Boot from RAID-1 with possible removal of 1 HD

Discussion in 'OS X Mountain Lion (10.8)' started by Muldert, Jul 26, 2013.

  1. Muldert, Jul 26, 2013
    Last edited: Jul 26, 2013

    Muldert macrumors member

    Apr 1, 2012
    So I was thinking.

    I have an iMac (2012, for that matter) with a 1TB HDD. Which is slow, as all HDD's are I'm afraid.
    I also have an external HD of 1TB with a USB3 connection.

    My first thought was I'd combine them in a RAID-0 (striped) for performance.

    But than I thought: what if I'd make it a RAID-1. Than I'd be able to remove the HD in the future (for whatever reason, booting perhaps). I would not have the writing performance (hence the RAID-1) but I will be able to have reading performance of up to 250 MBps, right?

    Tell me guys, I have the feeling I'm missing something. What is it?

    EDIT: Just a second question. I have another 3TB drive. What if I combine my Macintosh HD (1TB, 60MBps read) plus my external HD (1TB, 60MBps read) plus a partition of my second external (1TB of 3TB, 140MBps read) in a mirrored RAID? Will that increase speed up to 260 MBps?
  2. Aznboy1993 macrumors newbie


    Feb 25, 2012
    No your read (and write) performance will not change with a RAID1 (mirrored) configuration. In fact, you might experience some slowdowns due to overhead (depending on how good your controller is OR if you're using "software" RAID). Basically, a RAID1 configuration "mirrors your drives," so when you're writing data to the main drive your raid controller is also writing data to the other drive, on the fly. All of this is transparent and you, the end user, shouldn't notice a difference. The benefit of having a mirrored RAID array is that if one drive fails you can will essentially have a carbon copy of all of the data on the other drive and to rebuild the array you just need to put another same capacity drive in place of the dead one (or you can just use the single drive without the array).

    Having a RAID0 (striped) array would yield you a net increase in (mainly) sequential read/write performance. It's not twofold as you might think, but you do see a healthy increase in performance. The downside of a striped set is that it is volatile. Essentially, when one drive goes bad you lose ALL of your data (as the controller basically micromanages the data placement on both drives simultaneously, hence why you see an increase in overall performance).

    Regarding your second question: No you cannot do that because with a mirrored set you have to have 2*n (where n is an integer) [basically an even number] drives in order for it to function properly. This is because each drive must be cloned to another and if you have an odd number than one of them is essentially "left out." With regards to you using all 3 drives in a RAID0 configuration: you can do that, but I would advise against it. (Personally) I would only recommend RAIDing identical drives of identical storage capacities. And as mentioned above, the net increase in performance is non-linear, thus you can't just "add them up."

    I hope this helps you understand RAID some. If you want to read up more on RAID I would recommend checking this out. Let us know if you need anything else. Cheers! :)
  3. Muldert thread starter macrumors member

    Apr 1, 2012
    Thanks for the response! Definitely made some things clearer for me.

    I was under the assumption that when having a mirrored RAID set, that the processor could read data from two disks at once. So that writing speed would DEcrease but the reading speed would INcrease. Am I in the wrong here?
  4. jav6454 macrumors P6


    Nov 14, 2007
    1 Geostationary Tower Plaza
    Theoretically possible. However, RAID1 is built in mind that you'll always have two drives at minimum. So in case one fails, you have the other to back it up. So, you might remove the drive and it will work, but don't count on having fool proof if you remove one.

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