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6.3 / OPTICAL MEMORY 203

To calculate the new parity, the array management software must read the old
user strip and the old parity strip. Then it can update these two strips with the new
data and the newly calculated parity. Thus, each strip write involves two reads and
two writes.
In the case of a larger size I/O write that involves strips on all disk drives,
parity is easily computed by calculation using only the new data bits.Thus, the par-
ity drive can be updated in parallel with the data drives and there are no extra
reads or writes.
In any case, every write operation must involve the parity disk, which there-
fore can become a bottleneck.
RAID Level 5

RAID 5 is organized in a similar fashion to RAID 4.The difference is that RAID 5
distributes the parity strips across all disks. A typical allocation is a round-robin
scheme, as illustrated in Figure 6.8f. For an n-disk array, the parity strip is on a differ-
ent disk for the first n stripes, and the pattern then repeats.
The distribution of parity strips across all drives avoids the potential I/O bottle-
neck found in RAID 4.

RAID Level 6
RAID 6 was introduced in a subsequent paper by the Berkeley researchers
[KATZ89]. In the RAID 6 scheme, two different parity calculations are carried out
and stored in separate blocks on different disks. Thus, a RAID 6 array whose user
data require N disks consists of N + 2 disks.
Figure 6.8g illustrates the scheme. P and Q are two different data check algo-
rithms. One of the two is the exclusive-OR calculation used in RAID 4 and 5. But
the other is an independent data check algorithm.This makes it possible to regener-
ate data even if two disks containing user data fail.
The advantage of RAID 6 is that it provides extremely high data availability.
Three disks would have to fail within the MTTR (mean time to repair) interval to
cause data to be lost. On the other hand, RAID 6 incurs a substantial write penalty,
because each write affects two parity blocks. Performance benchmarks [EISC07]
show a RAID 6 controller can suffer more than a 30% drop in overall write perfor-
mance compared with a RAID 5 implementation. RAID 5 and RAID 6 read per-
formance is comparable.
Table 6.4 is a comparative summary of the seven levels.

6.3 OPTICAL MEMORY

In 1983, one of the most successful consumer products of all time was introduced:
the compact disk (CD) digital audio system. The CD is a nonerasable disk that can
store more than 60 minutes of audio information on one side.The huge commercial
success of the CD enabled the development of low-cost optical-disk storage tech-
nology that has revolutionized computer data storage. A variety of optical-disk
systems have been introduced (Table 6.5).We briefly review each of these.