IBM ServeRAID-4 Ultra160 Series User Reference page 38

Create an array using three of the
physical drives, leaving the fourth as a
hot-spare drive.
Then, create a logical drive within that
array.
The data is striped across the drives,
creating blocks.
Notice that the storage of the data
parity (denoted by
and it shifts from drive to drive.
A parity block (
representation of the data from the
other blocks in the same stripe. For
example, the parity block in the first
stripe contains data representation of
blocks 1 and 2.
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.
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IBM User's Reference: ServeRAID
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) also is striped,
.
) contains a
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-4 Ultra160 SCSI Controller
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