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

[Joel Becker <jlbec evilplan org>]

On Wed, Mar 30, 2011 at 02:49:58PM -0700, Mingming Cao wrote:
> On Wed, 2011-03-30 at 13:17 +1100, Dave Chinner wrote:
> > For direct IO, the IO lock is always taken in shared mode, so we can
> > have concurrent read and write operations taking place at once
> > regardless of the offset into the file.
> > 
> 
> thanks for reminding me,in xfs concurrent direct IO write to the same
> offset is allowed.

	ocfs2 as well, with the same sort of strategem (including across
the cluster).

> > 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. 

	Oh dear God no.  One of the major DIO use cases is to tell the
kernel, "I know I won't do that, so don't spend any effort protecting
me."

Joel

-- 

"I don't want to achieve immortality through my work; I want to
 achieve immortality through not dying."
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