Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753975AbZDNHQe (ORCPT ); Tue, 14 Apr 2009 03:16:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752062AbZDNHQX (ORCPT ); Tue, 14 Apr 2009 03:16:23 -0400 Received: from one.firstfloor.org ([213.235.205.2]:41328 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751925AbZDNHQW (ORCPT ); Tue, 14 Apr 2009 03:16:22 -0400 To: Bart Van Assche Cc: LKML Subject: Re: dm-multipath and write request ordering From: Andi Kleen References: Date: Tue, 14 Apr 2009 09:16:18 +0200 In-Reply-To: (Bart Van Assche's message of "Mon, 13 Apr 2009 13:22:49 +0200") Message-ID: <878wm3903h.fsf@basil.nowhere.org> User-Agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1645 Lines: 41 Bart Van Assche writes: > - Is it correct that round-robin load balancing can only work > correctly in such a setup with proper support for write barriers in > the device mapper ? No opinion on that one. > - Did I understand it correctly that the current dm implementation > only supports barriers when remapping a single underlying device ? And also only for dm_linear, so probably it doesn't work on dm_mp (haven't tested) even if it has only a single device. Barriers over multiple devices are very difficult, at least as long as the underlying protocol doesn't support shared barriers or the Linux barrier concept be extended. The problem is that you cannot express to the underlying device that some of its requests are dependent on requests going to other devices. > - Are there any plans to add barrier support to dm-multipath ? [note I'm not very familiar with Linux's dm_mp finer details, some details here might be wrong] I doubt it's really doable with RR. It might be possible if you do the IO primary on one device and only on error fall back to another device. Then during the fallback you could just synchronize all IO and solve the barrier problem this way and otherwise do barriers on the single active device only. But with multiple devices you would have the problem described above. -Andi -- ak@linux.intel.com -- Speaking for myself only. -- 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/