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I've got some generic RAID controller built into my motherboard and I want to build either a RAID 5 or RAID 0+1 array. What are the disadvantages of running unmatched drives? Like 4 different brand 7200rpm 500gb drives?

This would determine whether I look around for whatever used drives I could get my hands on vs. paying extra for a set of matched identical drives.

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the raid controller built into your motherboard isn't real hardware raid, it's "hardware-assisted" raid (aka "fakeRAID") and depends on drivers to work. you won't have any trouble using a software raid with unmatched drives, and there are a lot of reasons that would be better. –  quack quixote Jan 22 '10 at 5:37
    
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Unless performance is critical (high throughput needed), I don't think there is much of a downside. One risk of getting matched drives you could get drive made in the same batch and drive could fail relatively soon to each other. In that case, when one drive fails from normal wear and tear odds are another may be getting ready to fail. Either way, if the controller supports SMART, I would enable it because it could warn if a drive is about to fail. –  Scott McClenning Jan 22 '10 at 5:41
    
@Scott McClenning: SMART is not very useful in this regard. Of all the drives that have failed me, SMART has actually warned me (and I've been watching the stats pretty closely) in some 10% of the cases - in the remaining 90%, the drives might as well be on fire (heads audibly banging into the enclosure, i/o errors way through the roof, platters trying to spin up and down, smoke pouring out), and SMART would insist they were fine. –  Piskvor May 2 '12 at 14:32

1 Answer 1

Disadvantages of running RAID on unmatched drives:

  • You will inevitably have one drive as the slowest drive and one drive as the fastest drive. The slow drive is the bottleneck, so the unmatched RAID system will be slightly slower than a matched system with drives of the same average speed.

  • The hard drive failure times will likely have a wider dispersion, making it likely that your first failure will occur sooner (worse time-to-first-failure) than if you had matched drives.

  • You will inevitably have one drive pull the most electrical power. It is likely that the total power pulled by the unmatched RAID array will pull more electrical power than a matched system with drives of the same median power requirements, increasing the cost of powering the system and the cost of cooling the system.

Advantages of running RAID on unmatched drives:

  • The biggest advantage of unmatched drives is that the hard drive failure times will likely have a wider dispersion. That gives you more time to replace the failed drive and re-sync before the next one fails. If all the drives are so perfectly matched that they all wear out and fail nearly simultaneously, then you lose all the data.

  • If you already have some hard drives, often you will be unable to find exactly the same model of drive to round out a full set. It costs less to use the drives you have and buy another not-quite-identical drive or two than to buy a complete matched set.

As long as all the drives have the same storage capacity and very roughly the same speed, I think that overall the advantages of deliberately choosing unmatched drives outweigh the advantages of getting perfectly matched drives.

(Should I list those "advantages to running raid on unmatched drives" with this question, or is there a separate question for that?)

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if you'd like to continue with advantages, feel free. there's not a separate question for it (that i know of), but it's probably better to go ahead and post here than to create a separate question. –  quack quixote Jun 3 '10 at 19:16
    
Unmatched drives might have different firmware bugs, which might not be triggered at the some time. If you are lucky. –  fstx May 2 '12 at 14:43

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