Understanding Raid Level-5 Enhanced - IBM Netfinity ServeRAID-3H Installation And User Manual

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 (also called 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 is a very desirable choice, because it 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 four drives and, depending upon the level of firmware and the
stripe-unit size, supports a maximum of eight or 16 drives.
The following illustration is an example of a RAID level-5E logical drive.
You start with four physical drives.
Create an array using all four physical drives.
*
3
x 5
*
x
2 1
*
4
6
*
8 7
9
Chapter 1. Getting started