From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755922AbaCNJw6 (ORCPT ); Fri, 14 Mar 2014 05:52:58 -0400 Received: from cantor2.suse.de ([195.135.220.15]:55670 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755543AbaCNJw4 (ORCPT ); Fri, 14 Mar 2014 05:52:56 -0400 Message-ID: <5322D177.8050408@suse.de> Date: Fri, 14 Mar 2014 10:52:55 +0100 From: Hannes Reinecke User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Christoph Hellwig , Mike Snitzer CC: Jeff Moyer , Jens Axboe , Shaohua Li , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 1/1] block: rework flush sequencing for blk-mq References: <20140130132620.GA6031@infradead.org> <20140130132630.GB6031@infradead.org> <20140308155240.GA32297@infradead.org> <531B74B6.4070004@suse.de> <20140312102849.GA26509@infradead.org> <20140313161347.GA6598@redhat.com> <20140314092519.GA10139@infradead.org> <20140314093458.GA19451@infradead.org> In-Reply-To: <20140314093458.GA19451@infradead.org> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/14/2014 10:34 AM, Christoph Hellwig wrote: > On Fri, Mar 14, 2014 at 02:25:19AM -0700, Christoph Hellwig wrote: >> b) is a bit harder, but we should think hard about it when rewriting the >> multipath code to support blk-mq. Talking about which I think trying to >> use dm-multipath on any blk-mq device will go horribly crash and boom at >> the moment. > > Talking abnout crashing and burning.. Hannes, did you run this patch > past dm-devel and linux-scsi yet? Don't quite like it but the problem > seems real.. > No, I haven't. This issue is only exhibited if you try to run multipath on a non-SCSI device (in this case it was cciss). But then that project got abandoned, and there never was a machine with a multipathed cciss controller. Same issue with DASD; you _could_ potentially run multipath on DASD, but all recent mainframes have a feature called 'hyperpav', which essentially implements multipath support within the DASD driver. So running multipath here won't buy you anything. (Plus the DASD driver will only _ever_ return an I/O error after is has had a response from the storage array. Making it truly pointless to run multipathing ...) The only valid use case would be xDR. But then RH apparently has support for xDR even without that patch (otherwise they would have asked for it, being the good upstream citizen as they claim to be, right?) so it looks as if it's not needed there, neither. So this patch hasn't had any application in the real world and I haven't pursued with it upstream. But if Mike feels it'll be a good idea nevertheless I can easily send an updated version. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)