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Appendix B: Understanding RAID
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RAID 5 Arrays
A RAID 5 array is built from a minimum of three and a maximum of
sixteen disk drives, and uses data striping and parity (see below) 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 example below) 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 below.
In the example above, P represents the distributed parity data.
Disk Drive 1
Disk Drive 2
Disk Drive 3
Disk Drive 4
250 GB
250 GB
400 GB
400 GB
Disk Drives in Logical Drive
Disk Drive 1
Disk Drive 2
Disk Drive 3
Disk Drive 4
RAID 5 Logical Drive = 750 GB plus Parity
1
2
3
P
Not Used
Not Used
4
5
P
6
750
749
748
P
...
...
...
...
Unused Space: 150 GB
Unused Space: 150 GB
Drive Segment Size (Smallest Disk Drive)