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).
>
> 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.
Sure. But why isn't the aio_fsync(O_DSYNC) enough by itself?
It seems to me you should have something like this:
/* Flush pending aio_writes until they are dirty data,
and wait before the aio_fsync. */
qemu_aio_flush();
/* Call aio_fsync(O_DSYNC). */
raw_aio_flush(bs);
/* Wait for the aio_fsync to complete. */
qemu_aio_flush();
What am I missing?
-- Jamie
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