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

[Kevin Wolf <kwolf@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2487 bytes --]

Am 31.01.2018 um 15:12 hat Stefan Hajnoczi geschrieben:
> 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 <famz@redhat.com>
> > > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > > ---
> > > >  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.
> > > >  
> > > > +@item --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.

So you understand "concurrent writers" as at least two writers? Because
I wouldn't understood it as a writer that is concurrent to the
(read-only) qemu-img.

Maybe rephrase it as "...allowing other processes to write to the
image" then?

> 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.

share-rw=on is a -device option and only about the guest, not about the
backend. It is never unsafe if the guest can cope with external writers.
It just doesn't prevent qemu from locking the image file for image
formats.

Kevin

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 801 bytes --]