qcow2 compression [was Re: F20 System Wide Change: Visible Cloud]

[Matthew Miller]

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

Just for clarity, qcow2 can be compressed or uncompressed; these are
compressed.

> is any way of re-compressing the image with "-9" option so we are sure
> the size is the best we can offer.


I think this may just come down to deflate (zlib) vs lzma (xz). A comment in
the qemu-img source code says "best compression, small window, no zlib
header" -- I'm kind of guessing, but I expect that the small window size was
chosen on purpose for when the image is actually used.

> 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.

Probably not relevant, but I'm open for any approaches for improvement.

-- 
Matthew Miller  ☁☁☁  Fedora Cloud Architect  ☁☁☁  <mattdm at fedoraproject.org>