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right now on Wikipedia, it says RAID 1 (Mirroring) has a space efficiency of 1/n, and

Space efficiency is given as the amount of storage space available in an array of n disks, in multiples of the capacity of a single drive. For example, if an array holds n=5 drives of 250GB size, and efficiency is n-1, then available space is 4 × 250GB, or roughly 1TB.

so the more hard drives you have, the less capacity you have according to 1/n ? Can that be right? Even the "unit" is not correct... (n-1) is still in TB... 1/n is like 1 / TB. What is that? Shouldn't it be something like n / 2?

The screenshot is here:

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Here is a definition of efficiency: – Tobu Jun 26 '10 at 16:02
All information on Wikipedia is ALWAYS correct! ;) – TFM Jun 26 '10 at 16:31

Yes. In RAID 1, you have the total available capacity of the smallest member of the array. If you have 10x1TB drives, you will only have 1TB of usable space as the data is mirrored across all members.

Typically this doesn't matter, since a RAID 1 usually consists of 2 drives, which gives you 1/2 total capacity.

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so if "Space efficiency is given as the amount of storage space available in an array of n disks, in multiples of the capacity of a single drive" -- should it be n then? Given 6 hard drives of 200GB each, the amount of storage space is 1 times the capacity of a single drive, so 1 is the multiple, which is the space efficiency. – 太極者無極而生 Jun 26 '10 at 15:49
@Jian - Huh? 1 is talking about the capacity of 1 disk. If you have a 2 disk RAID 1 you have 1/2 total space available. If you have a 10 disk, you have 1/10 raw space available. – MDMarra Jun 27 '10 at 4:41

You are right.

The RAID0 setup is correctly specified as having n capacity. This is: n * (disk capacity) = (total capacity)

The RAID1 setup should then have a space efficiency of 1. That is: 1 * (disk capacity) = (total capacity)

This inconsistency suggests that the ratio of space efficiency between RAID0 and RAID1 is: n / (1/n) = n^2

Sidenote: I see the line you quoted above (from Wikipedia) has been changed to: "Space efficiency is given as an equation in terms of the number of drives, n, which results in a value between 0 and 1, representing the fraction of the sum of the drives' capacities that is available for use." and the space efficiency for RAID0 has been changed to 1

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