From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264347AbTLPECE (ORCPT ); Mon, 15 Dec 2003 23:02:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264358AbTLPECE (ORCPT ); Mon, 15 Dec 2003 23:02:04 -0500 Received: from vladimir.pegasys.ws ([64.220.160.58]:16390 "EHLO vladimir.pegasys.ws") by vger.kernel.org with ESMTP id S264347AbTLPECA (ORCPT ); Mon, 15 Dec 2003 23:02:00 -0500 Date: Mon, 15 Dec 2003 20:01:56 -0800 From: jw schultz To: Linux Kernel Mailing List Subject: Re: raid0 slower than devices it is assembled of? Message-ID: <20031216040156.GJ12726@pegasys.ws> Mail-Followup-To: jw schultz , Linux Kernel Mailing List References: <200312151434.54886.adasi@kernel.pl> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200312151434.54886.adasi@kernel.pl> User-Agent: Mutt/1.3.27i X-Message-Flag: If you're running Outlook, look out! Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 15, 2003 at 02:34:54PM +0100, Witold Krecicki wrote: > I've got / on linux-raid0 on 2.6.0-t11-cset-20031209_2107: > > /dev/md/1: > Version : 00.90.01 > Creation Time : Thu Sep 11 22:04:54 2003 > Raid Level : raid0 > Array Size : 232315776 (221.55 GiB 237.89 GB) > Raid Devices : 2 > Total Devices : 2 > Preferred Minor : 1 > Persistence : Superblock is persistent > > Update Time : Mon Dec 15 12:55:48 2003 > State : clean, no-errors > Active Devices : 2 > Working Devices : 2 > Failed Devices : 0 > Spare Devices : 0 > > Chunk Size : 64K > > Number Major Minor RaidDevice State > 0 8 3 0 active sync /dev/sda3 > 1 8 19 1 active sync /dev/sdb3 > UUID : b66633c2:ff11f60d:00119f8d:7bb9fc6c > Events : 0.357 > > Disks are two ST3120026AS connected to sii3112a controller, driven by sata_sil > 'patched' so no limit for block size is applied (it's not needed for it). > > 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 > > What seems strange to me is that second drive is faster than first one > (devices are symmetrical, sd[a,b]2 is swapspace (not mounted at time of > test), sd[a,b]1 is /boot (raid1)). > What is even stranger is that raid0 which should be faster than single drive, > is pretty much slower- what's the reason of that? Overhead+randomness would make an md stripe slower. This measurement is an indication of how fast the drive can sustain sequential data reads No Linux [R]AID improves sequential performance. How would reading 65KB from two disks in alternation be faster than reading continuously from one disk? There used to be some HW raid controllers that might have improved sequential performance by using stripe sizes of 512 bytes (every access hit all disks) but then you suffered near worst case latency with every non-cached read. -- ________________________________________________________________ J.W. Schultz Pegasystems Technologies email address: jw@pegasys.ws Remember Cernan and Schmitt