F20 System Wide Change: Visible Cloud

[Daniel P. Berrange]

On Mon, Jul 15, 2013 at 01:06:26PM +0200, Lukas Zapletal wrote:
> On Mon, Jul 15, 2013 at 12:22:22PM +0200, Jaroslav Reznik wrote:
> > downloadable cloud images (in qcow2 and raw.xz format) produced and released 
> 
> I know that it is convenient to expose qcow2 images uncompressed so
> users are able to upload them directly into clouds via httpd. Qcow2 is
> compressed already by default (is that zlib?) so I am wondering if there
> is any way of re-compressing the image with "-9" option so we are sure
> the size is the best we can offer.
> 
> Currently qemu-img does not offer that. Size comparison between qcow2 vs
> raw.xz shows that there are most likely some gaps we can fill in the
> qcow2 size.
> 
> Just throwing in some ideas. Maybe not that relevant for this, I
> apologize.

I hacked qemu code to use Z_BEST_COMPRESSION instead of Z_DEFAULT_COMPRESSION,
and got the following size results.

 - qcow2 built-in compression + default level
     237436928

 - qcow2 built-in compression + best (9) level
     236322816

 - qcow2 no-compression + default gzip
     218773883

 - qcow2 no-compression + best (9) gzip
     217264037

So the change from the default, to best level has no meaningful impact
on image size with built-in qcow2 compression.  Using gzip over a
non-compressed qcow2 file is better than qcow2's built-in compression.
Given that you can't directly boot the resulting image though, the win
is not enough to outweigh the downside.

Updating to qcow2 to support xz (or some other algo) as a built-in
compression choice would likely give better results. The downside though
is that the images would be incompatible with every existing deployed
qemu/xen version out there.

So I think the choice of qcow2 with built-in compression, and raw with
external xz compression are most reasonable at this point in time.

Regards,
Daniel
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