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Date: Thu, 21 Nov 2013 09:51:39 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu-img create: add -o nocow option
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To: Chunyan Liu <cyliu@suse.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

On Thu, Nov 21, 2013 at 11:33:56AM +0800, Chunyan Liu wrote:
> 2013/11/20 Stefan Hajnoczi <stefanha@gmail.com>
> 
> > On Wed, Nov 20, 2013 at 04:50:29PM +0800, Chunyan Liu wrote:
> > >  block/cow.c               |   22 ++++++++++++++++++++++
> > >  block/qcow.c              |   22 ++++++++++++++++++++++
> > >  block/qcow2.c             |   22 ++++++++++++++++++++++
> >
> > I think you can avoid modifying all the image formats:
> >
> > .bdrv_create() functions pass options to bdrv_create_file().  Therefore
> > an image format like qcow2 does not need to parse the nocow option
> > itself.  Only raw-posix.c:.bdrv_create() needs to know about the nocow
> > option.
> >
> >
> In existing code, options passed to bdrv_create_file contains no option in
> fact.
> 
> And if we pass all options to bdrv_create_file directly, raw-posix.c:
> .bdrv_create() will get NOCOW option but at the same time get SIZE option,
> it
> will create a file with total size. For cow/qcow/qcow2, I suppose it's not
> expected? In current code, bdrv_create_file will create a zero-sized image
> for
> cow/qcow/qcow2.

I see what the problem is: the loop that parses options does options++.
So by the time it has processed them the options pointer will be NULL or
point to a terminator option (option->name == NULL).

I was confused because there has been a patch series which changes
.bdrv_create() option parsing.  But I forgot it hasn't been merged.
https://lists.gnu.org/archive/html/qemu-devel/2013-08/msg01695.html

Kevin: Have you looked at .bdrv_create() QEMUOptionParameter removal
recently?  Otherwise I'm inclined to merge Chunyan's patch - we can
always refactor the code later when QEMUOptionParameter is removed.

Stefan