RAID level 6, the P + Q redundancy scheme, is much like RAID level 5, but
stores extra redundant information to guard against multiple disk failures.
Instead of using parity, level 6 uses error-correcting codes such as the Reed–
Solomon codes (see the bibliographical notes). In the scheme in Figure 10.3g,
2 bits of redundant data are stored for every 4 bits of data—unlike 1 parity
bit in level 5—and the system can tolerate two disk failures.