Thanks for it.
Acked-by: Pingfan Liu <piliu(a)redhat.com>
On 03/23/2020 09:28 PM, Hari Bathini wrote:
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(a)linux.ibm.com>
* The 4th point added in "Things to remember" section can be dropped,
if the fix proposed in Redhat Bug 1796941 is accepted upstream.
fadump-howto.txt | 86 +++++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 78 insertions(+), 8 deletions(-)
diff --git a/fadump-howto.txt b/fadump-howto.txt
index 7061f02..89675a1 100644
@@ -84,19 +84,25 @@ kexec-tools installed. If not, you install it via the following
# 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
# 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
-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"
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...
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
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
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`
+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.