Re: [Qemu-devel] image size - OS reports 720mb, but image file is 2.3 gig

[Paul Brook <paul@codesourcery.com>]

On Friday 11 February 2005 14:18, Joe Bogner wrote:
> Hello,
>
> I'm trying to create an win2k image that is as small as possible. I'd
> like to be able to copy the image back and forth between machines so
> that I have a complete win2k development image.
>
> According to win2k, the size is about 720mb. Originally win2k was
> reporting the size to be about 1.2gig, but I was able to reduce it
> through NTFS compression.
>
> Here's the dilemna. The physical image file is 2.3gig. I've run a
> qemu-img convert -f qcow <input> -O qcow <output>, which took the
> original 4.5 gig image down to 2.3 gig. With qemu-img compression, I
> was able to get it down to 1.7gig.
>
> Why is there such a large discrepency between what win2k reports and
> the physical image file on disk? Is there anything else I can do to
> reduce it, aside from compressing the physical image file?
>
> 1.7 gig is still quite a bit to be shuffling around between machines.

Even if files have been deleted/compressed the data will still be there on the 
disk. qemu can't tell the difference between "useful" data, and unused data 
that can be discarded. You need to make sure all the unused space on the 
image is actually zeroed.

One way to achieve this would be to shrink the guest partition to its minimum 
size, then create a second partition on the unused space and explicitly zero 
that out (using dd from knoppix if necessary).

Paul