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kexec-kdump-howto.txt: Add document about initramfs rebiuld 

Add some note about the limitation of kdumpctl's auto detect and rebuild
feature, and suggest the user to rebuild the initramfs manually on
major system change, and don't include the initramfs in disk images.

Put the note about system change in front part of the document so user
will less likely to miss it.

Signed-off-by: Kairui Song <kasong@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>
This commit is contained in:
Kairui Song 2019-05-10 15:09:44 +08:00
parent 1c1159a586
commit f544a12e78
1 changed files with 30 additions and 0 deletions
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kexec-kdump-howto.txt
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@ -136,6 +136,22 @@ perform postmortem analysis:
and so on...
Notes:
When kdump starts, the kdump kernel is loaded together with the kdump
initramfs. To save memory usage and disk space, the kdump initramfs is
generated strictly against the system it will run on, and contains the
minimum set of kernel modules and utilities to boot the machine to a stage
where the dump target could be mounted.
With kdump service enabled, kdumpctl will try to detect possible system
change and rebuild the kdump initramfs if needed. But it can not guarantee
to cover every possible case. So after a hardware change, disk migration,
storage setup update or any similar system level changes, it's highly
recommended to rebuild the initramfs manually with following command:
# kdumpctl rebuild
Saving vmcore-dmesg.txt
----------------------
Kernel log bufferes are one of the most important information available
@ -654,6 +670,20 @@ not been written in watchdog-core framework then this option will not have
any effect and module will not be added. Please note that only systemd
watchdog daemon is supported as watchdog kick application.
Notes for disk images:
Kdump initramfs is a critical component for capturing the crash dump.
But it's strictly generated for the machine it will run on, and have
no generality. If you install a new machine with a previous disk image
(eg. VMs created with disk image or snapshot), kdump could be broken
easily due to hardware changes or disk ID changes. So it's strongly
recommended to not include the kdump initramfs in the disk image in the
first place, this helps to save space, and kdumpctl will build the
initramfs automatically if it's missing. If you have already installed
a machine with a disk image which have kdump initramfs embedded, you
should rebuild the initramfs using "kdumpctl rebuild" command manually,
or else kdump may not work as expeceted.
Parallel Dumping Operation
==========================
Kexec allows kdump using multiple cpus. So parallel feature can accelerate