From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lists.gentoo.org (pigeon.gentoo.org [208.92.234.80]) by finch.gentoo.org (Postfix) with ESMTP id B37DC1381F3 for ; Sat, 22 Jun 2013 11:12:34 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id E0B5EE0B3B; Sat, 22 Jun 2013 11:12:27 +0000 (UTC) Received: from mail-ve0-f172.google.com (mail-ve0-f172.google.com [209.85.128.172]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by pigeon.gentoo.org (Postfix) with ESMTPS id 868C1E0B36 for ; Sat, 22 Jun 2013 11:12:26 +0000 (UTC) Received: by mail-ve0-f172.google.com with SMTP id jz10so7313205veb.31 for ; Sat, 22 Jun 2013 04:12:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:content-type; bh=CU1UXQCgpqsCnDM439B2MPyHzrYE/LppEQ2xO2FY8Gk=; b=ybJ9aGMjFJ4PvR4KHiRGlkWV1xVDle9QD7d/2Q7zOk1a4HNpTxOQXffIQTJ1rDmK1V GmTcn4mXF5esRiGuIe0WpliqO4C5Il0jWIyQ+VHnU7m9Ve1i3lNSVG6mPUinBizj8fK6 +X5wZWTn8vzI0DJ/JZCTQr7oJhPcWchdK+Z4vTc3t2Rpy+djPiYw2ESsiytvDr+mkxl6 iQ/kbmlQuRVeWWHtN/d/iY23kV4qPslAHEN8X28Pyc7WF81Tt/419+Tyf/NCi/2kVJJB P/zYqcRdmo8apQYl0tKwJVeOKmwfAC8OXuwT7z8Fr00FTiHZLFLdXo/dVq17BwxDJUUD xbIA== Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-amd64@lists.gentoo.org Reply-to: gentoo-amd64@lists.gentoo.org MIME-Version: 1.0 X-Received: by 10.52.113.162 with SMTP id iz2mr6544188vdb.0.1371899545558; Sat, 22 Jun 2013 04:12:25 -0700 (PDT) Sender: freemanrich@gmail.com Received: by 10.52.180.98 with HTTP; Sat, 22 Jun 2013 04:12:25 -0700 (PDT) In-Reply-To: References: Date: Sat, 22 Jun 2013 07:12:25 -0400 X-Google-Sender-Auth: LdYcAN-YM0MN0LOrOtsvunl5S50 Message-ID: Subject: Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value? From: Rich Freeman To: gentoo-amd64@lists.gentoo.org Content-Type: text/plain; charset=ISO-8859-1 X-Archives-Salt: 0141902a-9e2b-4a2e-8fd8-eea121daed65 X-Archives-Hash: d94b0a16f2bce02c6f217a8c808f04d2 On Sat, Jun 22, 2013 at 6:29 AM, Duncan <1i5t5.duncan@cox.net> wrote: > Rich Freeman posted on Fri, 21 Jun 2013 11:13:51 -0400 as excerpted: >> If you protect 1 drive of data with 25 drives of parity (call them >> mirrors or parity or whatever - they're functionally equivalent) then >> you need 25/26 drives to fail to lose 1 drive of data. > > Almost correct. DOH - good catch. Would need 26 fails. > AFAIK 13 drives of data with 13 mirrors wouldn't (normally) be called > raid1 (unless it's 13 individual raid1s)... That's why I commented that I find RAID "levels" extremely unhelpful. There is striping, mirroring, and RS parity, and every possible combination of the above. We have a special name raid5 for striping with one RS parity drive. We have another special name raid6 for striping with two RS parity drives. We don't have a special name for striping with 37 RS parity drives. Yet, all three of these are the same thing. I was referring to 13 data drives with one mirror each . If you lose two drives you could potential lose one drive of data. If you made that one big raid10 then if you lose two drives you could lose 13 drives of data. Both scenarios involve bad luck in terms of what pair goes. > You're right that at that level, you DO need a real backup, and it should > take priority over raid-whatever. HOWEVER, in addition to creating a > SINGLE raid across all those drives, it's possible to partition them up, > and create multiple raids out of the partitions, with one set being a > backup of the other. I wouldn't consider that a great strategy. Sure, it is convenient, but it does you no good at all if your computer burns up in a fire. Multiple-level redundancy just seems to be past the point of diminishing returns to me. If I wanted to spend that kind of money I'd probably spend it differently. However, I do agree that mdadm should support more flexible arrays. For example, my boot partition is raid1 (since grub doesn't support anything else), and I have it set up across all 5 of my drives. However, the reality is that only two get used and the others are treated only as spares. So, that is just a waste of space, and it is actually more annoying from a config perspective because it would be really nice if my system could boot from an arbitrary drive. Oh, as far as raid on partitions goes - I do use this for a different purpose. If you have a collection of drives of different sizes it can reduce space waste. Suppose you have 3 500GB drives and 2 1TB drives. If you put them all directly in a raid5 you get 2TB of space. If you chop the 1TB drives into 2 500GB partitions then you can get two raid5s - one 2TB in space, and the other 500GB in space. That is 500GB more data for the same space. Oh, and I realize I wrote raid5. With mdadm you can set up a 2-drive raid5. It is functionally equivalent to a raid1 I think, and I believe you can convert between them, but since I generally intend to expand arrays I prefer to just set them up as raid5 from the start. Since I stick lvm on top I don't care if the space is chopped up. Rich