From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 22 Jan 2002 12:46:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 22 Jan 2002 12:46:08 -0500 Received: from unthought.net ([212.97.129.24]:16842 "HELO mail.unthought.net") by vger.kernel.org with SMTP id ; Tue, 22 Jan 2002 12:46:01 -0500 Date: Tue, 22 Jan 2002 18:46:00 +0100 From: =?iso-8859-1?Q?Jakob_=D8stergaard?= To: Bill Davidsen Cc: "Alok K. Dhir" , linux-kernel@vger.kernel.org Subject: Re: Autostart RAID 1+0 (root) Message-ID: <20020122184600.C11697@unthought.net> Mail-Followup-To: =?iso-8859-1?Q?Jakob_=D8stergaard?= , Bill Davidsen , "Alok K. Dhir" , linux-kernel@vger.kernel.org In-Reply-To: <001201c1a03e$e654acd0$9865fea9@pcsn630778> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Mutt/1.2i In-Reply-To: ; from davidsen@tmr.com on Tue, Jan 22, 2002 at 12:15:28PM -0500 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 22, 2002 at 12:15:28PM -0500, Bill Davidsen wrote: > On Fri, 18 Jan 2002, Alok K. Dhir wrote: > > > I want to test using a software RAID 1+0 partition as root: md0 and md1 > > set up as mirrors between two disks each, and md2 set up as a stripe > > between md0 and md1. However, the RedHat 7.2 installer doesn't allow > > creating nested RAID partitions. > > Here's my understanding. If you are using hardware RAID you can do > anything your controller supports, and it looks like a single drive to the > CPU. But if you are looking for reliable boot, you need to use /boot as a > RAID-1 partition on the first two drives, and make that partition the > active partition (that may not be needed with your BIOS). I think he is referring to software RAID. And yes, it is indeed a problem that the RedHat installer cannot create nested RAIDs (at least, I too was unable to do that, so either it's impossible, or I'm equally blind). > This is because if the first disk fails totally, the 2nd will be used to > boot. You also should use an initrd image to be sure all you need to get > up is on that small mirrored partition. After that your other partitions > can be whatever pleases you. Also, GRUB/LILO only support booting from RAID-1 (or no RAID). ... > > > > Does the kernel support autostarting nested RAID partitions? > > Yes it does. If you have persistent superblocks on all arrays, they *should* autostart. If you boot from the 4G disk, does the array start properly ? Does it start properly even if you remove your /etc/raidtab ? Please check that you have the correct RAID levels either compiled into your kernel, or on an initrd. > > Is doing software 1+0 a bad idea anyway due to performance issues? > > It should outperform most other RAID configs under heavy load, but in most > cases RAID-5 is fine for system which don't need the absolute highest > performance. Note that the extra writes are queued and there are no extra > reads unless it is in recovery mode. RAID-1 can be faster, because there > are two copies of the data, if one drive is busy the other can be used. I > haven't checked to see that software RAID does that correctly and gets the > benefit. A performance improvement went into 2.4 at some stage - all newer 2.4 kernels will schedule reads to the mirror which has it's head nearest to where the read should occur. This works very well in my experience. -- ................................................................ : 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}...............: