Re: [Qemu-devel] [PATCH] aio-posix: Fix return value of aio_poll()

[Dietmar Maurer]

> > I currently use qemu_aio_set_fd_handler() to implement async output to
> > the backup stream. While that works, performance is much better
> > (during backup) if I use a separate thread to write the data.
> >
> > Is that known behavior, or should aio give about the same performance
> > as using a thread?
> 
> It depends, but this is what we saw for migration too.  You may also have
> less contention on the big QEMU lock if you use a thread.

But when I use a thread it triggers the bug in bdrv_drain_all(). So how can
I fix  bdrv_drain_all() if I use a separate thread to write data?

I currently use CoQueue to wait for the output thread.

Maybe there is some example code somewhere?
 
> > Or would I get better performance if I use Linux native AIO support?
> 
> That also depends, but in general it should be faster.

Is there an easy way to switch to native AIO? The code in block/linux-aio.c is 
only
usable inside block drivers? I simply need to write to a unix file descriptor.