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On Sat, Jun 22, 2013 at 6:29 AM, Duncan <1i5t5.duncan@×××.net> wrote: |
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> Rich Freeman posted on Fri, 21 Jun 2013 11:13:51 -0400 as excerpted: |
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>> If you protect 1 drive of data with 25 drives of parity (call them |
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>> mirrors or parity or whatever - they're functionally equivalent) then |
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>> you need 25/26 drives to fail to lose 1 drive of data. |
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> |
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> Almost correct. |
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DOH - good catch. Would need 26 fails. |
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> AFAIK 13 drives of data with 13 mirrors wouldn't (normally) be called |
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> raid1 (unless it's 13 individual raid1s)... |
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That's why I commented that I find RAID "levels" extremely unhelpful. |
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There is striping, mirroring, and RS parity, and every possible |
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combination of the above. We have a special name raid5 for striping |
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with one RS parity drive. We have another special name raid6 for |
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striping with two RS parity drives. We don't have a special name for |
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striping with 37 RS parity drives. Yet, all three of these are the |
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same thing. |
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I was referring to 13 data drives with one mirror each . If you lose |
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two drives you could potential lose one drive of data. If you made |
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that one big raid10 then if you lose two drives you could lose 13 |
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drives of data. Both scenarios involve bad luck in terms of what pair |
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goes. |
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> You're right that at that level, you DO need a real backup, and it should |
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> take priority over raid-whatever. HOWEVER, in addition to creating a |
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> SINGLE raid across all those drives, it's possible to partition them up, |
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> and create multiple raids out of the partitions, with one set being a |
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> backup of the other. |
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I wouldn't consider that a great strategy. Sure, it is convenient, |
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but it does you no good at all if your computer burns up in a fire. |
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Multiple-level redundancy just seems to be past the point of |
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diminishing returns to me. If I wanted to spend that kind of money |
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I'd probably spend it differently. |
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However, I do agree that mdadm should support more flexible arrays. |
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For example, my boot partition is raid1 (since grub doesn't support |
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anything else), and I have it set up across all 5 of my drives. |
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However, the reality is that only two get used and the others are |
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treated only as spares. So, that is just a waste of space, and it is |
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actually more annoying from a config perspective because it would be |
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really nice if my system could boot from an arbitrary drive. |
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Oh, as far as raid on partitions goes - I do use this for a different |
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purpose. If you have a collection of drives of different sizes it can |
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reduce space waste. Suppose you have 3 500GB drives and 2 1TB drives. |
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If you put them all directly in a raid5 you get 2TB of space. If you |
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chop the 1TB drives into 2 500GB partitions then you can get two |
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raid5s - one 2TB in space, and the other 500GB in space. That is |
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500GB more data for the same space. Oh, and I realize I wrote raid5. |
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With mdadm you can set up a 2-drive raid5. It is functionally |
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equivalent to a raid1 I think, and I believe you can convert between |
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them, but since I generally intend to expand arrays I prefer to just |
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set them up as raid5 from the start. Since I stick lvm on top I |
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don't care if the space is chopped up. |
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|
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Rich |