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". Max
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