From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1O9NIH-0006mQ-4E for qemu-devel@nongnu.org; Tue, 04 May 2010 14:56:33 -0400 Received: from [140.186.70.92] (port=58578 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O9NI8-0006iU-8u for qemu-devel@nongnu.org; Tue, 04 May 2010 14:56:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1O9NI5-00084H-AR for qemu-devel@nongnu.org; Tue, 04 May 2010 14:56:23 -0400 Received: from verein.lst.de ([213.95.11.210]:45168) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1O9NI4-00083H-S9 for qemu-devel@nongnu.org; Tue, 04 May 2010 14:56:21 -0400 Date: Tue, 4 May 2010 20:56:14 +0200 From: Christoph Hellwig Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH 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-Disposition: inline In-Reply-To: <20100420014635.GE21899@shareable.org> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jamie Lokier Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" , Rusty Russell , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, hch@lst.de 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.