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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, May 28, 2013 at 10:50:28AM +0800, qiaonuohan@cn.fujitsu.com wrote:
> The kdump-compressed format is *linux specific* *linux standard* crash dump
> format used in kdump framework. The kdump-compressed format is readable only
> with the crash utility, and it can be smaller than the ELF format because of
> the compression support.
> 
> Note, similar to 'dump-guest-memory':
> 1. The guest should be x86 or x86_64. The other arch is not supported now.
> 2. If the OS is in the second kernel, gdb may not work well, and crash can
>    work by specifying '--machdep phys_addr=xxx' in the command line. The
>    reason is that the second kernel will update the page table, and we can
>    not get the page table for the first kernel.
> 3. The cpu's state is stored in QEMU note.
> 4. The vmcore are able to be compressed with zlib, lzo or snappy. zlib is
>    available by default, and option '--enable-lzo' or '--enable-snappy'
>    should be used with configure to make lzo or snappy available.

Please see the coding style docs and run scripts/checkpatch.pl on your
patches:
http://git.qemu.org/?p=qemu.git;a=blob;f=CODING_STYLE;hb=HEAD
http://git.qemu.org/?p=qemu.git;a=blob;f=HACKING;hb=HEAD

Where does that code live that writes DISKDUMP files?  I can see the
diskdump.[ch] code.

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?

Stefan