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can be executed in parallel if the block of data to be accessed is distributed across
multiple disks.
With the use of multiple disks, there is a wide variety of ways in which the data
can be organized and in which redundancy can be added to improve reliability.This
could make it difficult to develop database schemes that are usable on a number of
platforms and operating systems. Fortunately, industry has agreed on a standardized
scheme for multiple-disk database design, known as RAID (Redundant Array of
3
Independent Disks). The RAID scheme consists of seven levels, zero through six.
These levels do not imply a hierarchical relationship but designate different design
architectures that share three common characteristics:
1. RAID is a set of physical disk drives viewed by the operating system as a sin-
gle logical drive.
2. Data are distributed across the physical drives of an array in a scheme known as
striping, described subsequently.
3. Redundant disk capacity is used to store parity information, which guarantees
data recoverability in case of a disk failure.
The details of the second and third characteristics differ for the different RAID lev-
els. RAID 0 and RAID 1 do not support the third characteristic.
The term RAID was originally coined in a paper by a group of researchers at
4
the University of California at Berkeley [PATT88]. The paper outlined various
RAID configurations and applications and introduced the definitions of the RAID
levels that are still used. The RAID strategy employs multiple disk drives and dis-
tributes data in such a way as to enable simultaneous access to data from multiple
drives, thereby improving I/O performance and allowing easier incremental in-
creases in capacity.
The unique contribution of the RAID proposal is to address effectively the
need for redundancy. Although allowing multiple heads and actuators to operate
simultaneously achieves higher I/O and transfer rates, the use of multiple devices
increases the probability of failure. To compensate for this decreased reliability,
RAID makes use of stored parity information that enables the recovery of data lost
due to a disk failure.
We now examine each of the RAID levels.Table 6.3 provides a rough guide to
the seven levels. In the table, I/O performance is shown both in terms of data trans-
fer capacity, or ability to move data, and I/O request rate, or ability to satisfy I/O re-
quests, since these RAID levels inherently perform differently relative to these two

3 Additional levels have been defined by some researchers and some companies, but the seven levels
described in this section are the ones universally agreed on.
4 In that paper, the acronym RAID stood for Redundant Array of Inexpensive Disks. The term
inexpensive was used to contrast the small relatively inexpensive disks in the RAID array to the alterna-
tive, a single large expensive disk (SLED). The SLED is essentially a thing of the past, with similar disk
technology being used for both RAID and non-RAID configurations. Accordingly, the industry has
adopted the term independent to emphasize that the RAID array creates significant performance and
reliability gains.