Re: NewStore update

[Christoph Hellwig <hch@infradead.org>]

On Sat, Feb 21, 2015 at 09:53:45AM -0800, Sage Weil wrote:
> Ah, thanks. I guess in the buffered case though we won't block normally 
> anyway (unless we've hit the bdi dirty threshold).  So it's probably 
> either aio direct or buffered write + aio fsync, depending on the cache 
> hints?

buffered I/O will also block on:

 - acquiring i_mutex (do you plan on having parallel writers to the same
   file?)
 - reading in the page for read-modify-write cycles
 - waiting for writeback to finish for a previous write to the page

In adition to all the other ways even O_DIRECT aio could block (most
importantly block allocation)

I have a hacked prototype to do non-blocking writes similar to the
non-blocking reads we've been discussion on fsdevel for the last half
year.