Raid 5

Discussion in 'Mac Basics and Help' started by trainguy77, Nov 1, 2005.

  1. trainguy77 macrumors 68040

    Nov 13, 2003
    with RAID 5 setup, that had 5 hard drives in it, each 72 GB. How many drives can i lose until it can't still run. And how many drives can I lose without data lose? Is it 1 even though I have 5 drives? Does my data capacity rise with more drives in RAID 5?
  2. CanadaRAM macrumors G5


    Oct 11, 2004
    On the Left Coast - Victoria BC Canada
    A RAID 5 array can have as few as 3 drives. It consists of a striped set with parity (redundant) data striped across all drives.

    You can lose one and only one drive without data loss. A proper RAID 5 will continue to work at reduced efficiency after the loss of one disk, because it has to reconstruct the missing drive's data from the parity data on the other drives.

    The advantage of adding more drives to a RAID is that you net out more usable space.

    RAID 1 Mirror = 2 x 100 Gb = 100 Gb usable space = 50%

    RAID 3 or 5: 3 drives = 3 x 100 = 200 Gb space = 66.7%
    RAID 3 or 5: 5 drives = 5 x 100 = 400 Gb space = 80%

    There is a point of diminishing marginal returns, however, as the more drives you have, the higher the probability of two drives failing within the time window to replace and rebuild the data of the first failed one.

    If you want higher fault tolerance, you have to accept greater space wastage and start mirroring striped sets with other striped sets.

    But remember RAID != Backup. RAID protects only against drive failure.

    If you have a virus, crash or a dumb@$$ moment that results in the deletion, corruption or overwriting of data, it is done to the primary data and the parity data immediately and equally - and you are equally in the s#@^=#ouse as if you had but one drive.
  3. Eniregnat macrumors 68000


    Jan 22, 2003
    In your head.
    You can loose 1 drive, with out down time or data loss. Anything more

    At work, I run a few RAID systems, actualy 2 redundant, and one non redundant, and ...ok more than a few. Our primary media server is doubly redundant. I run 2 RAID5 pizzaboxes. Each RAID5 is a mirror of the other. Down time is not an option for me. Rebuilding a RAID5 is a pain in the ass, and can take a long time, for me up to 22hrs. If you ever have to rebuild the RAID, if a drive falls out or fails, stop what your doing, and let it rebuild with out taxing the system.

    If your not running the server edition of OSX, the RAID rebuild and management features are difficult to use at best, unless you love comand line, or are using a thrid-party controller or managment software.
  4. Anonymous Freak macrumors 603

    Anonymous Freak

    Dec 12, 2002
    To clarify what CanadaRAM said, in a RAID-5, you lose one drive's worth of capacity. So with 3 drives, you lose 33%, with four drives, 25%, five drives 20%, etc. Again, as he also mentioned, you run into diminishing returns, as more drives means more likelihood of two drive failures.

    If you want to really protect against data loss, you'll have a RAID-5 with a 'hot spare'. This is an extra drive attached to the RAID controller that sits idle until a drive fails. Then the RAID controller immediately reconstructs the full RAID using this spare drive. (The array will be slow as molasses, as it has to figure out what belongs on the 'missing' drive, and write that to the spare; while servicing normal reads and writes.)

    For example, a great way to set up an Xserve RAID is to have each 7-drive side as a 6-drive RAID-5 plus hot spare, then RAID-0 the two sides. (In case you weren't aware, the Xserve has each side show up separately to the computer. It can't span across all 14 drives in hardware.) You'll end up 'losing' four drives worth of capacity, (28%) but you'll be able to lose two drives on each side before you lose data. (And unless three drives on one side fail exactly simultaneously, you should have time to replace the one or two dead ones.)
  5. superbovine macrumors 68030


    Nov 7, 2003
    Yes you can only loss 1 drive. however, sometimes when a drive goes down it takes another with it that still works. depending on your raid controller will tell you how you can bring the working one back up. just fyi if you ever have two drives go down at once. that is about the 3rd thing you should check.
  6. trainguy77 thread starter macrumors 68040

    Nov 13, 2003
    Thanks, the server I have at work, will rebuild automatically. So that is good. I have had me system backing up every night. Just in case. Thanks for the info. So maybe i should order 1 or 2 spare drives just in case.....I also know it can still run when it is short one drive as i have lost 1 drive before.

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