Re: Is it normal to get bigger qcow2 image after blockcopy?

[Alberto Garcia]

On Tue, Nov 07, 2023 at 05:32:22PM +0800, Han Han wrote:
> >>> Before blockcopy, check source image:🔄  ❓
> >>>
> >>   # qemu-img info -U
> >>> /var/lib/avocado/data/avocado-vt/images/jeos-27-x86_64.qcow2🔄  ❓
> >>>
> >>
> > Was this a compressed qcow2 image when you started? maybe you started with
> > an appliance image?
> >
> Hi, Nir, I have a question here: How to check if a qcow2 image is
> compressed?

$ qemu-img map /path/to/image.qcow2

If it says "File contains external, encrypted or compressed clusters"
and the image is not encrypted then that's probably it.

> The qcow2 file /var/lib/avocado/data/avocado-vt/images/copy.qcow2 is
> created without compress options, while the qemu-img info for it
> shows `compression type: zlib`. Is that mean the image is compressed
> as well?

No, in qcow2 there is actually no such thing as a compressed image as
opposed to an uncompressed image.

qcow2 images contain a collection of data clusters, and each
individual cluster can be either compressed or uncompressed.

That zlib that qemu-img reports is the algorithm that is used for
compressed clusters *IF* they exist, but it's perfectly possible that
an image does not have any compressed cluster.

Berto