From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO Date: Mon, 2 Jan 2012 11:15:18 -0500 Message-ID: <20120102161518.GA28940@infradead.org> References: <1324429254-28383-1-git-send-email-minchan@kernel.org> <20111222234135.GB7056@barrios-laptop.redhat.com> <4F01D750.7040304@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Stefan Hajnoczi , Minchan Kim , Rusty Russell , Chris Wright , Jens Axboe , Stefan Hajnoczi , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , Vivek Goyal To: Paolo Bonzini Return-path: Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:53385 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752728Ab2ABQPh (ORCPT ); Mon, 2 Jan 2012 11:15:37 -0500 Content-Disposition: inline In-Reply-To: <4F01D750.7040304@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Jan 02, 2012 at 05:12:00PM +0100, Paolo Bonzini wrote: > On 01/01/2012 05:45 PM, Stefan Hajnoczi wrote: > >By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT > >to hint that seek time optimizations may be sub-optimal. NBD and > >other virtual/pseudo device drivers set this flag. Should virtio-blk > >set it and how does it affect performance? > > By itself is not a good idea in general. > > 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? 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.