From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:47565 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932469AbcI2RAM (ORCPT ); Thu, 29 Sep 2016 13:00:12 -0400 Subject: Re: [Nbd] [PATCH][V3] nbd: add multi-connection support To: Wouter Verhelst References: <1475092892-8230-1-git-send-email-jbacik@fb.com> <20160929095204.mexr6wpypo3bl6mx@grep.be> <87908d95-0b7c-bc3f-f69d-94d006829daf@fb.com> <20160929164100.akytbkbtvziwaqqj@grep.be> CC: , , , , From: Josef Bacik Message-ID: Date: Thu, 29 Sep 2016 12:59:18 -0400 MIME-Version: 1.0 In-Reply-To: <20160929164100.akytbkbtvziwaqqj@grep.be> Content-Type: text/plain; charset="windows-1252"; format=flowed Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On 09/29/2016 12:41 PM, Wouter Verhelst wrote: > On Thu, Sep 29, 2016 at 10:03:50AM -0400, Josef Bacik wrote: >> So think of it like normal disks with multiple channels. We don't send flushes >> down all the hwq's to make sure they are clear, we leave that decision up to the >> application (usually a FS of course). > > Well, when I asked earlier, Christoph said[1] that blk-mq assumes that > when a FLUSH is sent over one channel, and the reply comes in, that all > commands which have been received, regardless of which channel they were > received over, have reched disk. > > [1] Message-ID: <20160915122304.GA15501@infradead.org> > > It is impossible for nbd to make such a guarantee, due to head-of-line > blocking on TCP. > Huh I missed that. Yeah that's not possible for us for sure, I think my option idea is the less awful way forward if we want to address that limitation. Thanks, Josef