Re: [Qemu-devel] [Qemu-block] Raw notes from a small block layer/QAPI/something pre-christmas meeting

[Peter Krempa <pkrempa@redhat.com>]

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On Wed, Dec 20, 2017 at 19:15:55 +0100, Markus Armbruster wrote:
> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
> > On Fri, Dec 15, 2017 at 05:38:00PM +0100, Max Reitz wrote:
> >
> >> Image creation in qemu-system-* vs. qemu-img:
> >>   In order to get proper introspection for qemu-img create, we need a
> >>   QAPI schema.  If we have a QAPI schema, we might as well add
> >>   blockdev-create to QMP.
> >>   As long as we do not have a really-none (null, void, ...) machine type
> >>   for qemu-system-*, launching such a process just for creating an image
> >>   will bring quite a bit of overhead (e.g. with -M none -accel qtest).
> >>   However, as for libvirt, this is not exactly a regression since
> >>   libvirt currently cannot create images at all (apart from implicitly
> >>   through drive-mirror etc.).  Further work on voidifying qemu-system-*
> >>   will improve performance.
> >
> > In terms of the I/O operations involved, image creation is a already a
> > pretty slow process, particularly if pre-allocation is used which is
> > common.  So even QEMU's current slow (circa 300ms) startup time is a
> > complete non-issue for image creation IMHO - it'll be dwarfed by the
> > time to actually create the image. 
> >
> >>   On the other side, we can also add QAPI introspection to qemu-img.
> >>   (qemu-img already links to QAPI, so this should not be too hard.)
> >>   qemu-img will also need command-line introspection, though.
> >
> > I figure the qapi-ificiation is the hard & time consuming bit of
> > work. Once that's done exporting it via both qemu-img & qemu-system*
> > is quite straighforward.
> 
> qemu-system-*: trivial.
> qemu-img, via command line: straightforward
> qemu-img, via QMP: more difficult, since QMP is entangled with HMP,
>     character devices, ...
> 
> If libvirt really wants to use QMP for the job, *and* doesn't want to
> use the qemu-system-* that's running a guest, the easy solution is
> running another qemu-system-* without a guest.

QMP is necessary to have for very-long operations (blockdev-mirror), but
for image creation the command line will be enough. Provided that
interface for -blockdev and "create image" will be similar enough.

Especially the fact that qemu-img create does not really like 'json:{}'
thus some image options are impossible to pass (multiple hosts for
gluster, ...).

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