From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 1 May 2002 12:35:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 1 May 2002 12:35:54 -0400 Received: from unthought.net ([212.97.129.24]:1672 "HELO mail.unthought.net") by vger.kernel.org with SMTP id ; Wed, 1 May 2002 12:35:54 -0400 Date: Wed, 1 May 2002 18:35:53 +0200 From: =?iso-8859-1?Q?Jakob_=D8stergaard?= To: Kent Borg Cc: Arjan van de Ven , Jaime Medrano , linux-kernel@vger.kernel.org Subject: Re: raid1 performance Message-ID: <20020501183553.D31556@unthought.net> Mail-Followup-To: =?iso-8859-1?Q?Jakob_=D8stergaard?= , Kent Borg , Arjan van de Ven , Jaime Medrano , linux-kernel@vger.kernel.org In-Reply-To: <3CCE9038.F4C830B4@redhat.com> <20020430102148.D4470@borg.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Mutt/1.2i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 30, 2002 at 10:21:48AM -0400, Kent Borg wrote: > On Tue, Apr 30, 2002 at 01:38:16PM +0100, Arjan van de Ven wrote, very > roughly: > [that RAID 1 is only as fast in reading as the fastest disk because of > seeking over alternate blocks, and ] > > > The only way to get the "1 thread sequential read" case faster is by > > modifying the disk layout to be > > > > Disk 1: ACEGIKBDFHJ > > Disk 2: ACEGIKBDFHJ > > > > where disk 1 again reads block A, and disk 2 reads block B. To read > > block C, disk 1 doesn't have to move it's head or read a dummy block > > away, it can read block C sequention, and disk 2 can read block D > > that way. > > > > That way the disks actually each only read the relevant blocks in a > > sequential way and you get (in theory) 2x the performance of 1 disk. > > I am confused. > > Assuming a big enough read is requested to allow a parallelizing to > two disks, why can't the second disk be told not to read alternate > blocks but to start reading sequential blocks starting half way up the > request? This is *not* as simple as it sounds. Believe me, I spent a week trying... However, with ext2 (and other filesystems as well), a large sequential file read is *not* sequential on the disk. You should actually see better performance on RAID-1 than on a single disk for very large reads, becuase some of the lookups needed (block indirection or whatever) will be run by the "best" disk in the given situation. > > Also, why does hdparm give me significantly faster read numbers on > /dev/md than it does on /dev/hd? I had assumed > there was parallelizing going on. Does this mean I would get a speed > improvement if I ran my single disk notebook as a single disk RAID 1 > because there is some bigger or better buffering going on in that code > even without parallelizing? hdparm is not a good benchmark for this. Use bonnie, bonnie++, tiotest, or even 'dd' with *huge* files. -- ................................................................ : jakob@unthought.net : And I see the elder races, : :.........................: putrid forms of man : : Jakob Østergaard : See him rise and claim the earth, : : OZ9ABN : his downfall is at hand. : :.........................:............{Konkhra}...............: