RAID5 Cheat

Posted on 2011-10-27
Last Modified: 2012-05-12
I have a RAID5 set up with 3 HDD of 250GB each. If I methodically replace one drive at a time with a larger drive and let the RAID rebuild each time, will I eventually get to a larger RAID array and maintain my data?
Question by:rhondotbob
    LVL 46

    Assisted Solution

    Nope.  Not only that, but you put your data at risk.   All it takes is one bad block at any time and you lose a chunk of data, and some RAID controllers will even abort the rebuild, so you will be required to contract a recovery specialist.

    Do it the right way, backup, blow away, build it the way you want, restore.
    LVL 46

    Expert Comment

    (P.S.  the free space will show up as unallocated.  It won't expand a file system.  It won't expand a logical device.  The controller just thinks you had a disk failure and  put in a replacement drive bigger than you needed.   If  you used X blocks before, then you will end up with using X blocks on the replacement disk for the same LUN, not X + Y blocks.
    LVL 33

    Expert Comment

    [dlethe]'s right.  The controller would just carve out 250GB of your 500GB drive and you'd be stuck with the same amount of storage space.

    Author Comment

    THough I might be able to cheat. Oh well, client is paying for it. So what I have here is easy money. Thanks
    LVL 14

    Accepted Solution

    Your best bet would be to copy the data to another drive, replace the 3 RAID drives with bigger ones (creating a new RAID set), and then copying the data back.  
    LVL 15

    Expert Comment

    While you temporarily put the data at risk, its a calculated risk.  Clearly you would want to have a known-good backup prior to performing any work on the array.

    The additional space would show up as unallocated and you would need to configure the unallocated space on both the array controller and on the operating system using the new space.

    Also, if this is a SATA or SAS array, make sure the controller can support drives larger than 400 GB before performing the work.  Sometimes this simply requires a firmware upgrade.  Sometimes you have to replace the array controller.

    Good luck and here's hoping you have no issues when performing the upgrade.
    LVL 46

    Expert Comment

    But if you take a backup, then it will require much less time to build an array then multiple rebuilds plus expansion (assuming this was even possible).  Not only that, if your controller supports background init, then you could kick off the recovery and run it concurrently with a background init, so it builds the new array once, while restoring at the same time.  All without any risk of data loss.  (Assuming your backup is good).

    So doing it the right way is much safer, could take minutes instead of days, resizes the file system at the same time ... and even if you have some catastrophic failure, your original disk drives with the data are still good.
    LVL 55

    Expert Comment

    "Nope" is not the correct answer; the procedure you outline is supported on some cards but not on others, so the first question should be "what's the controller?"

    It does leave you without parity for an extended period of time of course, and it does take ages but it may be valid. It was common practice with some controllers, but that was when disks were smaller so rebuilds took less time.

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