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Hardware Thread, Drives failure in Raid 5 in Technical; There was a post about RAID5 and 6 recently on the Spiceworks community - basically saying that RAID5 is poor ...
  1. #16

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    There was a post about RAID5 and 6 recently on the Spiceworks community - basically saying that RAID5 is poor practice now due to the probability of a URE occuring, especially on a SATA array. (The article linked was here - Data Storage: The Myth of Redundancy - Datamation).

    Basically, the chance of an unrecoverable error occurring on a RAID array during rebuild is 50% on a 6TB array which uses standard SATA disks. SAS disks generally have a higher rating for UREs, so it isn't strictly relevant for them at the moment (but in a few years time when we hit the array sizes of them too it will be).

    The article goes to say that RAID 0 is sometimes more reliable than RAID 5 - which in itself is a ridiculous claim, however the original point does stand somewhat - that RAID5 is not really an ideal choice any more.

    RAID 6 adds a slight extra level of redundancy, but doesn't increase the reliability - the chance of the array going belly up during rebuild still exists.

    So, the end result is advice to go with a RAID 10 array instead. If you are looking at a 3 disk array, switching to 4 disks isn't a huge extra cost either - and the size of the array would be the same at the end of it.

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    Quote Originally Posted by Hawkeyez View Post
    Yep, slim, but can happen. Member of my team here had his Raid5 fail at home, only for a 2nd disk to fail while the 1st disk was still rebuilding!.

    Like others have said, still need to have a backup solution, just in case that 1 in a million happens, and 2 disks fail.
    It might be one in a million, but the MTBF of any one drive in an array will be just the same as the other drives, so if one drive fails after several years service the chances are that another one will fail soon!

    Plus, if there are factors which have contributed to the failure of one drive, (temperature, shock, power failure, etc), those same factors will also apply to other drives in the array.

    NN

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    RAID5 can only cope with (a maximum of) 1 HDD failing.

    As sonofsanta mentioned, having a HDD configured as a spare/hotswap just means the RAID contoller will automatically use that drive (and rebuild the RAID) instead of you manually removing the failed one and inserting a working HDD.

    It makes no difference (as far as redundancy is concerned) as to how may HDDs are in the RAID5 Array, it can still only cope with one HDD failing; plus there is a 50% performance hit whilst a HDD has failed.

    Where people express concern with RAID5 is if another HDD fails before the original HDD has been replaced AND the RAID has not been rebuilt; but the same could be said for RAID1.

    Only RAID6 can cope with any 2 HDD failures (but write performance is 50% slower than RAID5), and RAID 10, 50 etc can cope with 1 per sub-array.

    You could go on and specify multiple RAID Controllers etc, but it is a balance of performance, redundancy and cost and more importantly backup strategy.
    Last edited by MYK-IT; 5th March 2012 at 08:58 AM.

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    There's an interesting paper by Adam Leventhal about how unsuitable RAID-5 is and RAID-6 will be...

    Triple-Parity RAID and Beyond

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