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    SLCentralArticlesArticles Oct 23rd, 2017 - 12:36 PM EST
    AGW #29: RAID Crazed
    Author: JonnyGURU
    Date Posted: February 12th, 2002

    RAID Crazed

    Things were a bit punchy this Friday in the tech room. Of course, come Fridays, things are always a bit punchy because we know we get to be away from off the wall questions from newbie computer folk for two days.

    This Friday, one of the techs, Joe, was spouting off about different combination RAID levels. Yes, sometimes even the small talk for techs is technical. This one just got wildly out of control on an extreme tangent.

    Combination RAID levels are levels of RAID that are combined with other RAID levels. For example, if you have several RAID 1 arrays, where an even number of drives are striped together where the data is redundant for fault tolerance, and then stripe those together in a RAID 0, this is called RAID 1+0. If you had several RAID 0 arrays, where a number of drives are striped together to form one large drive, and the stripe those arrays together in a RAID 1 array where half of those drives are a parity of the other half, then you have RAID 0+1. See?

    If you don't completely grasp the concept of RAID and combining RAIDs, SLCentral editor Tom Solinap breaks it down really well in this article from last year.

    Where the conversation was going was what I now believe to be an attempt to make stew of my brain. First Joe lay down the rules to make sure everyone understood. He covered the following with everyone and anyone caught with "deer in the headlight eyes" at this point was immediately sent out of the room to take out the trash.

    Typically RAID arrays are defined up to level 5. Level 5 is a raid of drives who's capacity is the size of the drives used (assuming they are all the same), multiplied by the number of drives, minus the size of one drive. Then, the parity is written across the entire array. There's also a level 6 which is similar to 5 except for that the parity is striped twice across the array, so your capacity is the size of the drives used, multiplied by the number of drives, minus the size of two drives. This improves the fault tolerance to two down drives as opposed to one with your typical RAID 5. Of course, if you lose more than one drive in your array at a time, it's time to give up in the IT industry and get a job as a shepherd.

    We were doing well at this point as we only lost one man during the explanation of RAID 5 and 6.

    Now we start laying down the rules of RAID 0+5 and 5+0, as well as 0+6 and 6+0. Take your RAID 0. Multiple drives are seen as one drive. Now take multiple RAID 0s and stripe them as a RAID 5 so there is a redundancy written across the array, or stripe them as a RAID 6 so the redundancy is written twice and you have RAID 0+5 and RAID 0+6.

    RAID 5+0 and 6+0 will be the opposite of this. Set up multiple RAID 5s or 6s and stripe them together as a RAID 0 and you have your RAID 5+0 and RAID 6+0.

    Now it's pop quiz time:

    Joe asks, "What's the minimum number of drives you can have for a RAID 0+5?"

    "Well, RAID 0 can be only 2 drives. For example, 2 20GB drives are seen as 1 40GB drive in RAID 0. RAID 5 needs to be at least 3 drives, so the answer is going to be 6."

    Joe asks, "Using the 20GB drives as an example, what would be the capacity of your 6 drive RAID 0+5?"

    "The RAID 0 would be 40GB with the two 20s. 3 multiplied by 40GB is 120GB, but you have to subtract 40 for the parity, so the answer is 80GB."

    Joe now asks, "What's your fault tolerance?"

    "Well certainly one drive can die and there'd be no issue at all. Two drives could die and the array would still be up, but if it's going to be two drives down, the two drives would have to be the two drives together on the same RAID 0 for it not to compromise the data. Odds are against losing two drives at the same time, but the odds are even slimmer that if two drives die they would be on the same array, so I'm going to call your bluff and still answer 1 drive."

    "Fair enough." This leads Joe to ask the following, "If you had six 20GB drives, what would improve fault tolerance while maintaining at least an 80GB capacity?"

    "Well, the first thing that comes to mind is RAID 0+1, but you lose HALF of your drives to parity, so that's not the answer because that will only give us 60 GB. If we striped each set of 3 drives as a raid 5 first, that would give us 2 arrays of 40GB, then we could stripe those as a RAID 0 and that capacity is doubled. Two drives can die and those drives can be on two different arrays of three, so my fault tolerance is improved slightly.

    If I went with a RAID 6, I can't combine the array with a RAID 0 because RAID 6 alone requires at least four drives. But, if I stripe the six 20GB drives together in a single RAID 6, I get my 80GB and my fault tolerance is improved in that any two drives can fail and I'm still running."

    Joe asks, "Using the minimum number of 20GB drives, what's that capacity and fault tolerance of a RAID 6+6?"

    "A RAID what plus what?" I ask as I start to look for my bottle of Sobe Strawberry Banana.

    Joe explains, "If you take multiple RAID 6s and stripe them together in a RAID 6, how many of these 20GB drives will you need minimum, what would the capacity of the entire array be and what would the maximum fault tolerance be including the off chance that more than one drive died on the same array?"

    I find my Sobe and look at the bottle. "You know what Joe? This stuff would be awesome with a shot of Malibu rum in it."

    You can feel free to answer Joe's last question in our forums ;)

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