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 <f...@redhat.com> > > > Signed-off-by: Kevin Wolf <kw...@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. 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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