Specifications

4–17
Compaq HSG80 Array Controller ACS Version 8.4 Configuration and CLI Reference Guide
Figure 4–7. Parity Ensures Availability; Striping Provides Good Performance
Just as with stripesets, the I/O requests are broken into smaller “chunks” and striped
across the disk drives until the request is read or written. But, in addition to the I/O
data, chunks of parity data—derived mathematically from the I/O data—are also
striped across the disk drives. This parity data enables the controller to reconstruct the
I/O data if a disk drive fails. Thus, it becomes possible to lose a disk drive without
losing access to the data it contained. (Data could be lost if a second disk drive fails
before the controller replaces the first failed disk drive.)
For example, in a three-member RAIDset that contains disk drives 10000, 20000, and
30000, the first chunk of an I/O request is written to 10000, the second to 20000, then
parity is calculated and written to 30000; the third chunk is written to 30000, the
fourth to 10000, and so on until all of the data is saved.
CXO5509A
I/O Request
Chunk 1
2
4
3
D
i
s
k
1
0
0
0
0
Chunk 1
4
D
i
s
k
2
0
0
0
0
2
Parity
for
3 & 4
D
i
s
k
3
0
0
0
0
Parity
for
1 & 2
3