Re: [PATCH 2/2] ubifs: Allow O_DIRECT

[Chris Mason <clm@fb.com>]

On Tue, Aug 25, 2015 at 09:46:11AM +1000, Dave Chinner wrote:
> On Mon, Aug 24, 2015 at 01:19:24PM -0400, Jeff Moyer wrote:
> > Brian Norris <computersforpeace@gmail.com> writes:
> > 
> > > On Mon, Aug 24, 2015 at 10:13:25AM +0300, Artem Bityutskiy wrote:
> > >> Now, some user-space fails when direct I/O is not supported.
> > >
> > > I think the whole argument rested on what it means when "some user space
> > > fails"; apparently that "user space" is just a test suite (which
> > > can/should be fixed).
> > 
> > Even if it wasn't a test suite it should still fail.  Either the fs
> > supports O_DIRECT or it doesn't.  Right now, the only way an application
> > can figure this out is to try an open and see if it fails.  Don't break
> > that.
> 
> Who cares how a filesystem implements O_DIRECT as long as it does
> not corrupt data? ext3 fell back to buffered IO in many situations,
> yet the only complaints about that were performance. IOWs, it's long been
> true that if the user cares about O_DIRECT *performance* then they
> have to be careful about their choice of filesystem.
> 
> But if it's only 5 lines of code per filesystem to support O_DIRECT
> *correctly* via buffered IO, then exactly why should userspace have
> to jump through hoops to explicitly handle open(O_DIRECT) failure?
> Especially when you consider that all they can do is fall back to
> buffered IO themselves....

This is what btrfs already does for O_DIRECT plus compressed, or other
cases where people don't want their applications to break on top of new
features that aren't quite compatible with it.

-chris