GSLC · Question #310
Which of the following is a valid space efficiency of RAID 3?
The correct answer is A. 1 - 1/n. RAID 3 dedicates exactly one disk out of n to parity, leaving n-1 disks for data, which yields a space efficiency of (n-1)/n or equivalently 1 - 1/n.
Question
Which of the following is a valid space efficiency of RAID 3?
Options
- A1 - 1/n
- B1/n
- C1 - 2/n
- D1
How the community answered
(18 responses)- A89% (16)
- B6% (1)
- C6% (1)
Why each option
RAID 3 dedicates exactly one disk out of n to parity, leaving n-1 disks for data, which yields a space efficiency of (n-1)/n or equivalently 1 - 1/n.
In RAID 3, one disk is always reserved for parity regardless of array size, so the usable data capacity is (n-1) disks out of n total, expressed as the fraction (n-1)/n. This simplifies algebraically to 1 - 1/n, meaning efficiency improves as more disks are added. For example, a 5-disk RAID 3 array yields 80% (1 - 1/5) usable space.
1/n is the space efficiency of RAID 1 (full mirroring), where only one disk out of n holds unique data and all others are mirror copies.
1 - 2/n is the space efficiency of RAID 6, which reserves two disks worth of capacity for dual parity rather than the single parity disk used in RAID 3.
A space efficiency of 1 (100%) implies zero overhead for redundancy, which is only true for RAID 0 - RAID 3 always sacrifices one full disk to parity.
Concept tested: RAID 3 storage space efficiency formula
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