From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758781Ab2DJOVx (ORCPT ); Tue, 10 Apr 2012 10:21:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:6776 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754338Ab2DJOVv (ORCPT ); Tue, 10 Apr 2012 10:21:51 -0400 Date: Tue, 10 Apr 2012 10:21:48 -0400 From: Vivek Goyal To: Jens Axboe Cc: linux kernel mailing list , Moyer Jeff Moyer Subject: Re: [RFC PATCH] block: Change default IO scheduler to deadline except SATA Message-ID: <20120410142148.GG21801@redhat.com> References: <20120410133708.GE21801@redhat.com> <4F843C17.5050004@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4F843C17.5050004@kernel.dk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 10, 2012 at 03:56:39PM +0200, Jens Axboe wrote: > On 2012-04-10 15:37, Vivek Goyal wrote: > > Hi, > > > > I am wondering if CFQ as default scheduler is still the right choice. CFQ > > generally works well on slow rotational media (SATA?). But often > > underperforms on faster storage (storage arrays, PCIE SSDs, virtualized > > disk in linux guests etc). People often put logic in user space to tune their > > systems and change IO scheduler to deadline to get better performance on > > faster storage. > > > > Though there is not one good answer for all kind of storage and for all > > kind of workloads, I am wondering if we can provide a better default and > > that is change default IO scheduler to "deadline" except SATA. > > > > One can argue that some SAS disks can be slow too and benefit from CFQ. Yes, > > but default IO scheduler choice is not perfect anyway. It just tries to > > cater to a wide variety of use cases out of the box. > > > > So I am throwing this patch out see if it flies. Personally, I think it > > might turn out to be a more reasonable default. > > I think it'd be a lot more sane to just use CFQ on rotational single > devices, and default to deadline on raid or non-rotational devices. This > still isn't perfect, since less worthy SSDs still benefit from the > read/write separation, and some multi device configs will be faster as > well. But it's better. Hi Jens, Thanks. Taking a decision based on rotational flag makes sense. I am not sure that does one get the information that a block device is a single device or not. Especially with HBAs, SCSI Luns over Fiber, iSCSI Luns etc. I have few Scsi Luns exported to me backed by a storage array. Everything runs CFQ by default. And though disks in the array are rotational, they are RAIDed and AFAIK, this information is not available to driver. I am not sure if there is an easy way to get similar info for dm/md devices. > > The below patch is not a good idea. There's no clear distinction between > on what CFQ is now the default. Can it be thought of as that block layer default is "deadline" and driver can override it. Yes, I agree that it is not very clear though. Thanks Vivek