Re: [Xen-devel] [Qemu-devel] [PATCH] xen_disk: implement BLKIF_OP_FLUSH_DISKCACHE, remove BLKIF_OP_WRITE_BARRIER

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 26 Apr 2012, Konrad Rzeszutek Wilk wrote:
> On Wed, Apr 25, 2012 at 01:23:35PM +0200, Christoph Hellwig wrote:
> > On Wed, Apr 25, 2012 at 12:21:53PM +0100, Stefano Stabellini wrote:
> > > That is true, in fact I couldn't figure out what I had to implement just
> > > reading the comment. So I went through the blkback code and tried to
> > > understand what I had to do, but I got it wrong.
> > > 
> > > Reading the code again it seems to me that BLKIF_OP_FLUSH_DISKCACHE
> > > is supposed to have the same semantics as REQ_FLUSH, that implies a
> > > preflush if nr_segments > 0, not a postflush like I did.
> > 
> > It's worse - blkfront translates both a REQ_FLUSH or a REQ_FUA
> > into BLKIF_OP_FLUSH_DISKCACHE.
> 
> I think that is what remained of the BARRIER request.
> > 
> > REQ_FLUSH either is a pre flush or a pure flush without a data transfer,
> > and REQ_FUA is a post flush.  So to get the proper semantics you'll have
> > to do both, _and_ sequence it so that no operation starts before the
> > previous one finished.
> 
> If I were to emulate the SCSI SYNC command which one would it be?
> 
> I think REQ_FLUSH? In which I would think that the blkfront needs to
> get rid of the REQ_FUA part?
> 

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