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On 07/12/2015 12:25 PM, Alan McKinnon wrote: |
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> It's because the "RAID" abilities built into most motherboards are |
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> really shitty. Very little, if any, optimization going on, no real |
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> intelligence, and the whole thing just looks and feels like it's no more |
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> than 2 or more volumes shoved into one group. You would probably be |
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> better off adding both disks to ove LVM VG and telling the system to do |
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> simple striping or mirroring. |
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It can also be noticeable with a slower CPU, as all tasks are offloaded |
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to the CPU. If you are doing I/O heavy tasks (like compiling) it can be |
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very noticeable. Also beware Highpoint cards - quite a few of them are |
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no better than onboard RAID - their prices reflect this. |
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> Proper hardware RAID using a proper RAID adapter is a whole different |
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> story. Those tend to have proper firmware and control the disks |
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> properly. They do the optimizations you expect, they will reorder writes |
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> and do something sane with reads, and are worth the money is you need |
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> RAID. However, most still only do a subset of desirable RAID, usually 0, |
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> 1 and 5. |
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When I upgraded my mythtv backend with a 9750-8i (a real RAID card ~$800 |
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at the time I got it) the difference in I/O performance is staggering. |
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Mine supports RAID 6 and 10 as well. |
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> Linux software RAID, done in-kernel, is an amazing compromise. It's |
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> almost as efficient as good hardware RAID, and covers all bases and RAID |
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> types. Yes, it is somewhat more complex and you do have to use |
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> management utilities when working with it. It's similarly complex to |
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> administrating say LVM (not the same, just same order of magnitude) and |
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> comes with the benefit of NOT costing half a server motherboard :-) |
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It also has the benefit of being able to recover a RAID on a separate |
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machine. If your RAID HBA goes and you can't find a replacement, you're |
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screwed. |
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(Comments are from my first hand experience on my mythtv backend at |
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home, I'd tried Highpoint, motherboard fakeraid, and mdadm before |
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getting a real RAID HBA.) |
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Dan |
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