Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752047AbcJCH5N (ORCPT ); Mon, 3 Oct 2016 03:57:13 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:38687 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751554AbcJCH5F (ORCPT ); Mon, 3 Oct 2016 03:57:05 -0400 Date: Mon, 3 Oct 2016 00:57:01 -0700 From: Christoph Hellwig To: Wouter Verhelst Cc: Christoph Hellwig , Josef Bacik , Alex Bligh , Jens Axboe , "linux-block@vger.kernel.org" , Kernel Team , "nbd-general@lists.sourceforge.net" , "linux-kernel@vger.kernel.org" Subject: Re: [Nbd] [PATCH][V3] nbd: add multi-connection support Message-ID: <20161003075701.GA29457@infradead.org> 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> <2B49072B-6F83-4CD2-863B-5AB21E1F7816@fb.com> <20161003072049.GA16847@infradead.org> <20161003075149.u3ppcnk2j55fci6h@grep.be> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161003075149.u3ppcnk2j55fci6h@grep.be> User-Agent: Mutt/1.6.1 (2016-04-27) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1203 Lines: 25 On Mon, Oct 03, 2016 at 09:51:49AM +0200, Wouter Verhelst wrote: > Actually, I was pointing out the TCP head-of-line issue, where a delay > on the socket that contains the flush reply would result in the arrival > in the kernel block layer of a write reply before the said flush reply, > resulting in a write being considered part of the flush when in fact it > was not. The kernel (or any other user of SCSI/ATA/NVMe-like cache flushes) will wait for all I/O that needs to be in the cache for explicitly, so this is not a problem. > Can you clarify what you mean by that? Why is it an "odd flush > definition", and how would you "properly" define it? E.g. take the defintion from NVMe which also supports multiple queues: "The Flush command shall commit data and metadata associated with the specified namespace(s) to non-volatile media. The flush applies to all commands completed prior to the submission of the Flush command. The controller may also flush additional data and/or metadata from any namespace." The focus is completed - we need to get a reply to the host first before we can send the flush command, so anything that we require to be flushed needs to explicitly be completed first.