Appendix B: Understanding RAID ● 99
RAID 5 Arrays
A RAID 5 array is built from a minimum of three disk drives, and uses data striping and parity
data to provide redundancy. Parity data provides data protection, and striping improves
performance.
Parity data is an error-correcting redundancy that’s used to re-create data if a disk drive fails.
In RAID 5 arrays, parity data (represented by Ps in the next figure) is striped evenly across the
disk drives with the stored data.
Drive segment size is limited to the size of the smallest disk drive in the array. For instance, an
array with two 250 GB disk drives and two 400 GB disk drives can contain 750 GB of stored
data and 250 GB of parity data, as shown in this figure.
Disk Drive 1
Disk Drive 2
Disk Drive 3
Disk Drive 4
250 GB
250 GB
400 GB
400 GB
Drive Segment Size
(Smallest Disk Drive)
Disk Drive 2
Disk Drive 3
Disk Drive 4
Disk Drive 1
1 4 ... P
2 5 ... 748
3 P ... 749
P 6 ... 750
Unused Space = 150 GB
Disk Drives in Logical Drive RAID 5 Logical Drive = 750 GB plus Parity
Unused Space = 150 GB
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