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

[Daniel P. Berrange]

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.

>   Plan B:
>     libvirt can use qemu-img now with the currently supported options,
>     and as soon as libvirt needs anything better, we will have something
>     better done.
>     (Also, there is "qemu-img create -f $format -o help"!  Because
>      parsing help texts has worked so well in the past.)

Regards,
Daniel
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