Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S968684AbXHMNLc (ORCPT ); Mon, 13 Aug 2007 09:11:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S969897AbXHMIft (ORCPT ); Mon, 13 Aug 2007 04:35:49 -0400 Received: from dsl081-033-126.lax1.dsl.speakeasy.net ([64.81.33.126]:33447 "EHLO bifrost.lang.hm" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S969797AbXHMIfn (ORCPT ); Mon, 13 Aug 2007 04:35:43 -0400 Date: Mon, 13 Aug 2007 01:31:24 -0700 (PDT) From: david@lang.hm X-X-Sender: dlang@asgard.lang.hm To: David Greaves cc: Paul Clements , Jan Engelhardt , Al Boldi , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org, linux-raid@vger.kernel.org Subject: Re: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways? [compare with e.g. NBD + MD raid]) In-Reply-To: <46C01052.4050708@dgreaves.com> Message-ID: References: <200708121335.17267.a1426z@gawab.com> <20070812174549.GA2915@teal.hq.k1024.org> <46BFB6BB.80406@steeleye.com> <46C01052.4050708@dgreaves.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2155 Lines: 45 On Mon, 13 Aug 2007, David Greaves wrote: > david@lang.hm wrote: >> per the message below MD (or DM) would need to be modified to work >> reasonably well with one of the disk components being over an unreliable >> link (like a network link) >> >> are the MD/DM maintainers interested in extending their code in this >> direction? or would they prefer to keep it simpler by being able to >> continue to assume that the raid components are connected over a highly >> reliable connection? >> >> if they are interested in adding (and maintaining) this functionality then >> there is a real possibility that NBD+MD/DM could eliminate the need for >> DRDB. however if they are not interested in adding all the code to deal >> with the network type issues, then the argument that DRDB should not be >> merged becouse you can do the same thing with MD/DM + NBD is invalid and >> can be dropped/ignored >> >> David Lang > > As a user I'd like to see md/nbd be extended to cope with unreliable links. > I think md could be better in handling link exceptions. My unreliable memory > recalls sporadic issues with hot-plug leaving md hanging and certain lower > level errors (or even very high latency) causing unsatisfactory behaviour in > what is supposed to be a fault 'tolerant' subsystem. > > > Would this just be relevant to network devices or would it improve support > for jostled usb and sata hot-plugging I wonder? good question, I suspect that some of the error handling would be similar (for devices that are unreachable not haning the system for example), but a lot of the rest would be different (do you really want to try to auto-resync to a drive that you _think_ just reappeared, what if it's a different drive? how can you be sure?) the error rate of a network is gong to be significantly higher then for USB or SATA drives (although I suppose iscsi would be limilar) David Lang - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/