From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764371AbcIOLqM (ORCPT ); Thu, 15 Sep 2016 07:46:12 -0400 Received: from mail.avalus.com ([89.16.176.221]:60853 "EHLO mail.avalus.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751758AbcIOLqJ (ORCPT ); Thu, 15 Sep 2016 07:46:09 -0400 Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements From: Alex Bligh In-Reply-To: <20160915114005.GC23259@infradead.org> Date: Thu, 15 Sep 2016 12:46:07 +0100 Cc: Alex Bligh , Wouter Verhelst , Josef Bacik , "nbd-general@lists.sourceforge.net" , "linux-kernel@vger.kernel.org" , linux-block@vger.kernel.org, Markus Pargmann , kernel-team@fb.com Message-Id: References: <1473369130-22986-1-git-send-email-jbacik@fb.com> <20160909200203.phhvodsfs7ymukfp@grep.be> <20160915104935.ohuwgq2chsedz6fl@grep.be> <27B346AF-F144-4770-BE38-446A66E71326@alex.org.uk> <20160915112936.vb7zxe7k6rvczosg@grep.be> <20160915114005.GC23259@infradead.org> To: Christoph Hellwig X-Mailer: Apple Mail (2.3124) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id u8FBkHhm004876 > On 15 Sep 2016, at 12:40, Christoph Hellwig wrote: > > On Thu, Sep 15, 2016 at 01:29:36PM +0200, Wouter Verhelst wrote: >> Yes, and that is why I was asking about this. If the write barriers >> are expected to be shared across connections, we have a problem. If, >> however, they are not, then it doesn't matter that the commands may be >> processed out of order. > > There is no such thing as a write barrier in the Linux kernel. We'd > much prefer protocols not to introduce any pointless synchronization > if we can avoid it. I suspect the issue is terminological. Essentially NBD does supports FLUSH/FUA like this: https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt IE supports the same FLUSH/FUA primitives as other block drivers (AIUI). Link to protocol (per last email) here: https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes -- Alex Bligh