Raid.whats happening to me space?

in macOS edited January 2014
Ok i've got 5 HDs, 4 internal and one firewire, I want to raid 2 of them, 1 ibm 60gig and another 120gig. When i raid them i get speed improvement but it reckons the space is limited to 112gig. Even if i raid all of em it still says 112gig.

so whats going on with me space?

Wheres that 68gig gone to?

i wanna raid em for the speed but for the amount of space im loosing it aint worth it for me.


  • Reply 1 of 8
    I know diddly squat about raiding, but ther ma be some maximum size that can be addressed (like with ram, or how windows can only see partitions up to so large).
  • Reply 2 of 8
    pyr3pyr3 Posts: 946member
    I'm assuming that you're trying to stripe the drives, since redundant RAID does nothing for speed. I always thought that you had to have two identical drives, at least in terms of size, to be able to stripe them.
  • Reply 3 of 8
    gargoylegargoyle Posts: 660member
    I have been told that for the best performance use drives from the same manufacturer and the same model. Also, if you are using the inbuilt controller you should only have 1 drive on each controller in the array. Although you can have two drives on each the controller CANT write to both drives on the same IDE cable at the same time.

    Second. What RAID level are you using? Someone correct me if I am wrong but these the the most common.

    RAID 1 (Mirroring - Redundancy)

    The same data is written to both drives. Depending on your system this will result in fractionally slower writes, but potentially a decent read increase in performance.

    This requires 2 drives and you lose 50% of your disk space because the data is copied to the two drives.

    RAID 0 (Striping - No Redundancy)

    This requires at least 2 drives. The data is written across the drives and often gives quite a good performance increase. The space of the drives is added up but the drives/partitions you stripe MUST be the same size. eq 2 x 20Gig partitions Striped gives to 40Gig. If one drive fails YOU LOSE EVERYTHING stored in the array.

    RAID 5 (Striping with parity - Redundancy)

    As above but with a minimum of 3 drives. the data is stiped across n-1 (where n is the number of drives) and a parity calculation is stored on the last one. This often has the best performance to space ratio as you only lose 1 drives worth of space which makes it more efficient the more drives you have. If one drive fails you can replace it and have your data rebuilt from the other drives in the array.
  • Reply 4 of 8
    cybermonkeycybermonkey Posts: 604member
    Cheers for the info, I was trying to stripe them unfortunately i dont have two identical drives.

    many thanks to gargoyle now i understand the raiding process, Think i'll go back to a mere solo drive for OSX
  • Reply 5 of 8
    The examples of Raid-0 and Raid-1 are reversed.

    Raid-0 is striping, Raid-1 is mirroring. When striping two disks the maximum capacity is double that of the smallest disk, so striping a 60Gb and a 120gb drive will give increased performance, but a maximum of 120gb of space. Striping two 120gb drives would give 240gb of space and increased performance.

    Hope this helps

  • Reply 6 of 8
    gargoylegargoyle Posts: 660member
    Oops, thanks for that!
  • Reply 7 of 8
    alcimedesalcimedes Posts: 5,486member
    if you try to RIAD two differntly sized drives it will just use the smaller drive's size on the larger drive.

    my guess is that you're getting 120GB (60 GB drive plus 60GB from the 120GB drive) when you're done, and once formatted it's down to 112 GB.
  • Reply 8 of 8
    cybermonkeycybermonkey Posts: 604member
    cheers again.

    I've managed to raid (striped) the master of the first bus with the master of the second bus, 57.27gb formatted and 74.53gb formatted leaving with a raid set size of 114gb. leaving me with a loss of only 17gig. that i can just about live with untill my next HD purchase for my external firewire case which ill make sure i get another 80gb so i get the full 160.

    theres definately an improved speedup on me dual 867 g4. installation took only 10 mins, usually 25-30 on a single drive. And apps launch a lot faster.

    Again thanks for all yer feedback you've made me happy.

    I've also printed this thread for future reference.

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