From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH V5 4/4] virtio-blk: Add REQ_FLUSH and REQ_FUA support to bio path Date: Thu, 2 Aug 2012 08:27:42 +0200 Message-ID: <20120802062742.GA23573@lst.de> References: <1343888757-25723-1-git-send-email-asias@redhat.com> <1343888757-25723-5-git-send-email-asias@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1343888757-25723-5-git-send-email-asias@redhat.com> 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: Asias He Cc: Jens Axboe , kvm@vger.kernel.org, "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Tejun Heo , Shaohua Li , Christoph Hellwig List-Id: virtualization@lists.linuxfoundation.org On Thu, Aug 02, 2012 at 02:25:56PM +0800, Asias He wrote: > We need to support both REQ_FLUSH and REQ_FUA for bio based path since > it does not get the sequencing of REQ_FUA into REQ_FLUSH that request > based drivers can request. > > REQ_FLUSH is emulated by: > 1. Send VIRTIO_BLK_T_FLUSH to device > 2. Wait until the flush is finished There is no need to wait for the flush to finish if the REQ_FLUSH request has no data payload. Even if it has a payload waiting is highly suboptimal and it should use a non-blocking sequencing like it is done in the request layer. > > REQ_FUA is emulated by: > 1. Send the actual write > 2. Wait until the actual write is finished > 3. Send VIRTIO_BLK_T_FLUSH to device > 4. Wait until the flush is finished > 5. Signal the end of the write to upper layer The same comment about not blocking applies here as well.