10.3.3 RAID Levels
Mirroring provides high reliability, but it is expensive. Striping provides high
data-transfer rates, but does not improve reliability. Various alternative schemes
aim to provide redundancy at lower cost by combining disk striping with “parity”
bits (which we describe next). These schemes have different cost–performance
trade-offs. The schemes are classified into RAID levels, as in Figure 10.3. (In the
figure, P indicates error-correcting bits, and C indicates a second copy of the
data.) For all levels, the figure depicts four disks’ worth of data, and the extra
disks depicted are used to store redundant information for failure recovery.