Re: [PATCH] scripts/qcow2-to-stdout.py: Add script to write qcow2 images to stdout

[Manos Pitsidianakis <manos.pitsidianakis@linaro.org>]

On Wed, 12 Jun 2024 12:21, Alberto Garcia <berto@igalia.com> wrote:
>On Wed 12 Jun 2024 09:01:01 AM +03, Manos Pitsidianakis wrote:
>> Hello Alberto,
>
>Hello Manos!
>
>> > This is equivalent to using qemu-img to convert a file to qcow2 and
>> > then writing the result to stdout, with the difference that this
>> > tool does not need to create this temporary qcow2 file and therefore
>> > does not need any additional disk space.
>>
>> Can you expand on this a little bit? Would modifying qemu-img to write
>> to stdout if given, say, - instead of a file output path be enough to
>> make this script unnecessary?
>
>Yes, it would be enough. Allowing qemu-img convert to write to stdout
>would indeed be very nice for the end user but it's a bit of a niche use
>case and it's also not a trivial task so I don't think that it's worth
>the effort. The output files that you pass to qemu-img convert need to
>be seekable because the only way to produce a qcow2 file without doing
>that is by precalculating all the metadata in advance before starting to
>write anything (that's why this script reads the input file twice).
>
>This is fundamentally different to what qemu-img convert does, which is
>to read the input file from start to finish and write it to the output
>file, relying on the relevant driver's existing write operations. All
>those assume random access to the output file.
>
>qemu-img is also much more generic in the sense that it supports many
>different output formats and image options.
>
>In contrast, writing the algorithm for a basic subset of qcow2 is quite
>simple and that's why I think that it makes sense to do it in a separate
>tool.
>
>Berto

Thanks for the complete explanation! It makes sense. Maybe add it to the 
commit message as well, it's informative.

Manos