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The kdump procedure

The received warning means the kdump operation might fail and the crashdump parameter should be configured correctly. This is the procedure of kdumping:

1. The normal kernel is booted with crashkernel=… as a kernel option, reserving some memory for the kdump kernel. The memory reserved by the crashkernel parameter is not available to the normal kernel during regular operation. It is reserved for later use by the kdump kernel.
2. The system panics.
3. The kdump kernel is booted using kexec, it used the memory area that was reserved w/ the crashkernel parameter.
4. The normal kernel’s memory is captured into a vmcore.

Note: Not reserving enough memory for the kdump kernel can lead to the kdump operation failing.

Configuring crashkernel

RHEL6.0 and RHEL6.1 kernels

RHEL6.2 (and later) kernels

Starting with RHEL6.2 kernels crashkernel=auto should be used. The kernel will automatically reserve an appropriate amount of memory for the kdump kernel. crashkernel is configured   in file /etc/grub.conf

Keep in mind that it is an algorithmically calculated memory reservation and might not meet the needs of all systems (Especially for configurations with lots of IO cards and loaded drivers). So always make sure that memory reserved by crashkernel=auto is sufficient for the target machine by testing kdump. If it is not, reserve more memory by syntax crashkernel= XM (X is amount of memory to be reserved in mega bytes).

Additionally some improvements have been made in the RHEL6.2 kernel which have reduced the overall memory requirements of kdump.

Note: It is recommended to test and verify that kdump is working on all systems after installation of all applications. The memory reserved by crashkernel=auto takes only typical RHEL configurations into account. Some hardware and larger configurations with many option cards may not work well with with crashkernel=auto, in this case the use of crashkernel=512M or more may be a recommended size to start.

   Prior to the 6.3GA release, crashkernel=auto will only reserve memory on systems with 4GB or more physical memory. If the system has less than 4GB of memory the memory must be reserved by explicitly requesting the reservation size, for example: crashkernel=128M. Since the 6.3GA release (kernel-2.6.32-279.el6), this limit has been lowered to 2GB.

    Some environments still require manual configuration of the crashkernel option, for example if dumps to very large local filesystems are performed.

 RHEL 7

crashkernel is configured in the GRUB_CMDLINE_LINUX line in /etc/default/grub:

GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=rhel00/root rd.lvm.lv=rhel00/swap
 rhgb quiet"
GRUB_DISABLE_RECOVERY="true"

After modifying /etc/default/grub, regenerate the GRUB2 configuration using the edited default file. If your system uses BIOS firmware, execute the following command:

# grub2-mkconfig -o /boot/grub2/grub.cfg

On a system with UEFI firmware, execute the following instead:

# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

Note:

RHEL7 with crashkernel=auto will only reserve memory on systems with 2GB or more physical memory. If the system has less than 2GB of memory the memory must be reserved by explicitly requesting the reservation size, for example: crashkernel=128M.  Some environments still require manual configuration of the crashkernel option, e.g A 2TB oracle databasehost crash ,kdump not work,  to modify crashkernel=2048M, then it work fine.

how much memory was reserved for the kdump kernel?

This is available when executing cat /proc/cmdline. Even when the kernel was started with crashkernel=auto then /proc/cmdline will contain the computed value that got reserved.

• cat /proc/cmdline
• cat /sys/kernel/kexec_crash_size

Where to find generated vmcores

When the hypervisor comes back up, the vmcore can be found by default under /var/crash/: (SLES is /var/log/dump)

# egrep '^path' /etc/kdump.conf
path /var/crash

The crash tool in kernel-debuginfo for analysis.

How to test kump work?

# echo c > /proc/sysrq-trigger

Note:  This command will cause the operating system to crash, do not test during the run-time of the industry branch.

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