From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx3.redhat.com (mx3.redhat.com [172.16.48.32]) by int-mx1.corp.redhat.com (8.11.6/8.11.6) with ESMTP id j29M93n28580 for ; Wed, 9 Mar 2005 17:09:03 -0500 Received: from mail.utsl.gen.nz (210-54-92-184.ipnets.xtra.co.nz [210.54.92.184]) by mx3.redhat.com (8.12.11/8.12.11) with ESMTP id j29M8u9Q004170 for ; Wed, 9 Mar 2005 17:08:56 -0500 Message-ID: <422F73E8.4070707@vilain.net> Date: Thu, 10 Mar 2005 11:08:40 +1300 From: Sam Vilain MIME-Version: 1.0 Subject: Re: [linux-lvm] Building up a RAID5 LVM home server (long) [0x05B52F13] References: <422F679C.8010003@boyko.net> In-Reply-To: <422F679C.8010003@boyko.net> Content-Transfer-Encoding: 7bit Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Andy Boyko Cc: LVM general discussion and development Andy Boyko wrote: >>The underlying principle, is keeping the MD layer as simple as possible, >>so that you've got nice little resilient building blocks for your Volume >>Group. Never use concatenation, striping or RAID5 at the MD layer if >>you've got LVM, it's simply in the wrong place! > Could you by any chance explain this? Assuming, say, five identical > disks, is it really wrong to make an MD RAID-5 out of them, and then > put an LVM LV atop that, for later extension? I'm feeling pretty > dopey, 'cause I'm not immediately grasping what would be an alternative > MD/LVM/EVMS/whatever approach, assuming you do fundamentally want N-1 > worth of available space from those disks. What am I missing? Sorry, I shouldn't have said "never". All I'm saying is that most of the time, you think you want this but in balance you probably want to simply buy a few more disks, or bigger disks, instead of worrying about an extra X% of space, which invariably will just increase backup requirements anyway. If you do have an equally sized (ideally also homogenous brand/model) set of disks, *and you don't overly care about write performance or excessive performance degradation in the case of a single failure*, then sure, RAID 5 setup as you say is the right solution. This is just an old saying repeated. RAID - Fast, Cheap, Reliable. Pick any two. fast+cheap = raid 0/striping fast+reliable = raid 1 or 10 cheap+reliable = raid 5 Sam.