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

From: Max Reitz
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 13:31:01 +0100
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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".


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