From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [dm-devel] Re: Is there a grand plan for FC failover? Date: Sat, 31 Jan 2004 10:30:37 +0100 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20040131093037.GR11683@suse.de> References: <401521A7.5030808@thekelleys.org.uk> <1075131446.2290.29.camel@mulgrave> <20040128100236.D11527@vienna.EGENERA.COM> <1075309052.2254.6.camel@mulgrave> <20040128130040.E11527@vienna.EGENERA.COM> <20040128124756.A7232@beaverton.ibm.com> <1075328066.2534.10.camel@mulgrave> <20040128165534.A9202@beaverton.ibm.com> <20040130194826.GC31596@reti> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ns.virtualhost.dk ([195.184.98.160]:40618 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S264278AbUAaJbK (ORCPT ); Sat, 31 Jan 2004 04:31:10 -0500 Content-Disposition: inline In-Reply-To: <20040130194826.GC31596@reti> List-Id: linux-scsi@vger.kernel.org To: Joe Thornber Cc: dm-devel@sistina.com, James Bottomley , "Philip R. Auld" , Simon Kelley , SCSI Mailing List 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