# rpm-ostree kargs --append='crashkernel=256M'
OKD runs on FCOS. You can follow these procedures to troubleshoot problems related to the operating system.
kdump service, included in
kexec-tools, provides a crash-dumping mechanism. You can use this service to save the contents of the system’s memory for later analysis.
kdump service is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.
For more information about the support scope of Red Hat Technology Preview features, see https://access.redhat.com/support/offerings/techpreview/.
FCOS ships with
kexec-tools, but manual configuration is required to enable
Perform the following steps to enable
kdump on FCOS:
To reserve memory for the crash kernel during the first kernel booting, provide kernel arguments by entering the following command:
# rpm-ostree kargs --append='crashkernel=256M'
Optional: To write the crash dump over the network or to some other location, rather than to the default local
/var/crash location, edit the
/etc/kdump.conf configuration file.
Network dumps are required when using LUKS.
For details on configuring the
kdump service, see the comments in
/etc/kdump.conf, and the
kdump.conf manual page.
kdump systemd service:
# systemctl enable kdump.service
Reboot your system:
# systemctl reboot
kdump has loaded a crash kernel by checking that the
kdump.service has started and exited successfully and that
cat /sys/kernel/kexec_crash_loaded prints
kdump service is intended to be enabled per node to debug kernel problems. Because there are costs to having
kdump enabled, and these costs accumulate with each additional
kdump-enabled node, it is recommended that
kdump only be enabled on each node as needed. Potential costs of enabling
kdump on each node include:
Less available RAM due to memory being reserved for the crash kernel.
Node unavailability while the kernel is dumping the core.
Additional storage space being used to store the crash dumps.
Not being production-ready because the
kdump service is in Technology Preview.
If you are aware of the downsides and trade-offs of having the
kdump service enabled, it is possible to enable
kdump in a cluster-wide fashion. Although machine-specific machine configs are not yet supported, you can perform the previous steps through a
systemd unit in a
MachineConfig object on day-1 and have kdump enabled on all nodes in the cluster. You can create a
MachineConfig object and inject that object into the set of manifest files used by Ignition during cluster setup. See "Customizing nodes" in the Installing → Installation configuration section for more information and examples on how to use Ignition configs.
MachineConfig object for cluster-wide configuration:
Create a Butane config file,
99-worker-kdump.bu, that configures and enables kdump:
variant: openshift version: 4.8.0 metadata: name: 99-worker-kdump (1) labels: machineconfiguration.openshift.io/role: worker (1) openshift: kernel_arguments: (2) - crashkernel=256M storage: files: - path: /etc/kdump.conf (3) mode: 0644 overwrite: true contents: inline: | path /var/crash core_collector makedumpfile -l --message-level 7 -d 31 - path: /etc/sysconfig/kdump (4) mode: 0644 overwrite: true contents: inline: | KDUMP_COMMANDLINE_REMOVE="hugepages hugepagesz slub_debug quiet log_buf_len swiotlb" KDUMP_COMMANDLINE_APPEND="irqpoll nr_cpus=1 reset_devices cgroup_disable=memory mce=off numa=off udev.children-max=2 panic=10 rootflags=nofail acpi_no_memhotplug transparent_hugepage=never nokaslr novmcoredd hest_disable" KEXEC_ARGS="-s" KDUMP_IMG="vmlinuz" systemd: units: - name: kdump.service enabled: true
|2||Provide kernel arguments to reserve memory for the crash kernel. You can add other kernel arguments if necessary.|
|3||If you want to change the contents of
|4||If you want to change the contents of
Use Butane to generate a machine config YAML file,
99-worker-kdump.yaml, containing the configuration to be delivered to the nodes:
$ butane 99-worker-kdump.bu -o 99-worker-kdump.yaml
Put the YAML file into manifests during cluster setup. You can also create this
MachineConfig object after cluster setup with the YAML file:
$ oc create -f ./99-worker-kdump.yaml
See the Capturing the Dump section in the Fedora documentation for kdump.
See the Dump Analysis section in the Fedora documentation for kdump.
It is recommended to perform vmcore analysis on a separate Fedora system.
kdump.conf(5) — a manual page for the
/etc/kdump.conf configuration file containing the full documentation of available options
kexec(8) — a manual page for the
Red Hat Knowledgebase article regarding kexec and kdump
If a machine cannot be provisioned, Ignition fails and FCOS will boot into the emergency shell. Use the following procedure to get debugging information.
Run the following command to show which service units failed:
$ systemctl --failed
Optional: Run the following command on an individual service unit to find out more information:
$ journalctl -u <unit>.service