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Raid 5 capacity question

Posted on 2007-11-20
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Last Modified: 2013-11-14
  I would like to destroy an empty LUN and use it's drives, to create two new luns. The problem is my calculations for capacity do not match what Navisphere is telling me for the current LUN, so I am not willing to destroy this LUN until I know my calculations are correct and the drives will each be large enough after spliting.

Current LUN has 9 drives with these details.

Raw Capacity = 268.40
User Capacity = 238.55

It's currently a raid 5 LUN....   SO

The described math I am using says multiply the smallest drive * number of drives - 1

238.55 * 9 - 1 = 1908

Navisphere shows all 9 drives to have a user Capacity of 1122.97
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Question by:Qwan
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4 Comments
 
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Expert Comment

by:bhnmi
ID: 20324062
You lose a 1/3 of total space on a raid 5.
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rindi earned 25 total points
ID: 20324143
If all the disks in the array are the same size, you loose the capacity of one of those disks to the array. (The above answer is only correct if there are exactly 3 same disks in the array).

If the disks in the array aren't the same size, you additionally loose the difference of the additional disk sizes (If you have 1 100GB Disk and the other 4 disks in the array, are 150GB, then you waste the additional 50GB space of each of the 4 150GB Disks, the array will have a capacity of 400GB).
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Assisted Solution

by:garycase
garycase earned 25 total points
ID: 20324200
No, you do not lose 1/3rd of the space on a RAID 5 array !! ==> you lose ONE drive worth of space :-)    The only time that equals 1/3rd of the total space is with a 3-drive array.   Add more drives, and the "wasted" space becomes proportionally less as the # of drives increase.

However, if I understand the question correctly, the 9 "drives" currently shown are not physical drives => but are the logical drives within a current RAID-5 array ... is that correct?

If so, how much storage you would get from a reconfiguration of that array depends on how you structure the new array.   If you build two distinct RAID 5 arrays, you'll lose one drive for each array.   But I can't think of any scenario that would have the 1122.97 listed above.   Can you provide specific details on the PHYSICAL drives associated with the arrays you want to build?
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Author Comment

by:Qwan
ID: 20324717
 The 9 drives above were actually part of a raid group. The raid group would have shown me the correct values for the drives attached, but in my case we had two LUNs carved from this one raid group so the numbers were inconsistant.

Thank you.
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