b229a9773c
(1) explain early kdump a little clearer in "Introduction"
(2) move the notes out to a new "Notes" section for readability and
add a note about need of reconfiguration after kernel update
(3) change journalctl -x option to -b option because -x is unnecessary
and -b will make it very faster if persistent journal is available
(4) shorten the example messages for readability
(5) add a note to "Limitation" about the earliness of early kdump
Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>
Acked-by: Lianbo Jiang <lijiang@redhat.com>
96 lines
3.5 KiB
Plaintext
96 lines
3.5 KiB
Plaintext
Early Kdump HOWTO
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Introduction
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------------
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Early kdump is a mechanism to make kdump operational earlier than normal kdump
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service. The kdump service starts early enough for general crash cases, but
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there are some cases where it has no chance to make kdump operational in boot
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sequence, such as detecting devices and starting early services. If you hit
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such a case, early kdump may allow you to get more information of it.
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Early kdump is implemented as a dracut module. It adds a kernel (vmlinuz) and
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initramfs for kdump to your system's initramfs in order to load them as early
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as possible. After that, if you provide "rd.earlykdump" in kernel command line,
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then in the initramfs, early kdump will load those files like the normal kdump
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service. This 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 --force --add earlykdump
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NOTE: Recommend to backup the original system initramfs before performing
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this step to put it back if something happens during boot-up.
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3. add rd.earlykdump in grub kernel command line.
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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 -b | grep early-kdump
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Then, you will see some useful logs, for example:
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- if early kdump is successful.
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Mar 09 09:57:56 localhost dracut-cmdline[190]: early-kdump is enabled.
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Mar 09 09:57:56 localhost dracut-cmdline[190]: kexec: loaded early-kdump kernel
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- if early kdump is disabled.
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Mar 09 10:02:47 localhost dracut-cmdline[189]: early-kdump is disabled.
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Notes
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-----
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- The size of early kdump initramfs will be large because it includes vmlinuz
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and kdump initramfs.
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- 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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- If you install an updated kernel and reboot the system with it, the early
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kdump will be disabled by default. To enable it with the new kernel, you
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need to take the above steps again.
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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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- Early kdump loads a crash kernel and initramfs at the beginning of the
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process in system's initramfs, so a crash at earlier than that (e.g. in
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kernel initialization) cannot be captured even with the early kdump.
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