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

[Marcelo Tosatti <marcelo@kvack.org>]

On Fri, Mar 28, 2008 at 04:40:54PM +0000, Paul Brook wrote:
> 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.

qemu_aio_flush() will wait for all pending AIO requests (including
aio_fsync) to complete.

Or do you mean something else?