Posted on 2005-05-08
Last Modified: 2010-04-26
What can happen to a RAID 5 system if the power fails between the write update to the data block and the write update to check block so that only one of the two is successfully written? What could be done to prevent this from happening?
Question by:krithi
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    many raid controllers have an onboard battery to prevent this from happening
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    Expert Comment

    You should also be using a UPS to prevent power from suddently going off, most UPS will link to the PC , so when theres a power failure, it will sent signal to the computer and then it will have time to save the files before shutting down.

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    by:Leon Fester
    Firstly, RAID5 by design, can handle that sort of failure, the reason for using at least 3 drives for a RAID5 implementations is because of the math functions involved in creating the data. In a 3 drive configuration, the general format is:

    Drive1, Drive2, Drive3
    data,    data,    CRC
    data,    CRC,    data
    CRC,    data,    data

    i.e. Data is written in such a manner that the check block is never written consecutively to the same drive. That way using the XOR function it is able to recreate 1 block of data, whether it is the check block or the data block using the information on the other drive.

    A RAID5 system is still able to run should a single drive in the configuration fails. So losing a block of data, to the drive provided that at least n-1 of the drives have been written to, should ensure that your data is available. If, for example a drive fails on a live server. The System will still backup the data across the RAID5 implementation using the above format. Wherever there is a drive missing, it will skip that drive and continue writing data across all drives as per normal. When the faulty drive has been replace your RAID system will rebuild the missing data itself.

    So onto the second part of your question, ensure that write behind is not enabled on any of the drives in your RAID system. This should help to prevent this scenario from presenting itself.
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