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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
