From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Brook Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH Date: Tue, 20 Apr 2010 14:22:58 +0100 Message-ID: <201004201422.58456.paul@codesourcery.com> References: <20100218222220.GA14847@redhat.com> <20100420014635.GE21899@shareable.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: Jamie Lokier , "Michael S. Tsirkin" , Rusty Russell , virtualization@lists.linux-foundation.org, hch@lst.de, kvm@vger.kernel.org To: qemu-devel@nongnu.org Return-path: Received: from mail.codesourcery.com ([38.113.113.100]:55223 "EHLO mail.codesourcery.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754643Ab0DTNXA (ORCPT ); Tue, 20 Apr 2010 09:23:00 -0400 In-Reply-To: <20100420014635.GE21899@shareable.org> Sender: kvm-owner@vger.kernel.org List-ID: > 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