Re: [Qemu-discuss] Info about compressed images

[Alberto Garcia]

On Fri, Jan 13, 2017 at 06:57:53PM +0100, Gianni Costanzi wrote:
> Hi,
> I'm new here and I've started using QEMU when I've started playing
> with Unetlab/EVE Network simulator.
> 
> I have some questions about compression and images with a compressed
> backing file:

I think there's some misunderstanding here about compressed images in
QEMU. I'll try to clarify:

* You create a compressed image with 'qemu-img convert -c'. That is a
  copy of the original image with all the clusters compressed.

* The compression is read-only: QEMU will read the compressed clusters,
  but everything that it writes will be uncompressed (also if you
  rewrite compressed clusters).
  
* Therefore, there's no such thing as an image with compression
  enabled. In QEMU you don't compress an image, you compress
  individual clusters of data. An image can have a mix of compressed
  and uncompressed clusters.

Berto