IBM Netfinity ServeRAID-4H Ultra160 User Reference page 21

If a physical drive fails in the array,
the ServeRAID controller switches
read and write requests to the
remaining functional drive in the RAID
level-5 array, which is a hot-spare
drive.
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, 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-5E logical drive.
Start with four physical drives.
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Chapter 1. Getting started
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