Re: O_DIRECT, O_SYNC, or fsync() on NFS mounts?

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Sun, 2010-11-21 at 05:46 -0500, Christoph Hellwig wrote:
> On Fri, Nov 19, 2010 at 04:26:35PM -0500, Trond Myklebust wrote:
> > If the application requests O_DIRECT/O_SYNC or calls fsync(), we are
> > required by POSIX to ensure the data is safe on disk. The presence of an
> > NFS delegation does not change that requirement.
> 
> That's not quite correct.  O_DIRECT for one is not actually specific in
> Posix at all, and the documented Linux semantics only say that the
> pagecache should not be used (even if it sometimes is with various
> filesystems).  There is not guarantee that data actually is on disk or
> reachable, for that you need to add the O_SYNC/O_DYSNC flag in addition
> or use fsync/fdatasync.

True.

We treat the O_DIRECT case as being the same as O_DIRECT|O_SYNC because
we don't currently have a way to locate and track outstanding O_DIRECT
rpc calls, and so fsync() has no effect.

We do, however support aio/dio, and so people who want better writev()
syscall latency can use that...

Cheers
  Trond