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? >
ping?
