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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH] Asynchronous reliable and configurable cache
	flush
Date: Wed, 2 Apr 2008 13:26:21 +0100
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To: qemu-devel@nongnu.org
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>

On Tuesday 01 April 2008, Jamie Lokier wrote:
> Paul Brook wrote:
> > > @@ -1021,6 +1031,13 @@ static void ide_write_dma_cb(void *opaque, int
> > > ret)=20
> > > =A0 =A0 =A0/* end of transfer ? */
> > > =A0 =A0 =A0if (s->nsector =3D=3D 0) {
> > > +=A0=A0=A0=A0=A0=A0=A0if (!s->write_cache) {
> > > +=A0=A0=A0=A0=A0=A0=A0 =A0 =A0ret =3D bdrv_flush(s->bs);
> > > +=A0=A0=A0=A0=A0=A0=A0 =A0 =A0if (ret !=3D 0) {
> > > +=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0ide_dma_error(s);
> > > +=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0return;
> > > +=A0=A0=A0=A0=A0=A0=A0 =A0 =A0}
> > > +=A0=A0=A0=A0=A0=A0=A0}
> >
> > By my reading this is adding a synchronous flush to the end of an
> > async write operation, which in practice makes the whole operation
> > synchronous.
>
> Looks that way to me too.  It might be simplest to open the device
> with O_DSYNC when !s->write_cache and user actually wants fdatasync,
> so that async write can be used.  I suspect every platform with useful
> Posix AIO has O_DSYNC.

The cache is dynamically enabled/disabled by the target. This means you've =
got=20
to close and repopen the file every time it changes, which is likely to get=
=20
really hairy.

> Otherwise, chaining where the completion of aio_write triggers
> aio_fsync instead of reporting completion to the guest?

Yes, that should do it.

Paul