Re: [PATCH 07/13] aio: enabled thread based async fsync

[Benjamin LaHaise]

On Wed, Jan 20, 2016 at 12:29:32PM -0800, Linus Torvalds wrote:
> On Wed, Jan 20, 2016 at 11:59 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >>
> >> Are there other users outside of Solace? It would be good to get comments..
> >
> > I know of quite a few storage/db products that use AIO. The most
> > recent high profile project that have been reporting issues with AIO
> > on XFS is http://www.scylladb.com/. That project is architected
> > around non-blocking AIO for scalability reasons...
> 
> I was more wondering about the new interfaces, making sure that the
> feature set actually matches what people want to do..

I suspect this will be an ongoing learning exercise as people start to use 
the new functionality and find gaps in terms of what is needed.  Certainly 
there is a bunch of stuff we need to add to cover the cases where disk i/o 
is required.  getdents() is one example, but the ABI issues we have with it 
are somewhat more complicated given the history associated with that 
interface.

> That said, I also agree that it would be interesting to hear what the
> performance impact is for existing performance-sensitive users. Could
> we make that "aio_may_use_threads()" case be unconditional, making
> things simpler?

Making it unconditional is a goal, but some work is required before that 
can be the case.  The O_DIRECT issue is one such matter -- it requires some 
changes to the filesystems to ensure that they adhere to the non-blocking 
nature of the new interface (ie taking i_mutex is a Bad Thing that users 
really do not want to be exposed to; if taking it blocks, the code should 
punt to a helper thread).  Additional auditing of some of the read/write 
implementations is also required, which will likely need some minor changes 
in things like sysfs and other weird functionality we have.  Having the 
flag reflects that while the functionality is useful, not all of the bugs 
have been worked out yet.

What's the desired approach to merge these changes?  Does it make sense 
to merge what is ready now and prepare the next round of changes for 4.6?  
Or is it more important to grow things to a more complete state before 
merging?

Regards,

		-ben

>           Linus

-- 
"Thought is the essence of where you are now."