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On Wed, Dec 6, 2017 at 7:13 PM, Frank Steinmetzger <Warp_7@×××.de> wrote: |
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> On Wed, Dec 06, 2017 at 06:35:10PM -0500, Rich Freeman wrote: |
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>> |
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>> IMO the cost savings for parity RAID trumps everything unless money |
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>> just isn't a factor. |
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> |
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> Cost saving compared to what? In my four-bay-scenario, mirror and raidz2 |
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> yield the same available space (I hope so). |
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> |
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Sure, if you only have 4 drives and run raid6/z2 then it is no more |
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efficient than mirroring. That said, it does provide more security |
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because raidz2 can tolerate the failure of any two disks, while |
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2xraid1 or raid10 can tolerate only half of the combinations of two |
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disks. |
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The increased efficiency of parity raid comes as you scale up. |
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They're equal at 4 disks. If you had 6 disks then raid6 holds 33% |
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more. If you have 8 then it holds 50% more. That and it takes away |
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the chance factor when you lose two disks. If you're really unlucky |
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with 4xraid1 the loss of two disks could result in the loss of 25% of |
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your data, while with an 8-disk raid6 the loss of two disks will never |
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result in the loss of any data. (Granted, a 4xraid1 could tolerate |
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the loss of 4 drives if you're very lucky - the luck factor is being |
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eliminated and that cuts both ways.) |
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If I had only 4 drives I probably wouldn't use raidz2. I might use |
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raid5/raidz1, or two mirrors. With mdadm I'd probably use raid5 |
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knowing that I can easily reshape the array if I want to expand it |
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further. |
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-- |
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Rich |
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