From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757163Ab1IGVbs (ORCPT ); Wed, 7 Sep 2011 17:31:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49201 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757079Ab1IGVbq (ORCPT ); Wed, 7 Sep 2011 17:31:46 -0400 Date: Wed, 7 Sep 2011 17:31:37 -0400 From: Vivek Goyal To: Christoph Hellwig Cc: Konrad Rzeszutek Wilk , Jeremy Fitzhardinge , jbeulich@novell.com, linux-kernel@vger.kernel.org, JBeulich@suse.com Subject: Re: Help with implementing some form of barriers in 3.0 kernels. Message-ID: <20110907213137.GJ31726@redhat.com> References: <20110907174832.GN32190@dumpdata.com> <20110907181740.GG31726@redhat.com> <20110907201620.GB26548@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110907201620.GB26548@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 07, 2011 at 04:16:20PM -0400, Christoph Hellwig wrote: > [Hmm, for some reason I never manage to receive Konrads mails directly, > but only get the replies, or copies via the list] > > On Wed, Sep 07, 2011 at 02:17:40PM -0400, Vivek Goyal wrote: > > On Wed, Sep 07, 2011 at 01:48:32PM -0400, Konrad Rzeszutek Wilk wrote: > > > Hey Christoph, > > > > > > I was wondering what you think is the proper way of implementing a > > > backend to support the 'barrier' type requests? We have this issue were > > > there are 2.6.36 type guests that still use barriers and we would like > > > to support them properly. But in 3.0 there are no barriers - hence > > > the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER? > > > > I think WRITE_FLUSH_FUA is not same as WRITE_BARRIER. Because it does > > not ensure request ordering. A request rq2 which is issued after rq1 (with > > WRITE_flush_FUA), can still finish before rq1. In the past WRITE_BARRIER > > would not allow that. > > > > So AFAIK, WRITE_flush_fua is not WRITE_BARRIER. > > Indeed. And while most guests won't care some will. E.g. reiserfs > which is the standard filesystem in most SuSE guests, which happen to > be fairly popular with Xen. > > I'd suggest you look at the pre-2.6.36 barrier implementation and see > if you can move that into xen-blkfront. > > For the qemu side doing this is a bit easier as you'll just have to wait > for all pending aio requests to complete. The current qemu xen disk > code gets thus horribly wrong, though. I have a basic question. In old guest why BARRIER handling on request queue is not sufficient for sequencing and ordering of requests and why xen-blkfront and qemu have to do something about it. I am also wondering if virtio-blk have similar issues? Thanks Vivek