Re: [dm-devel] [Lsf] Preliminary Agenda and Activities for LSF

[Matthew Wilcox <matthew wil cx>]

On Wed, Mar 30, 2011 at 02:49:58PM -0700, Mingming Cao wrote:
> > Direct IO semantics have always been that the application is allowed
> > to overlap IO to the same range if it wants to. The result is
> > undefined (just like issuing overlapping reads and writes to a disk
> > at the same time) so it's the application's responsibility to avoid
> > overlapping IO if it is a problem.
> 
> I was thinking along the line to provide finer granularity lock to allow
> concurrent direct IO to different offset/range, but to same offset, they
> have to be serialized. If it's undefined behavior, i.e. overlapping is
> allowed, then concurrent dio implementation is much easier. But not sure
> if any apps currently using DIO aware of the ordering has to be done at
> the application level. 

Yes, they're aware of it.  And they consider it a bug if they ever do
concurrent I/O to the same sector.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."