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 12:19:05PM +0200, Kevin Wolf wrote:
> Am 06.09.2017 um 10:50 hat Richard W.M. Jones geschrieben:
> > Commit 16b48d5d66d2 ("file-posix: Add 'locking' option") added this
> > option, but as it was not documented in the -help output it was not
> > easily possible to tell if a particular qemu binary supports it.
> > 
> > Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> > ---
> >  qemu-options.hx | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 9f6e2adfff..f8f95eb498 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -796,7 +796,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
> >      "       [,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n"
> >      "       [,serial=s][,addr=A][,rerror=ignore|stop|report]\n"
> >      "       [,werror=ignore|stop|report|enospc][,id=name][,aio=threads|native]\n"
> > -    "       [,readonly=on|off][,copy-on-read=on|off]\n"
> > +    "       [,readonly=on|off][,copy-on-read=on|off][,locking=off|auto|on]\n"
> >      "       [,discard=ignore|unmap][,detect-zeroes=on|off|unmap]\n"
> >      "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]]\n"
> >      "       [[,iops=i]|[[,iops_rd=r][,iops_wr=w]]]\n"
> 
> 'locking' is a driver-specific option and not universally available for
> all images, so it shouldn't be included here.

Indeed this patch is wrong, please ignore it.

However I couldn't work out the incantation to disable locking for a
qcow2 overlay backed by a file which is locked by another qemu
process:

  ...
      -drive file=/home/rjones/d/libguestfs/tmp/libguestfsSOXEiU/overlay1,cache=unsafe,format=qcow2,file.locking=off,id=hd0,if=none \
      -device scsi-hd,drive=hd0 \
  ...
  qemu-system-x86_64: -device scsi-hd,drive=hd0: Failed to get shared "write" lock
  Is another process using the image?
  ...

I'm guessing I need another level of indirection to get to the backing
file, but file.file.locking=off did not work either.

I think the error message there is wrong as well since it doesn't
refer to the right command line option nor tell you which file is
locked.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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