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6
Overview
Understanding RAID
RAID (Redundant Array of Independent Disks) is a system composed of many hard
drives; that is, multiple physical drives form a single virtual drive to be recognized
by the system. Different RAID modes have different advantages and disadvantages.
RAID 0 (Striping)
RAID 0 must be composed of a pair of hard drives at least. When data is written into
the whole hard drive, it will be equally striped and written into each hard drive of the
array. Thus the access speed becomes quicker. The effect of RAID 0 is to proportion
the data to a number of hard drives. More hard drives mean more read/write heads,
and therefore higher speed. Though RAID 0 is quick in read/write speed, it has no
data redundancy, and accordingly has no fault tolerance. It is suggested to compose
RAID 0 with hard drives of the same capacity. Because the capacity of striped disk
array is the multiplication of the smallest hard drive capacity with the number of
hard drives. For example, a 100GB hard drive and a 120GB hard drive unite into
RAID 0. The total capacity is 200GB (100GB×2).
RAID 1 (Mirroring)
RAID 1 must be composed of hard drives in even numbers. The RAID controller
willdivide the hard drives into a pair, and write data simultaneously into the two hard
drives. The two hard drives contain the same data. When one hard drive’s data are
damaged, you can replace the failed hard drive, and the RAID controller will restore
the data by the backup on the other hard drive. RAID 1 provides fault tolerance. It
is suggested to compose RAID 1with hard drives of the same capacity. Because the
capacity of mirrored disk array is that of the smallest hard drive. For example, a
100GB hard drive and a 120GB one unite into RAID 1. The total capacity is 100GB.
JBOD (Just a Bunch of Disks or Spanning)
JBOD is not a real RAID setting. It simply combines a bunch of disks into a single
virtual drive. It doesn’t have the functions of quick read/write speed and error
tolerance. However, compared with RAID 0, JBOD doesn’t waste any hard drive
capacity. Its capacity is the total of all hard drive capacities. For example, set a
100GB hard drive and a 120GB one as JBOD. The total capacity is 220GB.
