Re: Help with implementing some form of barriers in 3.0 kernels.

[Konrad Rzeszutek Wilk]

On Wed, Sep 07, 2011 at 02:17:40PM -0400, Vivek Goyal wrote:
> On Wed, Sep 07, 2011 at 01:48:32PM -0400, Konrad Rzeszutek Wilk wrote:
> > Hey Christoph,
> > 
> > I was wondering what you think is the proper way of implementing a
> > backend to support the 'barrier' type requests? We have this issue were
> > there are 2.6.36 type guests that still use barriers and we would like
> > to support them properly. But in 3.0 there are no barriers - hence
> > the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER?
> 
> I think WRITE_FLUSH_FUA is not same as WRITE_BARRIER. Because it does
> not ensure request ordering. A request rq2 which is issued after rq1 (with
> WRITE_flush_FUA), can still finish before rq1. In the past WRITE_BARRIER
> would not allow that.
> 
> So AFAIK, WRITE_flush_fua is not WRITE_BARRIER.

Ok, any thoughts on how to emulate it then perhaps? Mark each request after
rq1 with WRITE_FUA? .. But then how long should the _FUA bit be set - perhaps
until the rq1 has completed?

> 
> Thanks
> Vivek
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