IBM ServeRAID-4 Ultra160 Series User Reference page 35

When you replace a failed drive, the ServeRAID controller can rebuild all the RAID
level-1E and RAID level-5E logical drives automatically onto the replacement
physical drive. However, any data stored in a failed RAID level-0 logical drive is
lost.
Although the risk of data loss is present, you might want to assign RAID level-0 to
one of the logical drives to take advantage of the speed this RAID level offers. You
can use this logical drive to store data that you back up each day and for which
safety is not of primary importance, that is, data that you can re-create easily. You
also might want to use a RAID level-0 logical drive when you require maximum
capacity.
Understanding RAID level-1
RAID level-1 provides 100 percent data redundancy and requires only two physical
drives. With RAID level-1, the first half of a stripe is the original data; the second
half of a stripe is a mirror (that is, a copy) of the data, but written to the other drive
in the RAID level-1 array.
Because the data is mirrored, the capacity of the logical drive when assigned RAID
level-1 is 50 percent of the array capacity.
The following illustration shows an example of a RAID level-1 logical drive.
Start with two physical drives.
Create an array using the two physical drives.
Then, create a logical drive within that array.
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Chapter 3. Understanding RAID technology