From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758550Ab1LOBpD (ORCPT ); Wed, 14 Dec 2011 20:45:03 -0500 Received: from mga03.intel.com ([143.182.124.21]:63607 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758439Ab1LOBpA (ORCPT ); Wed, 14 Dec 2011 20:45:00 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.71,315,1320652800"; d="scan'208";a="85863194" Date: Thu, 15 Dec 2011 09:34:56 +0800 From: Wu Fengguang To: NeilBrown Cc: "Li, Shaohua" , "Ted Ts'o" , "linux-ext4@vger.kernel.org" , Jan Kara , LKML , "linux-fsdevel@vger.kernel.org" , "linux-raid@vger.kernel.org" , Jens Axboe Subject: Re: ext4 data=writeback performs worse than data=ordered now Message-ID: <20111215013456.GB17920@localhost> References: <20111214133400.GA18565@localhost> <20111214143014.GB18080@thunk.org> <1323910977.22361.423.camel@sli10-conroe> <20111215010010.GA14805@localhost> <20111215122759.7ce0b7b5@notabene.brown> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111215122759.7ce0b7b5@notabene.brown> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 15, 2011 at 09:27:59AM +0800, NeilBrown wrote: > On Thu, 15 Dec 2011 09:00:10 +0800 Wu Fengguang > wrote: > > > > I found sometimes one disk hasn't any request inflight, but we can't > > > send request to the disk, because the scsi host's resource (the queue > > > depth) is used out, looks we send too many requests from other disks and > > > leave some disks starved. The resource imbalance in scsi isn't a new > > > problem, even 3.1 has such issue, so I'd think writeback introduces new > > > imbalance between the 12 disks. In fact, if I limit disk's queue depth > > > to 10, in this way the 12 disks will not impact each other in scsi > > > layer, the performance regression fully disappears for both writeback > > > and order mode. > > > > I observe similar issue in MD. The default > > > > q->nr_requests = BLKDEV_MAX_RQ; > > > > is too small for large arrays, and I end up doing > > > > echo 1280 > /sys/block/md0/queue/nr_requests > > > > in my tests. > > And you find this makes a difference? > > That is very surprising because md devices don't use requests (and really use > the 'queue' at all) and definitely don't make use of nr_requests. Ah OK. Hope that I was wrong. I've just kicked off the tests to make sure. Thanks, Fengguang