From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alasdair G Kergon Subject: Re: [dm-devel] [PATCH 1/2] blkdev: fix merge_bvec_fn return value checks Date: Sat, 6 Mar 2010 02:20:15 +0000 Message-ID: <20100306022015.GU27852@agk-dp.fab.redhat.com> References: <20100303191613.GB18480@redhat.com> <87lje9kx9i.fsf@openvz.org> <20100303200734.GW5768@kernel.dk> <20100304121931.GB5768@kernel.dk> <20100305073059.GI5768@kernel.dk> <20100306085651.1a571571@notabene.brown> <20100305222748.GT27852@agk-dp.fab.redhat.com> <20100306105237.0a8621ed@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20100306105237.0a8621ed@notabene.brown> Sender: linux-kernel-owner@vger.kernel.org To: Neil Brown Cc: device-mapper development , Dmitry Monakhov , Mikulas Patocka , linux-kernel@vger.kernel.org, Mike Snitzer , jens.axboe@oracle.com List-Id: dm-devel.ids On Sat, Mar 06, 2010 at 10:52:37AM +1100, Neil Brown wrote: > But my first suggestion, that splitting could be made easier, still stands. In full agreement we should try to make it easier. > And do you honour merge_bvec_fn's of underlying devices? A quick grep > suggests you do only for dm-linear and dm-crypt. We implemented it to address some specific performance problems reported by people using those targets. So far there hasn't been any pressing need to implement it for other targets. > This suggests to me that it > is actually a hard interface to support completely in a stacked device, so we > might be better off without it. All this code is hard - but very useful. The direction I want to explore when I finally get some time to work on this is one which will probably make more use of, and extend, merge_bvec. > that it makes sense to only require that the lowest level does the > splitting, as that is where the specifics on what might be required exists. We have a hybrid model at the moment - some stuff dealt with top-down, other stuff bottom-up. This thread describes just one of several problems related to this. In the next incarnation of device-mapper I hope we'll find a way to eliminate all of them together. Maybe I'll finally get time later this year to start working on these ideas properly. Alasdair