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Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/so
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting |
Date: |
Wed, 20 Dec 2017 19:15:55 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
"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
- [Qemu-block] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Max Reitz, 2017/12/15
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Markus Armbruster, 2017/12/18
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Kashyap Chamarthy, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Daniel P. Berrange, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Kashyap Chamarthy, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Kevin Wolf, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Daniel P. Berrange, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Peter Krempa, 2017/12/21
Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Daniel P. Berrange, 2017/12/20
- Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting,
Markus Armbruster <=
Re: [Qemu-block] [Qemu-devel] Raw notes from a small block layer/QAPI/something pre-christmas meeting, Vladimir Sementsov-Ogievskiy, 2017/12/22