Raid 4 Horizontal Row Parity - IBM N Series Hardware Manual

10.4.2 RAID 4 horizontal row parity

Figure 10-7 illustrates the horizontal row parity approach used in the traditional RAID 4
solution. It is the first step in establishing an understanding of RAID-DP and double parity.
Figure 10-7 RAID 4 horizontal parity
Figure 10-7 represents a traditional RAID 4 group that uses row parity. It consists of four data
disks (the first four columns, labeled D) and the single row parity disk (the last column,
labeled P). The rows represent the standard 4 KB blocks used by the traditional RAID 4
implementation. The second row is populated with sample data in each 4 KB block. Parity
calculated for data in the row is then stored in the corresponding block on the parity disk.
In this case, the way parity is calculated is to add the values in each of the horizontal blocks.
That sum is stored as the parity value (3 + 1 + 2 + 3 = 9). In practice, parity is calculated by an
exclusive OR (XOR) process, but addition is fairly similar and works as well for the purposes
of this example. If you need to reconstruct data from a single failure, the process used to
generate parity is reversed. If the first disk fails, RAID 4 re-creates the data value 3 in the first
column. It subtracts the values on the remaining disks from what is stored in parity (9 - 3 - 2 -
1 = 3). This example of reconstruction with single-parity RAID shows why data is protected up
to, but not beyond, one disk failure event.
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Chapter 10. Data protection with RAID Double Parity