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

[Marcelo Tosatti <marcelo@kvack.org>]

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).

So fdatasync() is not enough because data written via AIO may not
have been reflected as "dirty file data" through write() by the time
raw_flush() is called.

The SCSI and IDE drivers use flush() in response to a "flush cache"
request, which is used by the guest OS to implement barriers, for
example.