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: Tue, 4 May 2010 22:01:24 +0300 Message-ID: <20100504190124.GA29764@redhat.com> References: <20100218222220.GA14847@redhat.com> <20100420014635.GE21899@shareable.org> <20100504185614.GB24998@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jamie Lokier , Rusty Russell , kvm@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org To: Christoph Hellwig Return-path: Received: from mx1.redhat.com ([209.132.183.28]:64442 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932121Ab0EDTFq (ORCPT ); Tue, 4 May 2010 15:05:46 -0400 Content-Disposition: inline In-Reply-To: <20100504185614.GB24998@lst.de> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, May 04, 2010 at 08:56:14PM +0200, Christoph Hellwig wrote: > On Tue, Apr 20, 2010 at 02:46:35AM +0100, Jamie Lokier 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. > > No. The Linux virtio_blk guest driver either supports data integrity > by using FLUSH or can send down BARRIER requests which aren't much > help at all. It seems we use BARRIER when we get REQ_HARDBARRIER, right? What does the REQ_HARDBARRIER flag in request mean and when is it set? > Qemu only implements FLUSH anyway.