From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Rabbitson Subject: Re: In this partition scheme, grub does not find md information? Date: Tue, 29 Jan 2008 15:47:16 +0100 Message-ID: <479F3C74.1050605@rabbit.us> References: <479EAF42.6010604@pobox.com> <18334.46306.611615.493031@notabene.brown> <479F07E1.7060408@pobox.com> <479F0AAB.3090702@rabbit.us> <479F331F.7080902@msgid.tls.msk.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <479F331F.7080902@msgid.tls.msk.ru> Sender: linux-raid-owner@vger.kernel.org To: Michael Tokarev Cc: Moshe Yudkowsky , linux-raid@vger.kernel.org List-Id: linux-raid.ids Michael Tokarev wrote: > Raid10 IS RAID1+0 ;) > It's just that linux raid10 driver can utilize more.. interesting ways > to lay out the data. This is misleading, and adds to the confusion existing even before linux raid10. When you say raid10 in the hardware raid world, what do you mean? Stripes of mirrors? Mirrors of stripes? Some proprietary extension? What Neil did was generalize the concept of N drives - M copies, and called it 10 because it could exactly mimic the layout of conventional 1+0 [*]. However thinking about md level 10 in the terms of RAID 1+0 is wrong. Two examples (there are many more): * mdadm -C -l 10 -n 3 -o f2 /dev/md10 /dev/sda1 /dev/sdb1 /dev/sdc1 Odd number of drives, no parity calculation overhead, yet the setup can still suffer a loss of a single drive * mdadm -C -l 10 -n 2 -o f2 /dev/md10 /dev/sda1 /dev/sdb1 This seems useless at first, as it effectively creates a RAID1 setup, without preserving the FS format on disk. However md10 has read balancing code, so one could get a single thread sustained read at a speed twice what he could possibly get with md1 in the current implementation I guess I will sit down tonight and craft some patches to the existing md* man pages. Some things are indeed left unsaid. Peter [*] The layout is the same but the functionality is different. If you have 1+0 on 4 drives, you can survive a loss of 2 drives as long as they are part of different mirrors. mdadm -C -l 10 -n 4 -o n2 however will _NOT_ survive a loss of 2 drives.