Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

[Jens Axboe]

On Thu, Dec 11 2008, Andrea Arcangeli wrote:
> On Thu, Dec 11, 2008 at 06:20:09PM +0100, Gerd Hoffmann wrote:
> > Andrea Arcangeli wrote:
> > > On Thu, Dec 11, 2008 at 05:11:08PM +0100, Gerd Hoffmann wrote:
> > >> Yes.  But kernel aio requires O_DIRECT, so aio users are affected
> > >> nevertheless.
> > > 
> > > Are you sure? It surely wasn't the case...
> > 
> > Tons of docs say so, but might be they are wrong, I didn't check.
> 
> I guess those tons of docs are just wrong then ;). I see no mention of
> O_DIRECT in `man io_submit` at least... I seem to recall initially aio
> only worked without O_DIRECT... ;). It's quite the opposite, O_DIRECT
> works best with kernel aio, not the other way around. O_DIRECT
> read/writes look very much like non-O_DIRECT seeking reads. For
> seeking sync-reads kernel aio pays off as well as with O_DIRECT.

aio is only async with O_DIRECT, with buffered IO it's sync.

-- 
Jens Axboe