Building a RAID with slightly different drives - what would you do?

Discussion in 'Mac Pro' started by IceMacMac, May 27, 2011.

  1. IceMacMac macrumors 6502

    Jun 6, 2010
    I have 4 Drives I plan to put into an OWC Mercury Elite-AL Pro QX2. This will only be a TimeMachine volume.

    The catch? I just discovered that they are closely spec'd but aren't quite exactly matching.
    All 4 drives are Hitachi 2TB Hitachi Deskstar 7200RPM

    However, Differences:
    Pair #1 are Nov 2010 3.0 GB/s part # 10311 -- 32 MB Cache
    Pair #2 Jan 2011 6.0 GB/s part # 12115 --64 MB Cache

    Recommendations....Should I:
    1. Use whatever RAID format I want and not worry?
    2. Avoid striping them and use them as 4 volumes merely spanned together?
    3. Avoid putting them together no matter what?
  2. nanofrog macrumors G4

    May 6, 2008
    You do not have to use identical drives in RAID (applies to both software and hardware implementations).

    However, the capacity will be based on the smallest drive, so if you were running a 1TB, and 3x 2TB's, then your capacity would be 4TB in a stripe set for example. Spanning/concatenation would give you all the capacity at the performance of a single disk (but if a disk dies, you don't loose all the information - the remaining drives can be recovered via software).

    In your particular case however, as the capacity is the same (all 2TB models), the mixed drives won't have a negative impact on usable capacity, no matter the configuration you choose.

    As per stripe sets vs. spanning, I'd recommend spanning. But that particular unit has a RoC (RAID on a Chip, which is an inexpensive hardware RAID controller), so RAID 5 is also a possibility.

    The risk is up to you (no idea what your primary storage pool/s are), but do be cautious. Stripe sets are dangerous; if any disk in the set dies, all the data is gone. And even though this is for a backup location, I'm rather nervous of stripe sets used as backup (just seen too many problems with stripe sets over the years).
  3. IceMacMac thread starter macrumors 6502

    Jun 6, 2010

    Thanks for your advice. I'm going with it.

    I've used striped RAID 0s for over 10 years and been pretty fortunate. Only one problem to date.

    However, as I'm getting older (hopefully wiser) I'm trying to create buffers/safety margins in every part of my life.

    This volume will serve as one of my 3 backup solutions (1 clone, 1 time-machine and 1 off-site), but still no need to have the whole volume collapse with one bad drive. I went with RAID 5 as a compromise.

    3 Drives in tandem for speed...1 drive for safety.
  4. nanofrog macrumors G4

    May 6, 2008
    :cool: NP. :)

    I know users that haven't had problems, but those that eventually experienced one (really a matter of when, not if), learned the risks involved the hard way. :( Not worth the hassle IMO, especially if the information is critical/cannot be recovered (i.e. family photos and movies for home users to professionals that could lose their shirts on a job by missed deadlines, as well as potentially damage their ability to find future work), or cannot afford the time lost on a manual recovery.
  5. hfg macrumors 68040


    Dec 1, 2006
    Cedar Rapids, IA. USA
    I don't know for sure if the differences of your drives will affect RAID operation, but in general it is a good idea to have matched drives in a RAID array. I usually even purchase a spare when I buy the RAID set of drives so that should one fail, I will have a exact replacement.

    I have had some experience with the same OWC enclosure and the WD "Green" drives. I installed a new set of "green" drives as a RAID-5 system with the OWC enclosure. It worked fine most of the time, but occasionally I would be notified that one of the drives had failed and was removed from the array, and needed to be replaced immediately to preserve redundancy.

    If I reset the system, all the drives would pass self-test and continue to function fine for awhile, and then I again would have a failed-drive notice.

    The "green" drives normally spin at a reduced speed for less heat and energy consumption, and then spin up to full speed when they are accessed. Although I wouldn't want that in my primary OS drive, I do like that feature for backup drives which are idle most of the times. From a little research online I found others who had determined that occasionally one drive would not become "ready" as quickly as the rest ... and would subsequently be voted out of the array as a failed drive.

    Replacing the "green" drives with standard drives in the OWC resolved the problem completely and the RAID-5 has been working fine ever since. I re-used the "green" drives in a DROBO FS network backup system which is based on redundant data storage rather than conventional RAID, and they work fine there. I also use them in my HP MediaSmart Server which uses a similar storage strategy as the Drobo and they are fine there as well.

  6. IceMacMac thread starter macrumors 6502

    Jun 6, 2010
    Yes I was certainly aware of this...and thus my post.

    Interesting experiences with Green Drives. I'll have to keep an eye on that moving forward. Thanks for posting.

    Interesting way of looking at it...certain drives getting voted out. Snds like "Survivor Island" for hard drives.

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