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

[Fam Zheng <famz@redhat.com>]

On Thu, 02/01 11:44, Stefan Hajnoczi wrote:
> On Wed, Jan 31, 2018 at 03:22:49PM +0100, Kevin Wolf wrote:
> > Am 31.01.2018 um 15:12 hat Stefan Hajnoczi geschrieben:
> > > 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.
> 
> Thanks for the explanation.  Documentation on share-rw=on|off would be
> nice.
> 
> Maybe something like:
> 
>   By default the guest has exclusive write access to its disk image.
>   This is enforced by QEMU via file locking.  If the guest can safely
>   share the disk image with other writers the -device ...,share-rw=on
>   parameter can be used.  This is only safe if the guest is running
>   software, such as a cluster file system, that coordinates disk
>   accesses to avoid corruption.
> 
>   Note that share-rw=on only declares the guest's ability to share the
>   disk.  Some QEMU features, such as image file formats, require
>   exclusive write access to the disk image and this is unaffected by the
>   share-rw=on option.

I will add these paragraphs to the "image locking" seciton in
docs/qemu-block-drivers.texi.

Fam