From 0f391cca7c5dd26d5a12fa8e8eb003f934848d31 Mon Sep 17 00:00:00 2001
From: David Timber <dxdt@dev.snart.me>
Date: Mon, 18 Nov 2024 02:42:01 +0100
Subject: Add writeups/disable-kdump/disable-kdump.md

---
 writeups/disable-kdump/disable-kdump.md | 79 +++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 writeups/disable-kdump/disable-kdump.md

diff --git a/writeups/disable-kdump/disable-kdump.md b/writeups/disable-kdump/disable-kdump.md
new file mode 100644
index 0000000..791281e
--- /dev/null
+++ b/writeups/disable-kdump/disable-kdump.md
@@ -0,0 +1,79 @@
+# Disable crashkernel in CentOS and Rocky
+https://forums.rockylinux.org/t/how-do-i-remove-crashkernel-from-cmdline/13346
+
+You really don't need crashkernel in a VM. Reserving more than 100MB of memory
+just for kernel dump in the unlikely event of kernel panic is such a luxury that
+only bare-metal servers can have. Cloud VMs are often non-mission critical.
+Hypervisors usually run so stable that they hardly cause any of the guest VMs to
+crash.
+
+<pre>
+# dmesg | grep -E 'crashkernel|Memory'
+[    0.000000] crashkernel reserved: 0x00000000ec000000 - 0x0000000100000000 (320 MB)
+[    0.000000] Memory: 3535220K/4096000K available (13248K kernel code, 5484K rwdata, 11172K rodata, 6016K init, 11412K bss, <b>560780K reserved</b>, 0K cma-reserved)
+</pre>
+
+Imagine what you could do with all that 320MB of memory.
+
+## To disable
+Do the following to disable crashkernel entirely for the current and future
+versions of kernel.
+
+```sh
+# DO NOT automatically reset crashkernel settings (ie. never turn it back on)
+sed -E 's/^(\s+)?auto_reset_crashkernel\s.*$//' /etc/kdump.conf > /etc/kdump.conf.tmp &&
+	echo 'auto_reset_crashkernel no' >> /etc/kdump.conf.tmp &&
+	mv -f /etc/kdump.conf.tmp /etc/kdump.conf
+
+# Remove the 'crashkernel=' arg in the kernel cmdline (legacy)
+sed -E 's/\s?crashkernel=\w+\s?/ /' /etc/default/grub > /etc/default/grub.tmp &&
+	mv -f /etc/default/grub.tmp /etc/default/grub
+
+# Administrative settings
+# Prevents the crashkernel option from being added for the later versions
+kdumpctl stop
+kdumpctl rebuild
+systemctl disable --now kdump
+
+# Manually remove the cmdline option from the existing BLS entries
+grubby --update-kernel=ALL --remove-args=crashkernel
+```
+
+Then the kernel won't reserve memory for crashkernel on next boot. See that
+crashkernel memory is no longer being reserved:
+
+<pre>
+# dmesg | grep -E 'crashkernel|Memory'
+[    0.000000] Memory: 3862896K/4096000K available (13248K kernel code, 5484K rwdata, 11172K rodata, 6016K init, 11412K bss, <b>233104K reserved</b>, 0K cma-reserved)
+</pre>
+
+## The full picture
+All of the components work hand in hand for crashkernel. It's unnecessarily
+complicated, not well documented and extremely RHEL-specific.
+
+  1. The kernel reserves memory if `crashkernel=` option is present in the
+     cmdline. Whether the userspace programs are configured to untilise the
+     reserved memory is, as far as the kernel is concerned, irrelevant
+  1. The initramfs image has to be generated for kdump tools
+  1. `/etc/default/grub` is no longer used for `crashkernel=` option (the step
+     above is only for the legacy tools)
+  1. The undocumented subcommand of `kdumpctl` is used to test if crashkernel is
+     enabled in `/usr/lib/kernel/install.d/92-crashkernel.install` when the
+     cmdline for each boot loader entry is composed
+  1. After disabling the kdump service, the `crashkernel=` option has to be
+     removed from each BLS entry using `grubby`
+
+This is a typical example of software bloat. It's based on a bad assumption of
+RHEL devs that all the RHEL distros will usually run on systems with plenty of
+memory.
+
+## Other resources
+https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html/managing_monitoring_and_updating_the_kernel/enabling-kdumpmanaging-monitoring-and-updating-the-kernel#enabling-kdumpmanaging-monitoring-and-updating-the-kernel
+
+Says nothing about the internal script(`92-crashkernel.install`). No mention of
+removing the crashkernel cmdline option to stop memory allocation.
+
+https://bobcares.com/blog/rhel-disable-kdump/
+
+Outdated info. The crashkernel cmdline option in `/etc/default/grub` is no
+longer used. `grub2-mkconfig` cannot be used to update BLS entries.
-- 
cgit