Does FreeBSD support sparse kernel crash dumps?

Paul Marciano pm940 at
Fri Apr 7 23:01:53 UTC 2006

--- Peter Wemm <peter at> wrote:
> On Friday 07 April 2006 12:47 pm, Paul Marciano
> wrote:
> > Hello.  I read a while back about someone working
> on
> > supporting sparse kernel crash dumps (dumping only
> the
> > active kernel pages to the dump device as opposed
> to
> > all physical memory - for machines where the phys
> mem
> > is greater than the dump dev space.)
> >
> > Does anyone know the status of that project?  Was
> it
> > committed, or are there plans to commit it?
> I have a working prototype as of last night.  There
> should be something 
> committable in the next week or two.
> When I boot my 2GB machine and force a crash dump
> from single user mode, the 
> fully debuggable vmcore file is in the 40-50MB
> range.  A busy machine with 
> 12GB ram took about 150MB to dump.
> There are still some things to work out.  It was
> written for the amd64 kernel, 
> but can be ported to i386.
> -Peter

That's very timely news Peter.  Do you think your code
is easily back-portable to 5.4?  Are the changes
limited to dump_machdep.c or otherwise not dependent
on a great deal of -current updates?

Do you compress the data stream at all (e.g. gzip)?

I have a specific need on a CompactFlash based system.
 I have a 256MB IDE mode card on a machine with 512MB
physical memory.  I can probably commit 128MB of the
card as a dumpdev but I can't go beyond that.

Good luck with it.


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