From: Ted Ts'o Subject: Re: ext4 data=writeback performs worse than data=ordered now Date: Wed, 14 Dec 2011 09:30:14 -0500 Message-ID: <20111214143014.GB18080@thunk.org> References: <20111214133400.GA18565@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "linux-ext4@vger.kernel.org" , Jan Kara , Li Shaohua , LKML , "linux-fsdevel@vger.kernel.org" To: Wu Fengguang Return-path: Content-Disposition: inline In-Reply-To: <20111214133400.GA18565@localhost> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, Dec 14, 2011 at 09:34:00PM +0800, Wu Fengguang wrote: > Hi, > > Shaohua recently found that ext4 writeback mode could perform worse > than ordered mode in some cases. It may not be a big problem, however > we'd like to share some information on our findings. > > I tested both 3.2 and 3.1 kernels on normal SATA disks and USB key. > The interesting thing is, data=writeback used to run a bit faster > than data=ordered, however situation get inverted presumably by the > IO-less dirty throttling. Interesting. What sort of workloads are you using to do these measurements? How many writer threads; I assume you are doing sequential writes which are extending one or more files, etc? I suspect it's due to the throttling meaning that each thread is getting to send less data to the disk, and so there is more seeking going on with data=writeback, where as with data=ordered, at each journal commit we are forcing all of the dirty pages out to disk, one inode at a time, and this is resulting in a more efficient writeback compared to when the writeback code is getting to make its own choices about how much each inode gets to write out at at time. It would be interesting to see what would happen if in ext4_da_writepages(), we completely ignore how many pages are requested to be written back by the writeback code, and just simply write back all of the dirty pages, and see if that brings the performance back. - Ted