From: Jonathan McDowell Subject: Re: [PATCH] updated dm-switch Date: Thu, 6 Jun 2013 15:45:18 -0700 Message-ID: <20130606224518.GU19749@earth.li> References: Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: device-mapper development Cc: Mikulas Patocka , "Alasdair G. Kergon" List-Id: dm-devel.ids On Wed, May 22, 2013 at 05:14:18PM -0400, Mikulas Patocka wrote: > This is updated dm-switch. It has better documentation and better error > reporting in message function. ... > --- > > From: Jim Ramsay > > dm-switch is a new target that maps IO to underlying block devices > efficiently when there are a large number of fixed-sized address regions > but there is no simple pattern to allow for a compact mapping > representation such as dm-stripe. Interesting. I have been working on something similar recently, for not dissimilar reasons. However the approach I had been taking was to extend __choose_pgpath to take a pos parameter indicating the block being requested, and then add a new dm-pref-hint multipath driver, which effectively does a round-robin but prefers any path that has the "pref" bit. Currently the dm-pref-hint driver is extremely hacky and reaches down into the scsi_device to do an appropriate CDB along each path to get the pref-map, or I'd post it for comments. I'm interested in why the decision was made to have dm-switch as a separate device-mapper layer? Are there a corresponding set of patches to multipath-tools which know how to do the setup of each individual path? J. -- Is this real - that's the first thing I think every morning. This .sig brought to you by the letter N and the number 24 Product of the Republic of HuggieTag