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RAID Configuration Introduction
RAID (Redundant Array of Independent Disks) is a method for computer data storage
schemes that divide and/or replicate data among multiple hard drives. RAID can be
designed to provide increased data reliability (fault tolerance) or increased I/O (input/
output) performance, or both. The following RAID configurations are provided for
users.
There are three major key concepts in RAID:
1. Mirroring : The copying of data to more than one disk;
2. Striping : The splitting of data across more than one disk;
3. Error correction : Where redundant data is stored to allow problems to be detected
and possibly fixed (known as fault tolerance).
Different RAID levels use one or more of these techniques, depending on the system
requirements. The main aims of using RAID are to improve reliability, important for
protecting information that is critical to a business, for example a database of customer
orders; or where speed is important, for example a system that delivers video on
demand TV programs to many viewers.
The configuration affects reliability and performance in different ways. The problem
with using more disks is that it is more likely that one will go wrong, but by using
error checking the total system can be made more reliable by being able to survive
and repair the failure. Basic mirroring can speed up reading data as a system can
read different data from both the disks, but it may be slow for writing if it insists that
both disks must confirm that the data is correctly written. Striping is often used for
performance, where it allows sequences of data to be read off multiple disks at the
same time. Error checking typically will slow the system down as data needs to be
read from several places and compared. The design of RAID systems is therefore a
compromise and understanding the requirements of a system is important. Modern
disk arrays typically provide the facility to select the appropriate RAID configuration.
RAID is often used in high availability systems, where it is important that the system
keeps running as much of the time as possible.
