Re: [Qemu-block] [PATCH v6 2/2] qemu-img: Document --force-share / -U

[Stefan Hajnoczi]

On Tue, Jan 30, 2018 at 05:38:20PM +0100, Kevin Wolf wrote:
> Am 30.01.2018 um 15:23 hat Eric Blake geschrieben:
> > On 01/30/2018 12:34 AM, Fam Zheng wrote:
> > > Signed-off-by: Fam Zheng <address@hidden>
> > > Signed-off-by: Kevin Wolf <address@hidden>
> > > ---
> > >  qemu-img.texi | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/qemu-img.texi b/qemu-img.texi
> > > index 60a0e080c6..ec7e2f5d1e 100644
> > > --- a/qemu-img.texi
> > > +++ b/qemu-img.texi
> > > @@ -86,6 +86,13 @@ exclusive with the @var{-O} parameters. It is 
> > > currently required to also use
> > >  the @var{-n} parameter to skip image creation. This restriction may be 
> > > relaxed
> > >  in a future release.
> > >  
> > > address@hidden --force-share (-U)
> > > +If specified, @code{qemu-img} will open the image in shared mode, 
> > > allowing
> > > +concurrent writers. For example, this can be used to get the image 
> > > information
> > 
> > Actually, we only permit one writer at a time.  Would it be better to
> > say "allowing a concurrent writer"?
> 
> As far as qemu-img is concerned, multiple writers to the image are
> allowed. There may be further restrictions imposed by a writer so that a
> second writer isn't allowed, but with raw images and share-rw=on nothing
> prevents two qemu instances writing to the same image while qemu-img is
> accessing it read-only.

I agree with what you wrote and it's technically correct, but this is a
confusing place to mention concurrent writers.  They are a common source
of user error and that's why locking was introduced in the first place.

There should be a separate paragraph in docs/qemu-block-drivers.texi
explaining that share-rw=on can be used safely with format=raw if the
guests are configured to safely access a shared disk.  It should also
mention that share-rw=on is unsafe for image formats.

Stefan

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