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

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

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?

After some experimentation, I came up with this command which works:

  $ qemu-system-x86_64 -drive file.file.filename=overlay,file.driver=qcow2,file.backing.file.locking=off

Rich.

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