Kernel crash before dumpon

Kris Kennaway kris at
Thu Nov 27 20:07:48 PST 2008

Frank Solensky wrote:
> I'm trying to get a dump off a machine with a 7.1-beta2 kernel that's
> been crashing during the boot process, following the instructions on
> I've recompiled the kernel with "-g" but no vmcore file appears so I'm
> assuming that the crash occurs before dumpon is executed.
> The page includes the following suggestion on how I might be able to
> proceed:
>         Alternatively, the dump device can be hard-coded via the
>         dump clause in the config(5) line of a kernel configuration
>         file. This approach is deprecated and should be used only
>         if a kernel is crashing before dumpon(8) can be executed.
> I tried adding
>         config dump "/dev/ad4s3b"
> to the configuration file but that option appears to be no longer
> supported: the config command gives an error message of:
>         root/dump/swap specification obsolete
> Is the paragraph above obsolete?  If so, what's the preferred way to
> collect the dump?

Yes, it's obsolete.  Please file a PR to remove the outdated advice.

Unfortunately in FreeBSD 5 and later there is no way to dump prior to 
multi-user startup, because of how device discovery is now dynamic.  You 
can still use other tools like ddb/remote gdb etc.


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