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

[Markus Armbruster]

"Daniel P. Berrange" <address@hidden> 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.

>>   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