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
