Re: [Qemu-devel] [PATCH for-2.10] qemu-options: Document the -drive locking parameter.

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Sep 12, 2017 at 01:32:05PM +0200, Kevin Wolf wrote:
> Am 12.09.2017 um 11:45 hat Richard W.M. Jones geschrieben:
> > On Wed, Sep 06, 2017 at 01:38:45PM +0200, Kevin Wolf wrote:
> > > This command line fragment looks correct to me. For me, it seems to
> > > work. I'm starting a first qemu in the background with default locking
> > > options:
> > > 
> > >     $ x86_64-softmmu/qemu-system-x86_64 -hda /tmp/test.qcow2
> > > 
> > > And then starting a second one with a command line resembling yours:
> > > 
> > >     $ x86_64-softmmu/qemu-system-x86_64 -device virtio-scsi \
> > >       -drive file=/tmp/test.qcow2,cache=unsafe,format=qcow2,file.locking=off,id=hd0,if=none \
> > >       -device scsi-hd,drive=hd0
> > 
> > The problem is with overlays, where file.locking doesn't propagate to
> > the backing file.  Thus:
> > 
> >   $ qemu-system-x86_64 -drive file=backing,format=raw
> > 
> > while in another terminal:
> > 
> >   $ qemu-img create -b backing -f qcow2 overlay
> >   $ qemu-system-x86_64 -drive file=overlay,format=qcow2,file.locking=off
> >   qemu-system-x86_64: Failed to get shared "write" lock
> >   Is another process using the image?
> 
> locking=off isn't the right tool for the case. Try this:
> 
> $ qemu-system-x86_64 -drive file=overlay,if=none -device virtio-blk-pci,drive=none0,share-rw=on
> 
> Unless you're doing really evil things, just telling qemu that your
> guest can cope with concurrent writers to the same image is enough. This
> propagates through the whole chain as appropriate.

Our guest certainly *cannot* cope with multiple writers to the backing
disk (file "raw" in my example).  In fact that would be a disaster.

The overlay protects the backing disk from ever seeing any writes.

In our case because the initial qemu instance (which we don't control)
opened the disk ("raw") with an exclusive lock, our only choice for
monitoring that disk is to turn off locking.

Rich.

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