[dm-devel] Re: Is there a grand plan for FC failover?

[Jens Axboe]

On Fri, Jan 30 2004, Joe Thornber wrote:
> On Wed, Jan 28, 2004 at 04:55:34PM -0800, Patrick Mansfield wrote:
> > > We had the "where does the elevator go" discussion at the OLS bof.  I
> > > think I heard agreement that the current situation of between dm and
> > > block is suboptimal and that we'd like a true coalescing elevator above
> > > dm with a vestigial one for the mid-layer to use for queueing below.  I
> > > think this is a requirement for dm multipath to work well, but it's not
> > > a requirement for it actually to work.
> > 
> > If the performance is bad enough, it doesn't matter if it works.
> 
> It would be great to get some benchmarks to back up these arguments.
> eg, performance of dm mpath with a simple round robin selector,
> compared to a scsi layer implementation.  Lifting the elevator (or
> lowering dm) is a big piece of work that I wont even consider unless
> there is very good reason; the reason probably needs to be broader
> than just multipath too.  Even if we did decide to do this, it won't
> happen in 2.6.

I suspect the problem really isn't that huge in 2.6, since most
performance file systems are using mpage or building their own big
bio's. So in a sense, some of the merging already does happen above dm
(and the io scheduler).

-- 
Jens Axboe