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Appendix B: Understanding RAID ● 71
RAID 1 Arrays
A RAID 1 array is built from two disk drives, where one disk drive is a mirror of the other (the
same data is stored on each disk drive). Compared to independent disk drives, RAID 1 arrays
provide improved performance, with twice the read rate and an equal write rate of single disks.
However, capacity is only 50 percent of independent disk drives.
If the RAID 1 array is built from different- sized disk drives, the free space, drive segment size is
the size of the smaller disk drive, as shown in this figure.
RAID 1 Enhanced Arrays
A RAID 1 Enhanced (RAID 1E) array—also known as a striped mirror—is similar to a RAID 1
array except that data is both mirrored and striped, and more disk drives can be included. A
RAID 1E array can be built from three or more disk drives.
In this example, the large bold numbers represent the striped data, and the smaller, non-bold
numbers represent the mirrored data stripes.
Disk Drive 2
Disk Drive 1 250 GB
400 GB
Drive Segment Size
(Smallest Disk Drive)
Disk Drive 1
Disk Drive 2
Disk Drives in Logical Drive
RAID 1 Logical Drive = 250 GB
1 – 250
1 – 250
Unused Space = 150 GB
Disk Drive 2
Disk Drive 1
400 GB
Disk Drives in Logical Drive RAID 1E Logical Drive = 600 GB
400 GB
400 GBDisk Drive 3
Disk Drive 2
Disk Drive 1
Disk Drive 3
1 3
2
3
4
5
6
1
2
6
4
5