Re: [Qemu-devel] [RFC 0/4] qemu-img: add max-size subcommand

[Daniel P. Berrange]

On Fri, Mar 03, 2017 at 09:51:46PM +0800, Stefan Hajnoczi wrote:
> RFCv1:
>  * Publishing patch series with just raw support, no qcow2 yet.  Please review
>    the command-line interface and let me know if you are happy with this
>    approach.
> 
> Users and management tools sometimes need to know the size required for a new
> disk image so that an LVM volume, SAN LUN, etc can be allocated ahead of time.
> Image formats like qcow2 have non-trivial metadata that makes it hard to
> estimate the exact size without knowledge of file format internals.
> 
> This patch series introduces a new qemu-img subcommand that calculates the
> required size for both image creation and conversion scenarios.
> 
> The conversion scenario is:
> 
>   $ qemu-img max-size -f raw -O qcow2 input.img
>   107374184448
> 
> Here an existing image file is taken and the output includes the space 
> required
> for data from the input image file.
> 
> The creation scenario is:
> 
>   $ qemu-img max-size -O qcow2 --size 5G
>   196688

Hmm, so that appears to be indicating the amount of physical space that
a qcow2 image would take up before any data has been written to it.

That's not actually what I was thinking. I would like to know the maximum
possible physical space that a 5G qcow2 image would take up once data is
written to every sector. Obviously this is impossible to say if you allow
for internal snapshots, but I think it is fine to say that we ignore
internal snapshots for purposes of this command.

IOW, I would expect  something like:

$ qemu-img max-size -O qcow2 --size 5G
5905580032

(I just blindly assumed qcow2 metadata is a 10% overhead for sake of
illustrating the number here)


Regards,
Daniel
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