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Re: [Qemu-block] [Qemu-devel] [PATCH v1 3/6] qemu-img: add support for -

From: Daniel P. Berrange
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v1 3/6] qemu-img: add support for -n arg to dd command
Date: Wed, 1 Feb 2017 12:16:58 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Wed, Feb 01, 2017 at 01:13:39PM +0100, Max Reitz wrote:
> On 30.01.2017 19:37, Eric Blake wrote:
> > On 01/26/2017 07:27 AM, Daniel P. Berrange wrote:
> >> On Thu, Jan 26, 2017 at 08:35:30PM +0800, Fam Zheng wrote:
> >>> On Thu, 01/26 11:04, Daniel P. Berrange wrote:
> >>>> The -n arg to the convert command allows use of a pre-existing image,
> >>>> rather than creating a new image. This adds a -n arg to the dd command
> >>>> to get feature parity.
> >>>
> >>> I remember there was a discussion about changing qemu-img dd's default to 
> >>> a
> >>> "conv=nocreat" semantic, if so, "-n" might not be that useful. But that 
> >>> part
> >>> hasn't made it into the tree, and I'm not sure which direction we should 
> >>> take.
> >>> (Personally I think default to nocreat is a good idea).
> >>
> >> Use nocreat by default would be semantically different from real "dd"
> >> binary which feels undesirable if the goal is to make "qemu-img dd"
> >> be as consistent with "dd" as possible.
> >>
> >> It would be trivial to rewrite this patch to add support for the "conv"
> >> option, allowing the user to explicitly give 'qemu-img dd conv=nocreat'
> >> instead of my 'qemu-img dd -n' syntax, without changing default semantics.
> > 
> > Adding 'conv=nocreat' (and not '-n') feels like the right way to me.
> The original idea was to make conv=nocreat a mandatory option, I think.
> qemu-img was supposed error out if the user did not specify it.

I'm not really seeing a benefit in doing that - it would just break
existing usage of qemu-img dd for no obvious benefit.

FYI, I've implemented "conv" with "nocreat" and "notrunc" options
which i'll post once i've tested some more.

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