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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 12:28:54 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Wed, Feb 01, 2017 at 01:23:54PM +0100, Max Reitz wrote:
> On 01.02.2017 13:16, Daniel P. Berrange wrote:
> > 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.
> Well... Is there existing usage?

It shipped in 2.8.0 though, so imho that means we have to assume there
are users, and thus additions must to be backwards compatible from now

> The benefit would be that one could (should?) expect qemu-img dd to
> behave on disk images as if they were block devices; and dd to a block
> device will not truncate or "recreate" it.
> If you don't give nocreat, it's thus a bit unclear whether you want to
> delete and recreate the target or whether you want to write into it.
> Some may expect qemu-img dd to behave as if the target is a normal file
> (delete and recreate it), others may expect it's treated like a block
> device (just write into it). If you force the user to specify nocreat,
> it would make the behavior clear.
> (And you can always delete+recreate the target with qemu-img create.)
> It's all a bit complicated. :-/

If the goal is to be compatible with /usr/bin/dd then IIUC, the behaviour
needs to be

 - If target is a block device, then silently assume nocreat|notrunc
   is set, even if not specified by user

 - If target is a file, then silently create & truncate the file
   unless nocreat or notrunc are set

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