From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH Date: Wed, 21 Apr 2010 13:39:59 +0300 Message-ID: <20100421103959.GA31461@redhat.com> References: <20100218222220.GA14847@redhat.com> <20100420014635.GE21899@shareable.org> <201004201422.58456.paul@codesourcery.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: qemu-devel@nongnu.org, Jamie Lokier , Rusty Russell , virtualization@lists.linux-foundation.org, hch@lst.de, kvm@vger.kernel.org To: Paul Brook Return-path: Received: from mx1.redhat.com ([209.132.183.28]:48062 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754044Ab0DUKok (ORCPT ); Wed, 21 Apr 2010 06:44:40 -0400 Content-Disposition: inline In-Reply-To: <201004201422.58456.paul@codesourcery.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Apr 20, 2010 at 02:22:58PM +0100, Paul Brook wrote: > > Does this mean that virtio-blk supports all three combinations? > > > > 1. FLUSH that isn't a barrier > > 2. FLUSH that is also a barrier > > 3. Barrier that is not a flush > > > > 1 is good for fsync-like operations; > > 2 is good for journalling-like ordered operations. > > 3 sounds like it doesn't mean a lot as the host cache provides no > > guarantees and has no ordering facility that can be used. > > (3) allows the guest to queue overlapping transfers with well defined results. > I have no idea how useful this is in practice, but it's certainly plausible. > > Paul In theory, yes. At the moment, qemu only implements FLUSH and lguest only implements barrier without FLUSH. If you think it's useful, maybe start by using FLUSH+barrier in linux guest driver, that'd demonstrate how it's used. -- MST