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6.2 / RAID 201

for data transfer intensive applications with a high percentage of reads. Improve-
ment occurs if the application can split each read request so that both disk mem-
bers participate.

RAID Level 2

RAID levels 2 and 3 make use of a parallel access technique. In a parallel access
array, all member disks participate in the execution of every I/O request. Typically,
the spindles of the individual drives are synchronized so that each disk head is in the
same position on each disk at any given time.
As in the other RAID schemes, data striping is used. In the case of RAID 2
and 3, the strips are very small, often as small as a single byte or word. With RAID
2, an error-correcting code is calculated across corresponding bits on each data disk,
and the bits of the code are stored in the corresponding bit positions on multiple
parity disks. Typically, a Hamming code is used, which is able to correct single-bit
errors and detect double-bit errors.
Although RAID 2 requires fewer disks than RAID 1, it is still rather costly.
The number of redundant disks is proportional to the log of the number of data
disks. On a single read, all disks are simultaneously accessed. The requested data
and the associated error-correcting code are delivered to the array controller. If
there is a single-bit error, the controller can recognize and correct the error in-
stantly, so that the read access time is not slowed. On a single write, all data disks
and parity disks must be accessed for the write operation.
RAID 2 would only be an effective choice in an environment in which many
disk errors occur. Given the high reliability of individual disks and disk drives,
RAID 2 is overkill and is not implemented.

RAID Level 3
RAID 3 is organized in a similar fashion to RAID 2.The difference is that RAID 3
requires only a single redundant disk, no matter how large the disk array. RAID 3
employs parallel access, with data distributed in small strips. Instead of an error-cor-
recting code, a simple parity bit is computed for the set of individual bits in the same
position on all of the data disks.

REDUNDANCY In the event of a drive failure, the parity drive is accessed and data is
reconstructed from the remaining devices. Once the failed drive is replaced, the
missing data can be restored on the new drive and operation resumed.
Data reconstruction is simple. Consider an array of five drives in which X0
through X3 contain data and X4 is the parity disk.The parity for the ith bit is calculated
as follows:

X4(i) = X3(i) { X2(i) { X1(i) { X0(i)
where { is exclusive-OR function.
Suppose that drive X1 has failed. If we add X4(i) { X1(i) to both sides of the
preceding equation, we get
X1(i) = X4(i) { X3(i) { X2(i) { X0(i)