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

[Michael S. Tsirkin]

On Tue, Apr 20, 2010 at 02:22:58PM +0100, Paul Brook 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.
> 
> (3) allows the guest to queue overlapping transfers with well defined results.
> I have no idea how useful this is in practice, but it's certainly plausible.
> 
> Paul

In theory, yes.
At the moment, qemu only implements FLUSH and lguest only
implements barrier without FLUSH.

If you think it's useful, maybe start by using FLUSH+barrier
in linux guest driver, that'd demonstrate how it's used.


-- 
MST