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

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>

fadump-howto.txt

@@ -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.