Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH

[Michael S. Tsirkin]

On Tue, May 04, 2010 at 08:56:14PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 20, 2010 at 02:46:35AM +0100, Jamie Lokier wrote:
> > Does this mean that virtio-blk supports all three combinations?
> > 
> >    1. FLUSH that isn't a barrier
> >    2. FLUSH that is also a barrier
> >    3. Barrier that is not a flush
> > 
> > 1 is good for fsync-like operations;
> > 2 is good for journalling-like ordered operations.
> > 3 sounds like it doesn't mean a lot as the host cache provides no
> > guarantees and has no ordering facility that can be used.
> 
> No.  The Linux virtio_blk guest driver either supports data integrity
> by using FLUSH or can send down BARRIER requests which aren't much
> help at all.

It seems we use BARRIER when we get REQ_HARDBARRIER, right?
What does the REQ_HARDBARRIER flag in request mean and when is it set?

>  Qemu only implements FLUSH anyway.