Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request

[Paul Brook <paul@codesourcery.com>]

On Friday 28 March 2008, Marcelo Tosatti wrote:
> On Fri, Mar 28, 2008 at 03:07:03PM +0000, Jamie Lokier wrote:
> > Marcelo Tosatti wrote:
> > > Its necessary to guarantee that pending AIO writes have reached stable
> > > storage when the flush request returns.
> > >
> > > Also change fsync() to fdatasync(), since the modification time is not
> > > critical data.
> > > +    if (aio_fsync(O_DSYNC, &acb->aiocb) < 0) {
> > >
> > >      BDRVRawState *s = bs->opaque;
> > > -    fsync(s->fd);
> > > +    raw_aio_flush(bs);
> > > +    fdatasync(s->fd);
> > > +
> > > +    /* We rely on the fact that no other AIO will be submitted
> > > +     * in parallel, but this should be fixed by per-device
> > > +     * AIO queues when allowing multiple CPU's to process IO
> > > +     * in QEMU.
> > > +     */
> > > +    qemu_aio_flush();
> >
> > I'm a bit confused by this.  Why do you need aio_fsync(O_DSYNC) _and_
> > synchronous fdatasync() calls?  Aren't they equivalent?
>
> fdatasync() will write and wait for completion of dirty file data
> present in memory.
>
> aio_write() only queues data for submission:
>
>        The "asynchronous" means that this call returns as soon as the 
> request has  been  enqueued;  the  write may or may not have completed when
> the call returns. One tests for completion using aio_error(3).

Surely you should be using the normal aio notification to wait for the 
aio_fsync to complete before reporting success to the device.

Paul