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. > 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 Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|
