Understanding X-RAID and capacity

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geojay wrote:

I don't understand your logic I'm afraid. I understand that the 4TB drive will only function as a 2TB drive in this configuration so let's treat it as a 2TB drive and then we have four 2TB drives or 8TB total. I'm assuming everything is redundant so does that not give us 4TB of effective capacity?

 

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Nope.

With this disk configuration XRAID uses RAID-5.  Here's a (somewhat simplified) picture of how RAID-5 redundancy works.  It's a math trick.

Imagine 3 blocks of data - A, B, C. These blocks are stored on three of the disks.

Now compute P=A+B+C, and store that in the corresponding slot of the fourth disk.

If any disk fails, you can compute the missing block from the corresponding blocks on the other three disks.

A = P-B-C

B = P-A-C

C = P-A-B

P = A+B+C

So you are protected from a single disk failure.  The overhead is 25% in this case, not 50%.

XRAID is a more sophisticated when you have mixed disk sizes.  But even with mixed disk sizes, capacity = (sum of all drives)-largest