HSG80 Array Controller ACS Version 8.2 User Guide (EK-HSG80-UG. B01, July 1998)

Creating Storagesets 3–15
Using RAIDsets to Increase Performance and Availability
RAIDsets are enhanced stripesets—they use striping to increase I/O
performance and distributed-parity data to ensure data availability.
Figure 3–7 illustrates the concept of RAIDsets and parity data.
Figure 3–7 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