From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zdenek Kabelac Subject: Re: dd to a striped device with 9 disks gets much lower throughput when oflag=direct used Date: Fri, 27 Jan 2012 16:16:12 +0100 Message-ID: <4F22BFBC.1000709@redhat.com> References: <20120127085226.GA16520@infradead.org> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: dm-devel@redhat.com List-Id: dm-devel.ids Dne 27.1.2012 16:03, Richard Sharpe napsal(a): > On Fri, Jan 27, 2012 at 12:52 AM, Christoph Hellwig wrote: >> On Thu, Jan 26, 2012 at 05:06:42PM -0800, Richard Sharpe wrote: >>> Why do I see such a big performance difference? Does writing to the >>> device also use the page cache if I don't specify DIRECT IO? >> >> Yes. Trying adding conv=fdatasync to both versions to get more >> realistic results. > > Thank you for that advice. I am comparing btrfs vs rolling my own > thing using the new dm thin-provisioning approach to get something > with resilient metadata, but I need to support two different types of > IO, one that uses directio and one that can take advantage of the page > cache. > > So far, btrfs gives me around 800MB/s with a similar setup (can't get > exactly the same setup) without DIRECTIO and 450MB/s with DIRECTIO. a > dm striped setup is giving me about 10% better throughput without > DIRECTIO but only about 45% of the performance with DIRECTIO. > You've mentioned you are using thinp device with stripping - do you have stripes properly aligned on data-block-size of thinp device ? (I think 9 disks are properly quite hard to align somehow on 3.2 kernel, since data block size needs to be power of 2 - I think 3.3 will have this relaxed to page size boundary. Zdenek