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