| KDUMP(7) | User Manuals | KDUMP(7) |
NAME¶
kdump - Saving kernel dumps in SUSE
SYNOPSIS¶
(not applicable)
DESCRIPTION¶
This manual page gives an overview about kdump configuration on SUSE.
Kdump is a technology to save the memory contents of a crashed system and save it to disk or network to analyse it later and find the cause of the crash. When the system crashes, the mechanism uses kexec to boot a normal Linux kernel (that has been loaded into the system previously) which then has access to the old memory contents via /proc/vmcore interface and can save that away.
After the memory has been saved, the system reboots (without kexec).
As mentioned above, that panic kernel has to be loaded into the system. That is accomplished via kexec(8) called by the kdump service at system bootup. To have memory for that panic kernel and also have RAM for the execution of that panic kernel, one has to reserve kernel memory with a special kernel command line option (crashkernel). The option is added to the kernel command line by the kdump-commandline service, but any change will only have effect for the next boot.
AUTOMATIC CONFIGURATION¶
A simple method to use kdump on SUSE is to install the kdump package and enable the kdump.service:
# zypper install yast2-kdump # systemctl enable kdump
To update the bootloader with the default kdump commandline options and generate the kdump initrd, you can call
# systemctl start kdump
This will fail, because the currently running kernel has not been started with the crashkernel commandline option. However the service will also start the kdump-commandline.service, which will by default update the bootloader to provide the option on the next boot.
If you don’t perform this step, the service will fail during the next boot and kdump should be available on the second-next boot.
MANUAL SETUP¶
Bootloader configuration¶
It’s necessary to reserve a certain amount of memory in the normal system at boot time which will be used by kexec(8) to load the panic kernel. To achieve that, you have to add a parameter called crashkernel in bootloader configuration. The syntax is:
crashkernel=size[@offset]
The optional offset is the load offset, i.e. the physical base address on which the memory reservation area starts.
To find a suggested value for size, us the kdumptool calibrate command. The suggested value is the sum of the Low and High values in its output.
Example: crashkernel=256M (on a normal PC system)
Note
There’s also a more advanced syntax that makes the amount of memory dependent on system RAM. See the section called “Extended crashkernel commandline”.
You can either supply the a hand-crafted crashkernel option to the kdump-commandline.service via the KDUMP_CRASHKERNEL config option and leave the job of updating the bootloader to the service.
Or you can disable automatic bootloader updates by setting KDUMP_UPDATE_BOOTLOADER to "false" and managing the bootloader config yourself.
Enable kdump service¶
The kdump systemd service loads the kdump kernel at boot. To enable it, run
# systemctl enable kdump
Configure kdump¶
The default configuration should work out of the box. You can tweak several configuration options in the /etc/sysconfig/kdump configuration file.
Important
If you make changes in that configuration file, you always have to execute systemctl restart kdump manually to make that changes apply. If you don’t, that changes will only apply after system reboot.
See the section “CONFIGURATION” later and/or kdump(5) for a description of the configuration options.
TESTING¶
It perfectly makes sense to test the kdump configuration in a sane system state, i.e. not when the system really crashes but to trigger the dump manually. To perform that, use the SysRq mechanism, i.e. just execute
# echo s > /proc/sysrq-trigger # echo u > /proc/sysrq-trigger # echo c > /proc/sysrq-trigger
After that, the panic kernel should boot and the dump should be saved.
The most common problem is that the panic kernel will fail because the default amount of memory reserved by the crashkernel option is not sufficient. This typically demonstrates by either the panic kernel failing early at boot or running out of memory when running the kdump userspace processes (systemd, makedumpfile, ...). In that case you will want to manually set the KDUMP_CRASHKERNEL config option.
The default value is provided by the kdumptool_calibrate command and set in the bootloader by the kdumptool_commandline command.
You can use the values suggested by kdumptool_calibrate as a starting point for finding a correct value and setting KDUMP_CRASHKERNEL manually.
The value of KDUMP_CPUS influences the amount of memory required by kdump. You may want to decrease the default value to limit the memory required. Note that previously the default was to use a single CPU.
CONFIGURATION¶
The configuration file is /etc/sysconfig/kdump. Just edit this file with a plain text editor to adjust the settings. All variables are described in kdump(5). Here’s a brief overview about some variables that are worth tweaking.
Save Directory¶
The most important setting is where the dump should be saved. Different methods are available: local file, FTP, SFTP (SSH), NFS, CIFS.
The configuration variable KDUMP_SAVEDIR has to be filled with a URL to where the dump should be saved. The syntax is described in kdump(5).
Note
If you want to use SSH or SFTP with public key authentication, make sure to read the "Secure File Transfer Protocol" section in kdump(5).
Preventing running out of disk space¶
If you save the dumps to your local file system, you may want kdump to delete old dumps automatically. See the KDUMP_KEEP_OLD_DUMPS option.
If the partition has less than KDUMP_FREE_DISK_SIZE megabytes free disk space after saving the dump, the dump is not saved at all.
Important
That two options don’t apply to network dump targets.
Dump Filtering and Compression¶
Dump size can be reduced by
see KDUMP_DUMPFORMAT and KDUMP_DUMPLEVEL in kdump(5)
Notification¶
If you enable notification support, then you get an email when the system reboots after the dump has been copied. This only works for locally saved dumps. See KDUMP_NOTIFICATION_TO in kdump(5).
Debugging options¶
Normally the machine is rebooted when kdump finishes and all errors are ignored to get the production system runnigng as soon as possible,
To get a shell for debugging kdump itself, see the KDUMP_IMMEDIATE_REBOOT and KDUMP_CONTINUE_ON_ERROR options.
Also see the KDUMP_VERBOSE option to turn on more logging during kdump service start and during the dump itself.
To debug the creation of the kdump initrd, you may want to run mkdumprd -d.
Warning
If you use a VGA console and trigger the dump when X11 is running (i.e. your graphical desktop), you might not see any output. Use a serial console in that case, or try to trigger the dump from Linux console (i.e. press Ctrl-Alt-F1 in your graphical desktop and log in there).
ADVANCED CONFIGURATION¶
Trigger Kdump on NMI (i386/x86_64 only)¶
Some systems (mostly "Enterprise" servers) have a so-called NMI button (physically or via the remote management consoles) that triggers an NMI manually if the system hangs completely and even SysRQ does not work any more.
If you want to trigger a kdump in that case, you have to execute
# sysctl kernel.panic_on_unrecovered_nmi=1
manually or (if you want to make that a permanent setting) add
kernel.panic_on_unrecovered_nmi=1
in /etc/sysctl.conf.
Extended crashkernel commandline¶
While the "crashkernel=size[@offset]" syntax is sufficient for most configurations, sometimes it’s handy to have the reserved memory dependent on the value of System RAM — that’s mostly for distributors that pre-setup the kernel command line to avoid a unbootable system after some memory has been removed from the machine.
The syntax is:
crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset] range=start-[end]
while start is inclusive and end is exclusive.
For example:
crashkernel=512M-2G:64M,2G-:128M
This would mean:
SEE ALSO¶
kexec(8), kdump(5), makedumpfile(8), crash(8), The Kexec and Kdump chapter in the SUSE System Analysis and Tuning Guide
AUTHOR¶
Bernhard Walle <bwalle@suse.de>
| 08/22/2025 | kdump 1.0.2 |