Quick RAID question...

Discussion in 'Mac Accessories' started by ashok77, Sep 25, 2009.

  1. ashok77 macrumors member

    Jul 27, 2008

    I'm looking at getting an external enclosure (4-bays) and have started to consider an enclosure with RAID. before anyone gets to excited i'm not thinking of raid as a substitute to regular backups.

    that being said, i've come across the term - Online Capacity Expansion (OCE). i've been checking around and i seem to be getting mixed definitions or maybe they are the same thing but different ways of accomplishing the same thing. could someone please explain to me which one is correct or if the definition covers both explanations.

    v1. allows expansion of an array through the addition of another disk ie. 3 disk RAID 5 array ---> 4 disk RAID 5 array

    v2. allows expansion of the array but replacing smaller drives with larger drives ie. 4x500gb RAID 5 array --->4x1tb RAID 5 array

    it seems like there are a few devices that allow you to do either of these expansions (netgear nv+/nvx has x-raid, promise ns4600 allows for additional drives) . i'm looking for a device that will allow me to expand the individual drives (v2, replace smaller drive with larger drive, allow the RAID to rebuild and repeat with the additional drives). i like this method of expansion the most since it doesn't require moving the data off the array, adding new drives and copying the data back as well as allowing you to replace drives as they become bigger and cheaper.

    i'm beginning to think this feature is more for the higher end RAID devices but i can always dream. on that note the best alternative that i've read about seems to be the OWC Mercury Elite-AL Pro Qx2. i emailed tech and they said i have to copy all the data off the array, replace drives and move data back for their RAID. downside is you have to have all your drives and a large enough backup drive to move the data onto but it has a quad interface which is nice and for the price seems pretty good

    any other suggestions are welcome. oh, and by the way i have a mini that will be acting as a "server" so i want to avoid media servers to avoid to much redundancy. a solid enclosure with a boat load of options is all i want, does that sound like to much to ask?!?! :)

  2. nanofrog macrumors G4

    May 6, 2008
    Both should be possible.

    Please note that the first method will depend on the array type. A RAID0 for example, would have to have the data restored off a backup. But a RAID5/6/10 for example, would allow you to add the drive/s (pairs for 10), and expand without blowing the data on the array.

    The second method depends on all the drives being replaced with larger drives, and has to be done one at a time (levels 5 & 6 for example). 10 is a bit more complicated. The reason is simple. An array's capacity is based off the smallest drive, so for it to work, all the drives must be of increased capacity.

    If you need throughput, you want to use something that uses 1 port/drive (a 4 port eSATA card + 4 port eSATA inputs on the external enclosure as a minimum method; software based btw).

    If you want to run a parity based array (5/6/50/60), don't go with a software/Fake RAID method, as it won't address the write hole issue. For that, you'd need a proper RAID card (they contain an NVRAM) solution. It would also be highly recommended to get a UPS as well, if you don't already have one.

    Hope this helps. :)
  3. ashok77 thread starter macrumors member

    Jul 27, 2008
    hey nanofrog.

    thanks for the response, it's exactly what i was looking for to help me understand thigns. now i just have to go out and find what i'm looking for!

    thanks again!
  4. nanofrog macrumors G4

    May 6, 2008
    :cool: NP. :)

    Just be warned, it usually takes quite a bit of research, so don't rush it. It tends to result in bad solutions if you do. :eek: ;)

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