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Re: [Qemu-discuss] Info about compressed images

From: Gianni Costanzi
Subject: Re: [Qemu-discuss] Info about compressed images
Date: Wed, 25 Jan 2017 10:33:32 +0100

Thank you very much Alberto,
I've really misunderstood how compression worked. BTW it seems good to compress some big base-images.

Thank you very much for having explained the concept very well :)

Best regards,
   Gianni Costanzi

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On Wed, Jan 25, 2017 at 10:21 AM, Alberto Garcia <address@hidden> wrote:
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.


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