Re: blkfront problem in pvops kernel when barriers enabled

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Sep 07, 2011 at 10:34:49AM -0700, Jeremy Fitzhardinge wrote:
> On 09/06/2011 06:47 PM, Konrad Rzeszutek Wilk wrote:
> > (on 3.1rc2) Looking to xenstore now there is 'feature-flush-cache=1' and
> > no 'feature-barrier'. So it is ok.
> > <scratches head>
> >
> > I can only think of 2.6.38-3 XenOLinux doing it - and it is a bug
> > to do it. It really ought to _not_ advertise 'feature-barrier' and
> > instead advertise 'feature-flush-cache'.
> 
> Does that mean that older guests which don't understand flush-cache will
> be left with no way to force writes to stable storage?  Seems to me that

Correct.
> even if the backend would prefer flush-cache, it should also advertise
> barriers.

But doing it incorrectly is bad - really bad.

> 
> However, that raises the question of how to express the preferred
> mechanism if multiple are available.  You could assume that flush-cache
> is always preferred if available, but that's pretty clunky.

That is how I did it in the frontend.
> 
>     J