdump
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----------------------------
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We assume if you're reading this document, you should already have kexec-tools
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installed.
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You can rebuild the initramfs with earlykdump support with below steps:
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1. start kdump service to make sure kdump initramfs is created.
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# systemctl start kdump
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NOTE: If a crash occurs during boot process, early kdump captures a vmcore
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and reboot the system by default, so the system might go into crash loop.
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You can avoid such a crash loop by adding the following settings, which
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power off the system after dump capturing, to kdump.conf in advance:
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final_action poweroff
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failure_action poweroff
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For the failure_action, you can choose anything other than "reboot".
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2. rebuild system initramfs with earlykdump support.
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# dracut --add earlykdump
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3. add rd.earlykdump in grub kernel command line.
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Note:
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[1]. Early kdump initramfs size will be large because it includes vmlinuz and
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kdump initramfs. And for step 2 if you are sure to overwrite system initramfs
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you can backup the original initramfs and use "--force" option.
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[2]. Early kdump inherits the settings of normal kdump, so any changes that
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caused normal kdump rebuilding also require rebuilding the system initramfs
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to make sure that the changes take effect for early kdump. Therefore, after
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the rebuilding of kdump initramfs is completed, provide a prompt message to
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tell the fact.
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After making said changes, reboot your system to take effect. Of course, if you
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want to disable early kdump, you can simply remove "rd.earlykdump" from kernel
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boot parameters in grub, and reboot system like above.
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Once the boot is completed, you can check the status of the early kdump support
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on the command prompt:
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# journalctl -x|grep early-kdump
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Then, you will see some useful logs, for exapmle:
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1. if early kdump is successful.
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Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: early-kdump is enabled.
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Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: kexec: loaded early-
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kdump kernel
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2. if early kdump is disabled.
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Mar 09 10:02:47 localhost.localdomain dracut-cmdline[189]: early-kdump is disabled.
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Limitation
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----------
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At present, early kdump doesn't support fadump.
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