IBM 86655RY - Netfinity 7600 - 8665 Hardware Maintenance Manual page 209

Understanding RAID level-5 Enhanced: RAID level-5 Enhanced (RAID level-5E)
requires a minimum of four physical drives. RAID level-5E is also firmware-specific.
You can think of RAID level-5E as "RAID level-5 with a built-in spare drive."
Reading from and writing to four disk drives is more efficient than three disk drives
and an idle hot spare and therefore improves performance. Additionally, the spare
drive is actually part of the RAID level-5E array, as shown in the following example.
With such a configuration, you cannot share the spare drive with other arrays. If you
want a spare drive for any other array, you must have another spare drive for those
arrays.
Like RAID level-5, this RAID level stripes data and parity across all of the drives in
the array. When an array is assigned RAID level-5E, the capacity of the logical drive is
reduced by the capacity of two physical drives in the array (that is, one for parity and
one for the spare).
RAID level-5E offers both data protection and increased throughput, in addition to
the built-in spare drive.
Note: For RAID level-5E, you can have only one logical drive in an array. When
using RAID level-5E, you can have a maximum of seven logical drives on the
controller.
RAID level-5E requires a minimum of 4 drives and supports a maximum of 16 drives.
The following illustration is an example of a RAID level-5E logical drive.
Start with four physical drives.
Create an array using all four physical drives.
Then, create a logical drive (labeled as 1) within the array.
Notice that the distributed spare drive is the free space (labeled as 2) shown below
the logical drive.
Installing and configuring ServeRAID controllers
199