Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Richard W.M. Jones]

On Mon, Dec 07, 2009 at 11:49:32AM +0000, Daniel P. Berrange wrote:
> On Mon, Dec 07, 2009 at 11:31:47AM +0000, Richard W.M. Jones wrote:
> > On Mon, Dec 07, 2009 at 11:30:14AM +0000, Daniel P. Berrange wrote:
> > > On Mon, Dec 07, 2009 at 11:19:54AM +0000, Jamie Lokier wrote:
> > > > 
> > > > No, the question is whether it makes sense to provide a 'shared'
> > > > option on the command line, or simply to always map:
> > > > 
> > > >      image opened read only => F_FDLCK
> > > >      image opened writable => F_WRLCK
> > > > 
> > > > and provide only a single command line option: 'lock'.
> > > 
> > > That doesn't work in the case of setting up a clustered filesystem
> > > shared between guests. That requires that the disk be opened writable,
> > > but with a shared (F_RDLOCK) lock.
> > 
> > I think Jamie's point is that you might as well use no locking at all
> > in this configuration.  It's hard to see what lock=shared is
> > protecting you against.
> 
> This is saying that there is no need to protect 'shared writers'
> from 'exclusive writers', which is not true.
[example snipped]

OK so the case for lock=shared is where you have a shared clustered
resource, *and* an uber-admin-tool which may require exclusive access
to the resource.  I now understand, and that seems to make sense
(albeit a rather rare case).

It has to be said the only reason I implemented the shared mode in the
original patch, was because both POSIX and Win32 (LockFileEx) have the
distinction between shared and exclusive, in other words 'coz it was
easy :-)

Rich.

-- 
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