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

On Tue, Apr 26, 2016 at 10:16:49PM -0400, Theodore Ts'o wrote:

On Tue, Apr 26, 2016 at 01:14:51AM -0700, Christoph Hellwig wrote:

quoted

I've been doing an audit of our direct I/O implementations, and most
of them does some form of transparent fallback, including some that
only pretend to support O_DIRECT, but do anything special for it at all,
while at the same time we go through greast efforts to check a file
system actualy supports direct I/O, leading to nasty no-op ->direct_IO
implementations as we even got that abstraction wrong.

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.

That's fine with me, but there ought to be some way for a program to
query whether a particular file / file system is one where DIO is
supported, and if so, what the alignment requirements would be.

Yes, that's called XFS_IOC_DIOINFO. We've been saying that this
should be promoted to the VFS for some time, though it might be
better to re-implement it with a different structure that includes
padding and a flags field....

That
way applications who care can get the information they need (and we
can use it for xfstests's _require_odirect :-).

Just return EOPNOTSUPP to XFS_IOC_DIOINFO if direct io is not
supported?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com