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Thread: Raid 5

  1. #1
    Junior Member
    Join Date
    Aug 2002

    Question Raid 5

    w/ RAID 5, disk striping w/ parity using 3 drives, i've heard that when there's data loss or an error that the data will be "reconstructed in memory". So please clarify.

    thnx in advance for all your help

  2. #2
    Join Date
    Apr 2002
    RAID is known as Redundant Array of Inexpensive Disks or Independent Disks. As you which !

    RAID 5 is disk striping w/parity using a minimum of 3 drives as you described it ! It uses a minimum of three drives since it writes the data into 64k packets distributed accross the 3 drives.

    It first writes one 64k packet to one drive, then another 64k packet to another and a parity checksum to the last one. The parity being a math resulting of complex matrix using both 64k data packets written to the other drives.

    Here is a example of how a file of 768K could be distributed to a four disk RAID 5

    ===== ===== ===== =====
    64k 64k Parity 64k
    64k Parity 64k 64k
    Parity 64k 64k Parity
    64k 64k Parity 64k
    64k Parity

    If you have hot swap, which helps a lot, and loose a drive. You would then take the drive off, put it back in and the drive would rebuild it self. So if it needs to rebuild a parity, it will create a new one using the 2 64k packets on the other drives. If it lost a 64k packet, it will recreate using another complex metrix that will regenerate the data by using the 64k packet remaining and the parity that was generated by the two original packets.

    So you have redundancy at the cost of one drive space.

    Also, it is always best to have hotswap system and a RAID Hardware controller which can manage automaticaly a lot of the manual steps you would do using a RAID software.

    I hope it helps !


  3. #3
    The answer is the controller takes care of the rebuild.
    The trick to RAID is use more than 3 drives, the controller will reconstruct your data in the background. The problem is what to do when Raid 5 is not available; 3 drives reduced to 2. We can either go to one big drive or go to a mirroring situation both drives have the same data. If you have 4 or more drives the reconstruction will remain RAID 5, if the volume is able to contain the data of the original drive.
    Considering the cost of a good controller, and the mininal cost of the drives I built my disks with 6 drives. 5 on line (4 data 1 parity) and one hot swap. If you lack the space in the server, buy 6 drives and built RAID 5 with 5 keep the sixth for a rebuild if necessary.

    Hope this helps

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