Re: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens for mode where DIO doesn't work

[Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>]

On Tue, Apr 26, 2016 at 11:25:26PM -0400, Theodore Ts'o wrote:
> On Wed, Apr 27, 2016 at 12:27:46PM +1000, Dave Chinner wrote:
> > > At this point I wonder if we should simply treat O_DIRECT as a hint
> > > and always allow it, and just let the file system optimize for it
> > > (skip buffering, require alignment, relaxed Posix atomicy requirements)
> > > if it is set.
> > 
> > I thought that's how most filesystems treated it, anyway. i.e.
> > anything they can't do via direct IO, they fell back to buffered IO
> > to complete (e.g. for allocation or append writes, etc). Hence why I
> > suggested the fallback rather than erroring out....
> 
> No, some file systems return EINVAL on the open.  In fact that's what
> the _require_odirect test in xfstests relies upon....

Sure, but that doesn't change the fact that many of the filesystems
that "support O_DIRECT" don't always do O_DIRECT - they
transparently do buffered IO instead and hence are treating O_DIRECT
as a hint once the file has been opened.

Cheers,

Dave.
-- 
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org