zeroing out the cloud image filesystem

[Bill Nottingham]

Matthew Miller (mattdm at fedoraproject.org) said: 
> On Wed, May 15, 2013 at 11:18:04AM -0400, Matthew Miller wrote:
> > > 2) I also commented out the "Zeroing out empty space" postinstall stuff,
> > > because it drastically increases the image build time for not much benefit,
> > > IMHO.
> > One time image build cost vs. whatever benefit multipled by every time the
> > image is used. :)
> 
> To put some numbers behind it, the compressed qcow2 image with the dd to
> zero empty space is 215M out of appliance-creator. Without it, it's 242M.
> 
> If we use a smaller blocksize, or an incrementally shrinking one, we can
> shrink it by another megabyte, at the cost of even more build time. I think
> that's probably across the worth-it threshold, though.
> 
> Using virt-sparsify seems to take even longer, but does get it down to 208M.
> That requires libguestfs, which we can't do in the current build system, but
> will be able to in the future one. (Feature rescheduled for F20.)

Why not use fstrim on the image file?

Bill