Required software¶

Following software packages are required for kdump:

There is no special kernel-kdump required like in earlier versions of SUSE LINUX Enterprise. The technical reason is that the normal kernel is relocatable now and can be used as kdump kernel, i.e. it’s possible to load and execute the normal kernel at any address, not only the compiled-in address as before.

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 offset is the load offset, i.e. the physical base address on which the memory reservation area starts. Starting with version 2.6.27, it’s not necessary to specify that offset manually since the kernel chooses a suitable base address automatically.

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)

Enable kdump service¶

The kdump systemd service just loads the kdump kernel at boot. To enable it, use yast2 kdump or simply

# systemctl enable kdump

on a shell. You can also execute it manually with systemctl start 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.

See the section “CONFIGURATION” later and/or kdump(5) for a description of the configuration options.

Load the kdump kernel¶

As mentioned above, the kdump service takes the part of loading the kdump kernel. As kdump kernel, the normal system kernel is used, no special kernel image is required.

However, as initramfs, a special initramfs is built by mkdumprd(8). Normally, you don’t have to take care about that step since the init script checks if the initramfs is up to date (reading the configuration file modification time) and rebuilds it if necessary.

To manually load the kdump kernel (i.e, without the SUSE init script), you have to use the kexec(8) tool with the -p (panic kernel) parameter like:

# kexec -p /boot/vmlinuz-version --initrd=/boot/initrd-version-kdump
--reuse-cmdline

Save Directory¶

The most important setting is where the dump should be saved. Following methods are available:

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).

If you would like to use a special network interface, see the KDUMP_NETCONFIG setting.

Example:

Deletion of old dumps¶

If you save the dumps to your local file system, you may want that kdump deletes automatically old dumps. Set KDUMP_KEEP_OLD_DUMPS to the value how much old dumps should be preserved. To disable deletion of old dumps, set it to 0, and to delete all old dumps, set it to -1.

If the partition has less than KDUMP_FREE_DISK_SIZE megabytes free disk space after saving the dump, the dump is not copied at all.

Dump Filtering and Compression¶

The size of kernel dumps is uncompressed and unfiltered as large as your system has RAM. To get smaller files (for example, to send it to support), you can compress the whole dump file afterwards. However, the drawback is that the dump has to be uncompressed afterwards before opening, so the disk space needs to be there in any case.

To use page compression which compresses every page and allows dynamic uncompression with the crash(8) debugging tool, set KDUMP_DUMPFORMAT to compressed (which is actually the default).

To filter the dump, you have to set the KDUMP_DUMPLEVEL. Then not all memory is saved to disk but only memory that does not fulfil some criteria. I.e. you may want to leave out pages that are completely filled by zeroes as they don’t contain any useful information. The following table lists for each KDUMP_DUMPLEVEL the pages that are skipped, i.e. 0 produces a full dump and 31 is the smallest dump.

Automatic Reboot and Error handling¶

If you want to have your machine rebooted automatically after the dump has been copied, set KDUMP_IMMEDIATE_REBOOT to true. The variable KDUMP_CONTINUE_ON_ERROR controls if a shell should be opened if something goes wrong while saving the dump to be able to fixup manually. This is mostly for debugging.

In production, use that only if you have a serial console since VGA console and keyboard is not reliable in kdump environment.

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¶

If something goes wrong and you possibly opened a bug report, you may be asked to increase verbosity to report what’s going wrong. Also, this is useful if you would like to find the cause yourself.

First, you can increase KDUMP_VERBOSE. The maximum log level is 15. That gives both information when loading the dump kernel (i.e. the rckdump start command) and also when copying the dump in initrd. Log levels of 8 or above print a lot of debugging information and also turn on debugging in subcommands being executed, such as ssh or makedumpfile.

To debug the creation of the kdump initrd, you may want to run mkdumprd -d.

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:

Known Problems¶

There are known problems when using Kdump on Xen.

New Bugs¶

Please report bugs and enhancement requests at https://bugzilla.novell.com.