Which of the following effectively doubles the amount of hard drives needed but also provides redundancy?

A.
RAID Level 0

B.
RAID Level 1

C.
RAID Level 2

D.
RAID Level 5

Explanation:
RAID Level 1 is commonly called mirroring. It mirrors the data from one disk or set of disks by duplicating the
data onto another disk or set of disks. This is often implemented by a one-for-one disk to disk ratio: Each drive
is mirrored to an equal drive partner that is continually being updated with current data. If one drive fails, the
system automatically gets the data from the other drive. The main issue with this level of RAID is that the onefor-one ratio is very expensive — resulting in the highest cost per megabyte of data capacity. This level
effectively doubles the amount of hard drives you need, therefore it is usually best for smaller capacity systems.
Incorrect Answers:
A: RAID Level 0 requires a minimum of two disks so in that sense, it does double the minimum disk
requirement. However, if the minimum amount of disks you require to store your data is more than two, then
RAID level 0 does not double the disk requirement. For example, if you needed 4 disks to store all your data,
you could just create a 4-disk RAID. RAID level 0 also provides no redundancy.
C: RAID Level 2 defines a 39-disk system. This doesn’t double the amount of hard drives needed because it is
a fixed disk requirement.
D: RAID Level 5 does not double the amount of hard drives needed. RAID level 5 requires the equivalent of one
extra drive for parity data. For example, if 4 disks were needed for the amount of data to be stored, the RAID
would need 5 disks. If 10 disks were required for the amount of data to be stored, the RAID would need 11
disks in total.

Krutz, Ronald L. and Russell Dean Vines, The CISSP Prep Guide: Mastering the CISSP and ISSEP Exams,
2nd Edition, Wiley Publishing, Indianapolis, 2004, p. 144