IBM ServeRAID User Reference page 41

Then create a logical drive within that array.
The data is striped across the drives, creating blocks.
Notice that the data on the drive on the right is a copy
of the data on the drive on the left.
With RAID level-1, if one of the physical drives fails, the ServeRAID controller
switches read and write requests to the remaining functional drive in the RAID
level-1 array.
RAID level-1 offers the following advantages and disadvantages.
Understanding RAID level-1 Enhanced
RAID level-1 Enhanced (RAID level-1E) combines mirroring and data striping. This
RAID level stripes data and copies of the data across all of the drives in the array.
As with the standard RAID level-1, the data is mirrored, and the capacity of the
logical drive is 50% of the array capacity.
RAID level-1E has a similar profile to RAID level-1; it provides data redundancy and
high levels of performance, but the storage capacity is diminished. However, RAID
level-1E allows a larger number of physical drives to be used.
RAID level-1E requires a minimum of three drives and, depending upon the level of
firmware and the stripe-unit size, supports a maximum of 8 or 16 drives.
The following illustration is an example of a RAID level-1E logical drive.
Start with three physical drives.
Advantages
v 100% data redundancy
v High performance
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Disadvantages
Allows only 50% of the physical drive
storage capacity to be used
Chapter 3. Understanding RAID technology
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