Appendix B: Selecting the Best RAID Level
●
155
RAID 1 Logical Drives
A RAID 1 logical drive 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
logical drives 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 logical drive is built from different-sized disk drives, drive segment size is the size
of the smaller disk drive, as shown in this figure.
RAID 1 Enhanced Logical Drives
A RAID 1 Enhanced (RAID 1E) logical drive—also referred to as a
striped mirror
—is similar to
a RAID 1 logical drive except that data is both mirrored
and
striped, and more disk drives can
be included. A RAID 1E logical drive can be built from three or more disk drives.
In this figure, 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 GB
Disk Drive 3
Disk Drive 2
Disk Drive 1
Disk Drive 3
1 3
2
3
4
5
6
1
2
6
4
5
