From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Philip R. Auld" Subject: Re: Is there a grand plan for FC failover? Date: Wed, 28 Jan 2004 10:02:36 -0500 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20040128100236.D11527@vienna.EGENERA.COM> References: <401521A7.5030808@thekelleys.org.uk> <1075131446.2290.29.camel@mulgrave> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from roadrunner-base.egenera.com ([63.160.166.46]:4292 "EHLO coyote.egenera.com") by vger.kernel.org with ESMTP id S265985AbUA1PFA (ORCPT ); Wed, 28 Jan 2004 10:05:00 -0500 Content-Disposition: inline In-Reply-To: <1075131446.2290.29.camel@mulgrave>; from James.Bottomley@steeleye.com on Mon, Jan 26, 2004 at 09:37:25AM -0600 List-Id: linux-scsi@vger.kernel.org To: James Bottomley Cc: Simon Kelley , SCSI Mailing List Hi, Rumor has it that on Mon, Jan 26, 2004 at 09:37:25AM -0600 James Bottomley said: > On Mon, 2004-01-26 at 08:18, Simon Kelley wrote: > > > I'm very much in favour of using drivers which are developed in the > > kernel mainline but I have an application which needs failover so I > > might be forced back to the qlogic-distributed code. > > Yes, the direction coming out of KS/OLS last year was to use the dm or > md multi-path code to sit the failover driver on top of sd (or any other > block driver). > > The idea being that the Volume Manager layer is the most stack generic > place to do this type of thing. The thread on this is here: > > http://marc.theaimsgroup.com/?t=106005575400003 > There are some issues left un-resolved with this approach. The one's that come mind are: 1) load balancing when possible: it's not enough to be just a failover mechanism. 2) requiring a userspace program to execute for failover is problematic when it could be the root disk that needs failing over. 3) Handling partitions is a problem. I see multipath and md/RAID as two different animals. Multipathing is multiple ways to reach the same physical block. That is it's under the logical layer. While RAID is multiple ways to reach the same logical block. It's basically many-to-one vs one-to-many. Having multiple physical paths to a logical partition is a little counter-intuitive. That said, I'm all for getting Linux to have a decent multipath implementation, at what ever layer is agreed upon. I'd love to be able to use the native Linux multipathing and stop supporting the ones I've written. FWIW, Phil > James > > > - > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Philip R. Auld, Ph.D. Egenera, Inc. Principal Software Engineer 165 Forest St. (508) 858-2628 Marlboro, MA 01752