Re: [Qemu-devel] [PATCH RFC] block: Tolerate existing writers on read only BdrvChild

[Fam Zheng]

On Wed, 03/01 17:22, Kevin Wolf wrote:
> Am 01.03.2017 um 17:10 hat Fam Zheng geschrieben:
> > On Wed, 03/01 16:16, Kevin Wolf wrote:
> > > > I'm not sure about this because: 1) this is intrusive from a user PoV, 
> > > > many
> > > > scripts and upper layer tools will stop working;
> > > 
> > > Are you sure? I don't expect user scripts or even proper management
> > > tools to use qemu-io on running VMs. I do expect that some users are
> > > using 'convert -s' with running VMs despite our recommendation against
> > > it.
> > > 
> > > If they are aware that they're doing something that works only in the
> > > right circumstances, then breaking it isn't nice. But my gut feeling is
> > > that most of them don't know about the implications of accessing a live
> > > image. In this case breaking their use case and making them think about
> > > whether they want to add something like a --force option sounds like a
> > > good thing because they aren't caught by surprise when things go wrong
> > > eventually.
> > 
> > Yes, the use case is poor for qcow2, and your points stand there. But image
> > locking will be at the posix level, which has a wider range of users. I 
> > cannot
> > draw the same conclusion on raw images as easily.
> 
> Well, with raw, I'm even less concerned about breaking the commands
> related to internal snapshots. :-)

Yes, I'm agree with a --force there. For qemu-img map and qemu-io, personally I
think it's better to keep the default working. qemu-io is a expert mode tool,
whoever using it at all should already know what he's doing, --force doesn't add
much protection for the innocent.

Fam