Re: [RFC] relaxed barrier semantics

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On Thu, Aug 05, 2010 at 04:52:15PM +0200, Hannes Reinecke wrote:
> Chris Mason wrote:
> > On Thu, Aug 05, 2010 at 05:11:56PM +0400, Vladislav Bolkhovitin wrote:
> >> Chris Mason, on 08/02/2010 09:39 PM wrote:
> >>> I regret putting the ordering into the original barrier code...it
> >>> definitely did help reiserfs back in the day but it stinks of magic and
> >>> voodoo.
> >> But if the ordering isn't in the common (block) code, how to
> >> implement the "hardware offload" for ordering, i.e. ORDERED
> >> commands, in an acceptable way?
> >>
> >> I believe, the decision was right, but the flags and magic requests
> >> based interface (and, hence, implementation) was wrong. That's it
> >> which stinks of magic and voodoo.
> > 
> > The interface definitely has flaws.  We didn't expand it because James
> > popped up with a long list of error handling problems.  Basically how
> > do the hardware and the kernel deal with a failed request at the start
> > of the chain.  Somehow the easy way of failing them all turned out to be
> > extremely difficult.
> > 
> > Even if that part had been refined, I think trusting the ordering down
> > to the lower layers was a doomed idea.  The list of ways it could go
> > wrong is much much longer (and harder to debug) than the list of
> > benefits.
> > 
> > With all of that said, I did go ahead and benchmark real ordered tags
> > extensively on a scsi drive in the initial implementation.  There was
> > very little performance difference.
> > 
> Care to dig it up?
> I'd wanted to give it a try, and if someone already did some work in
> that area it'll make things easier here.
> 
> I still think that implementing ordered tags is the correct way of
> doing things, implementation details notwithstanding.
> 
> It looks better conceptually than using FUA, and would be easier
> from the request-queue side of things.
> (Or course, as the entire logic is pushed down to the SCSI layer :-)

You see, I'm torn between the dread of giving scsi such great
responsibility and the joy of sending a link for a bitkeeper patch
series from 2.4.x.

http://lwn.net/2002/0214/a/queue-barrier.php3

Have a lot of fun ;)

-chris

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