Re: Converting images to stdout

[Alberto Garcia]

On Mon, Nov 20, 2023 at 05:23:27PM -0600, Eric Blake wrote:
> > I'm interested in this use case, and I think that the method would be
> > as simple as this:
> > 
> > 1. Decide a cluster size for the output qcow2 file.
> > 2. Read the input file once to determine which clusters need to be
> >    allocated in the output file and which ones don't.
> > 3. That infomation is enough to determine the number and contents of
> >    the refcount table, refcount blocks, and L1/L2 tables.
> > 4. Write the qcow2 header + metadata + allocated data to stdout.
> 
> It may also be possible to write a qcow2 file that uses the external
> data bit, so that you are only writing the qcow2 header + metadata,
> and reusing the existing input file as the external data.

Sure, although I'm not so certain about the use case here... also, the
input file might not be raw.

> > Because this would be an external tool it would only support
> > a qcow2 file with the default options. Other features like
> > compression would be out of scope.
> 
> Why is compression not viable?  Are you worried that the qcow2
> metadata (such as refcounts) becomes too complex?

Yeah, mostly... also, since the output file would be streamable it's
trivial to pipe it through gzip or whatever.

> I've also wondered how easy or hard it would be to write a tool that
> can take an existing qcow2 file and defragment and/or compress it
> in-place (rather than having to copy it to a second qcow2 file).

That sounds a bit more complex, but I guess it's doable. But not
something that I need atm :)

Berto