From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH Date: Tue, 4 May 2010 20:56:14 +0200 Message-ID: <20100504185614.GB24998@lst.de> References: <20100218222220.GA14847@redhat.com> <20100420014635.GE21899@shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20100420014635.GE21899@shareable.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Jamie Lokier Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, hch@lst.de List-Id: virtualization@lists.linuxfoundation.org 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. Qemu only implements FLUSH anyway.