Re: [Qemu-devel] [PATCH v4 0/9] Make 'dump-guest-memory' dump in kdump-compressed format

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Jun 20, 2013 at 10:18:35AM +0800, Qiao Nuohan wrote:
> On 06/19/2013 09:49 PM, Stefan Hajnoczi wrote:
> >Where does that code live that writes DISKDUMP files?  I can see the
> >diskdump.[ch] code.
> 
> Sorry, I cannot catch what do you mean here.

Please link to the code that writes DISKDUMP kdump files on a physical
machine.  I only see the crash utility code to read the DISKDUMP code
but I haven't found anything in the Linux kernel, the crash utility, or
the kexec-utils code to actually write a DISKDUMP file.

> >
> >The file format is pretty bad: we need 4 temporary files and a lot of
> >data copying to write it out.
> >
> >Why not just compress an ELF file and teach the crash utility how to
> >decompress while reading the ELF?
> >
> >Also, did you look into simply outputting the ELF file without zero
> >pages?
> 
> What I want is a dump file with smaller size. And compressed format and with
> zero pages excluded can make it. I choose kdump-compressed format because it is
> a standard format and it can realize what I want.
> 
> Why 4 temporary files are need? dump-guest-memory may be called with a fd which
> is supposed to send data of dump to. If fd is opened on a pipe or etc which is
> unable to seek, then I need to cache the data.

I understand why you need temporary files, but my questions stand:

Have you looked at using ELF more efficiently instead of duplicating
kdump code into QEMU?  kdump is not a great format for the problem
you're trying to solve - you're not filling in the Linux-specific
metadata and it's a pain to write due to its layout.

Why can't you simply omit zero pages from the ELF?

Why can't you compress the entire ELF file and add straightforward
decompression to the crash utility?

Stefan