On Fri, Mar 28, 2008 at 06:03:25PM +0000, Jamie Lokier wrote:
> 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 is enough, fdatasync() becomes redundant.
> 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?
I don't think the first qemu_aio_flush() is necessary because the fsync
request will be enqueued after pending ones:
aio_fsync() function does a sync on all outstanding asynchronous
I/O operations associated with aiocbp->aio_fildes.
More precisely, if op is O_SYNC, then all currently queued I/O opera-
tions shall be completed as if by a call of fsync(2), and if op is
O_DSYNC, this call is the asynchronous analog of fdatasync(2). Note
that this is a request only — this call does not wait for I/O comple-
tion.
glibc sets the priority for fsync as 0, which is the same priority AIO
reads and writes are submitted by QEMU.
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