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fadump: update fadump-howto.txt with some troubleshooting help

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Add recommendations on how much memory is required for FADump. Also,
mention the optimizations applied to default initrd when FADump is
used and how to workaround it.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Pingfan Liu <piliu@redhat.com>
This commit is contained in:
Hari Bathini 2020-03-23 18:58:49 +05:30 committed by Kairui Song
parent ce0305d4f9
commit d624a0326f
1 changed files with 78 additions and 8 deletions
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fadump-howto.txt
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@ -84,19 +84,25 @@ kexec-tools installed. If not, you install it via the following command:
# yum install kexec-tools
To be able to do much of anything interesting in the way of debug analysis,
you'll also need to install the kernel-debuginfo package, of the same arch
as your running kernel, and the crash utility:
Make the kernel to be configured with FADump as the default boot entry, if
it isn't already:
# grubby --set-default=/boot/vmlinuz-<kver>
Boot into the kernel to be configured for FADump. To be able to do much of
anything interesting in the way of debug analysis, you'll also need to install
the kernel-debuginfo package, of the same arch as your running kernel, and the
crash utility:
# yum --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash
Next up, we need to modify some boot parameters to enable firmware assisted
dump. With the help of grubby, it's very easy to append "fadump=on" to the end
of your kernel boot parameters. Optionally, user can also append
'fadump_reserve_mem=X' kernel cmdline to specify size of the memory to reserve
for boot memory dump preservation.
of your kernel boot parameters. To reserve the appropriate amount of memory
for boot memory preservation, pass 'crashkernel=X' kernel cmdline parameter.
For the recommended value of X, see 'FADump Memory Requirements' section.
# grubby --args="fadump=on" --update-kernel=/boot/vmlinuz-`uname -r`
# grubby --args="fadump=on crashkernel=6G" --update-kernel=/boot/vmlinuz-`uname -r`
The term 'boot memory' means size of the low memory chunk that is required for
a kernel to boot successfully when booted with restricted memory. By default,
@ -145,7 +151,7 @@ perform postmortem analysis:
and so on...
Saving vmcore-dmesg.txt
----------------------
-----------------------
Kernel log bufferes are one of the most important information available
in vmcore. Now before saving vmcore, kernel log bufferes are extracted
from /proc/vmcore and saved into a file vmcore-dmesg.txt. After
@ -153,6 +159,57 @@ vmcore-dmesg.txt, vmcore is saved. Destination disk and directory for
vmcore-dmesg.txt is same as vmcore. Note that kernel log buffers will
not be available if dump target is raw device.
FADump Memory Requirements:
System Memory Recommended memory
--------------------- ----------------------
4 GB - 16 GB : 768 MB
16 GB - 64 GB : 1024 MB
64 GB - 128 GB : 2 GB
128 GB - 1 TB : 4 GB
1 TB - 2 TB : 6 GB
2 TB - 4 TB : 12 GB
4 TB - 8 TB : 20 GB
8 TB - 16 TB : 36 GB
16 TB - 32 TB : 64 GB
32 TB - 64 TB : 128 GB
64 TB & above : 180 GB
Things to remember:
1) The memory required to boot capture Kernel is a moving target that depends
on many factors like hardware attached to the system, kernel and modules in
use, packages installed and services enabled, there is no one-size-fits-all.
But the above recommendations are based on system memory. So, the above
recommendations for FADump come with a few assumptions, based on available
system memory, about the resources the system could have. So, please take
the recommendations with a pinch of salt and remember to try capturing dump
a few times to confirm that the system is configured successfully with dump
capturing support.
2) Though the memory requirements for FADump seem high, this memory is not
completely set aside but made available for userspace applications to use,
through the CMA allocator.
3) As the same initrd is used for booting production kernel as well as capture
kernel and with dump being captured in a restricted memory environment, few
optimizations (like not inclding network dracut module, disabling multipath
and such) are applied while building the initrd. In case, the production
environment needs these optimizations to be avoided, dracut_args option in
/etc/kdump.conf file could be leveraged. For example, if a user wishes for
network module to be included in the initrd, adding the below entry in
/etc/kdump.conf file and restarting kdump service would take care of it.
dracut_args --add "network"
4) If FADump is configured to capture vmcore to a remote dump target using SSH
or NFS protocol, the network interface is renamed to kdump-<interface-name>
if <interface-name> is generic, for example, *eth#, or net#. This problem
occurs because the vmcore capture scripts in the initial RAM disk (initrd)
add the kdump- prefix to the network interface name to secure persistent
naming. As the same initrd is used for production kernel boot, the interface
name is changed for the production kernel too.
Dump Triggering methods:
This section talks about the various ways, other than a Kernel Panic, in which
@ -250,3 +307,16 @@ initramfs for the time being. If you need modify "KDUMP_COMMANDLINE=" in
options are copied from /proc/cmdline. In general it is best to append
command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing
the original command line completely.
How to disable FADump:
Remove "fadump=on" from kernel cmdline parameters:
# grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="fadump=on"
If KDump is to be used as the dump capturing mechanism, update the crashkernel
parameter (Else, remove "crashkernel=" parameter too, using grubby):
# grubby --update-kernel=/boot/vmlinuz-$kver --args="crashkernl=auto"
Reboot the system for the settings to take effect.