Raid 1; Raid 0+1; Figure 2-5. Diagram Of A Raid 1 Write - Compaq StorageWorks TM RAID Array 3000 Pedestal Storage Subsystem Hardware User's Manual

RAID 1

RAID 1 (also known as mirroring or shadowing) takes data sent by the host
and duplicates it on all the drives in an array. The result is a high degree of
data availability, since you can lose all but one drive in the array and still have
full access to your data. This high degree of data availability comes at a price:
a RAID 1 array requires multiple drives to achieve the storage capacity of a
single drive. Figure 2–5 shows a RAID 1 write.

Figure 2-5. Diagram of a RAID 1 write

A RAID 1 array will show up on the monitor as degraded when at least one
drive fails, even if two or more members of the redundancy group remain in
good working order. As long as at least two working drives remain in the
array, you may continue to run the array in degraded mode without putting
data in jeopardy.

RAID 0+1

RAID 0+1 combines RAID 0 (striping) with RAID 1 (mirroring). In RAID
0+1 write, the controller breaks up the data block from the host into smaller
chunks, then writes the chunks to half the drives in the array, while writing
duplicate chunks to the remaining drives.
RAID Array Controller 2-15