Re: [Qemu-devel] DMG chunk size independence

[Peter Wu <peter@lekensteyn.nl>]

On Mon, Apr 24, 2017 at 05:19:48PM -0400, John Snow wrote:
> 
> 
> On 04/23/2017 05:03 AM, Ashijeet Acharya wrote:
> > Hi,
> > 
> > Great news!
> > I have almost completed this task and the results are looking
> > promising. I have not yet attended to the DMG files having bz2
> > compressed chunks but that should be easy and pretty similar to my
> > approach for zlib compressed files. So, no worries there.
> > 
> > For testing I am first converting the images to raw format and then
> > comparing the resulting image with the one converted using v2.9.0 DMG
> > driver and after battling for 2 days with my code, it finally prints
> > "Images are identical." According to John, that should be pretty
> > conclusive and I completely agree.
> > 
> 
> Yes, comparing a sample.dmg against a raw file generated from the 2.9.0
> qemu-img tool should be reasonably good evidence that you have not
> altered the behavior of the tool.
> 
> > Now, the real thing I wanted to ask was, if someone is aware of a DMG
> > file which has a chunk size above 64 MiB so that I can test those too.
> > If yes, please share the download link with me.
> > Currently I am testing the ones posted by Peter Wu while submitting
> > his DMG work in 2014.
> > Here -> https://lists.nongnu.org/archive/html/qemu-devel/2014-12/msg03606.html
> > 
> 
> Are any of those over 64MB? I assume you're implying that they aren't.
> 
> Maybe Peter knows?...

I don't know DMG with bzip2-compressed chunks over 64M. Looking through
more recent files, there is this log for "Install macOS Sierra
10.12(16A323)-B.dmg" which contains only zlib-compressed or raw data
where the uncompressed size (in the MISH block) is always at most 1MiB:
https://github.com/Lekensteyn/dmg2img/issues/1#issuecomment-273662984

In an Xcode_7.2.dmg file, the situation was similar, only zlib or raw
and also with a max uncompressed size of 1MiB (actually, an exact size
of 1MiB in both cases based on "sectorCount").

Perhaps bzip2-compressed chunks are not so common for larger disk images
since zlib is faster.
-- 
Kind regards,
Peter Wu
https://lekensteyn.nl