From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO Date: Mon, 02 Jan 2012 17:18:13 +0100 Message-ID: <4F01D8C5.1060300@redhat.com> References: <1324429254-28383-1-git-send-email-minchan@kernel.org> <20111222234135.GB7056@barrios-laptop.redhat.com> <4F01D750.7040304@redhat.com> <20120102161518.GA28940@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Stefan Hajnoczi , Minchan Kim , Rusty Russell , Chris Wright , Jens Axboe , Stefan Hajnoczi , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Vivek Goyal To: Christoph Hellwig Return-path: Received: from mx1.redhat.com ([209.132.183.28]:5986 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752944Ab2ABQSk (ORCPT ); Mon, 2 Jan 2012 11:18:40 -0500 In-Reply-To: <20120102161518.GA28940@infradead.org> Sender: kvm-owner@vger.kernel.org List-ID: On 01/02/2012 05:15 PM, Christoph Hellwig wrote: > > When QEMU uses O_DIRECT, the guest should not use QUEUE_FLAG_NONROT > > unless it is active for the host disk as well. (In doubt, as is the > > case for remote hosts accessed over NFS, I would also avoid NONROT > > and allow more coalescing). > > Do we have any benchmark numbers where QUEUE_FLAG_NONROT makes a > difference? Not that I know of. > I tried a few times, and the only constant measureable > thing was that it regressed performance when used for rotating devices > in a few benchmarks. Were you trying with cache=none or writeback? For cache=none, that's exactly what I'd expect. cache=writeback could be more interesting... Paolo