Re: [Qemu-devel] [PATCH] Asynchronous reliable and configurable cache flush

[Paul Brook <paul@codesourcery.com>]

On Tuesday 01 April 2008, Jamie Lokier wrote:
> Paul Brook wrote:
> > > @@ -1021,6 +1031,13 @@ static void ide_write_dma_cb(void *opaque, int
> > > ret) 
> > >      /* end of transfer ? */
> > >      if (s->nsector == 0) {
> > > +       if (!s->write_cache) {
> > > +           ret = bdrv_flush(s->bs);
> > > +           if (ret != 0) {
> > > +               ide_dma_error(s);
> > > +               return;
> > > +           }
> > > +       }
> >
> > By my reading this is adding a synchronous flush to the end of an
> > async write operation, which in practice makes the whole operation
> > synchronous.
>
> Looks that way to me too.  It might be simplest to open the device
> with O_DSYNC when !s->write_cache and user actually wants fdatasync,
> so that async write can be used.  I suspect every platform with useful
> Posix AIO has O_DSYNC.

The cache is dynamically enabled/disabled by the target. This means you've got 
to close and repopen the file every time it changes, which is likely to get 
really hairy.

> Otherwise, chaining where the completion of aio_write triggers
> aio_fsync instead of reporting completion to the guest?

Yes, that should do it.

Paul