From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262564AbTLPVG6 (ORCPT ); Tue, 16 Dec 2003 16:06:58 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262591AbTLPVG6 (ORCPT ); Tue, 16 Dec 2003 16:06:58 -0500 Received: from roc-24-93-20-125.rochester.rr.com ([24.93.20.125]:22771 "EHLO mail.kroptech.com") by vger.kernel.org with ESMTP id S262564AbTLPVGz (ORCPT ); Tue, 16 Dec 2003 16:06:55 -0500 Date: Tue, 16 Dec 2003 16:11:31 -0500 From: Adam Kropelin To: jw schultz , Linux Kernel Mailing List Subject: Re: raid0 slower than devices it is assembled of? Message-ID: <20031216161131.A6197@mail.kroptech.com> References: <200312151434.54886.adasi@kernel.pl> <20031216040156.GJ12726@pegasys.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5.1i In-Reply-To: <20031216040156.GJ12726@pegasys.ws>; from jw@pegasys.ws on Mon, Dec 15, 2003 at 08:01:56PM -0800 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 15, 2003 at 08:01:56PM -0800, jw schultz wrote: > On Mon, Dec 15, 2003 at 02:34:54PM +0100, Witold Krecicki wrote: > > Those are results of hdparm -tT on drives: > > > > /dev/md/1: > > Timing buffer-cache reads: 128 MB in 0.40 seconds =323.28 MB/sec > > Timing buffered disk reads: 64 MB in 1.75 seconds = 36.47 MB/sec > > /dev/sda: > > Timing buffer-cache reads: 128 MB in 0.41 seconds =309.23 MB/sec > > Timing buffered disk reads: 64 MB in 1.46 seconds = 43.87 MB/sec > > /dev/sdb: > > Timing buffer-cache reads: 128 MB in 0.41 seconds =315.32 MB/sec > > Timing buffered disk reads: 64 MB in 1.23 seconds = 52.04 MB/sec > > > > No Linux [R]AID improves sequential performance. How would > reading 65KB from two disks in alternation be faster than > reading continuously from one disk? Never say never: /dev/sda: Timing buffer-cache reads: 128 MB in 3.34 seconds = 38.38 MB/sec Timing buffered disk reads: 64 MB in 8.60 seconds = 7.44 MB/sec /dev/sdb: Timing buffer-cache reads: 128 MB in 3.40 seconds = 37.64 MB/sec Timing buffered disk reads: 64 MB in 8.60 seconds = 7.44 MB/sec <...plus four more just like them...> /dev/md0: Timing buffer-cache reads: 128 MB in 3.35 seconds = 38.17 MB/sec Timing buffered disk reads: 64 MB in 4.05 seconds = 15.79 MB/sec md0 is a simple RAID0 of all six disks. Yes, these disks are dirt slow to begin with (Andrew Morton once mentioned he had pencils that wrote faster than my disks) but apparently md manages to get some parallelism going, even for sequential reads. (This is 2.6.0-test11-bk8, FWIW.) --Adam