Re: Question about non asynchronous aio calls.

[Gleb Natapov <gleb@scylladb.com>]

On Wed, Oct 07, 2015 at 11:08:34AM -0400, Brian Foster wrote:
> > > Second one is harder. We do need to write past the end of a file, actually
> > > most of our writes are like that, so it would have been great for XFS to
> > > handle this case asynchronously.
> > 
> > You didn't say what kernel you're on, but these:
> > 
> > 9862f62 xfs: allow appending aio writes
> > 7b7a866 direct-io: Implement generic deferred AIO completions
> > 
> > hit kernel v3.15.
> > 
> > However, we had a bug report about this, and Brian has sent a fix
> > which has not yet been merged, see:
> > 
> > [PATCH 1/2] xfs: always drain dio before extending aio write submission
> > 
> > on this list last week.
> > 
> > With those 3 patches, things should just work for you I think.
> > 
> 
> These fix some problems in that code, but the "beyond EOF" submission is
> still synchronous in nature by virtue of cycling the IOLOCK and draining
> pending dio. This is required to check for EOF zeroing, and we can't do
> that safely without a stable i_size.
> 
> Note that according to the commit Eric referenced above, ordering your
> I/O to always append (rather than start at some point beyond the current
> EOF) might be another option to avoid the synchronization here. Whether
> that is an option is specific to your application, of course.
> 
Our IO should be always append IIRC, the above explains why most aio we
do is truly async, but may be somewhere there is a reordering and then
we see synchronous behaviour. Will have to check it.

--
			Gleb.

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