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

On Wed, Apr 27, 2016 at 12:27:46PM +1000, Dave Chinner wrote:

quoted

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....

    		     	     	      	     - Ted