From: Wu Fengguang Subject: Re: ext4 data=writeback performs worse than data=ordered now Date: Thu, 15 Dec 2011 13:02:34 +0800 Message-ID: <20111215050233.GA8959@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 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 To: NeilBrown Return-path: Content-Disposition: inline In-Reply-To: <20111215122759.7ce0b7b5@notabene.brown> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.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. Yes it is: /sys/block/md0/queue/nr_requests cannot be modified at all... Sorry for the noise! Fengguang