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.
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