6.2. Overview of High Availability Techniques
To increase the reliability and availability of multi-disk storage systems it is common to add data redundancy in order to survive the data loss of a failed disk. Two basic paradigms for data redundancy exist: (1) mirroring the original data  and (2) constructing parity information . Both of these basic techniques can be refined into several alternative variations, each with its own specific characteristics. Figure 6.2 shows five broad classes of reliability techniques. Most algorithms that have been reported in the literature can be subsumed under one of these five classes. We now proceed to give an overview of each class in turn.