Re: [Qemu-block] [Qemu-devel] [PATCH v1 3/6] qemu-img: add support for -n arg to dd command

[Daniel P. Berrange]

On Wed, Feb 01, 2017 at 01:31:01PM +0100, Max Reitz wrote:
> On 01.02.2017 13:28, Daniel P. Berrange wrote:
> > 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
> > on.
> 
> Depends. I don't think there are too many users, so we could still
> justify a change if there's a very good reason for it.
> 
> I do agree that it's probably not a very good reason, though.
> 
> >> 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
> 
> Yes. But you could easily argue that every image file is a "block device".

IMHO that would be a bad idea as it would mean different behaviour
from dd vs qemu-img dd, when run on raw files.

If we assume nocreat|notrunc behaviour by default, then we would  likely
need to invent new "creat|trunc" flags to let people turn the previous
behaviour back on, which would diverge from 'dd' command.

Regards,
Daniel
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