Re: [PATCH] Implement barrier support for single device DM devices

[Andi Kleen <andi@firstfloor.org>]

On Fri, Feb 15, 2008 at 04:07:54PM +0300, Michael Tokarev wrote:
> Alasdair G Kergon wrote:
> > On Fri, Feb 15, 2008 at 01:08:21PM +0100, Andi Kleen wrote:
> >> Implement barrier support for single device DM devices
> >  
> > Thanks.  We've got some (more-invasive) dm patches in the works that
> > attempt to use flushing to emulate barriers where we can't just
> > pass them down like that.
> 
> I wonder if it's worth the effort to try to implement this.

DM in theory has some more knowledge for optimization. e.g. for example if 
it knows that a stream of requests hits only a single device then
it can just pass the barriers through again and only flush when there
is really a request dependency between different devices.  File systems can't
do it that fine grained; it's either all or nothing.

I don't know if doing it fine grained will much difference in performance
though. The only way to find out would be to try it.

-Andi