Downsides to running RAID on unmatched drives

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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.

Best Answer

  • 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?)