Compaq 3000 manual RAID 0+1, Diagram of a RAID 1 Write

Chapter 2. RAID Array Controller

2.6.2 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 comes at a price: a RAID 1 array requires multiple drives to achieve the storage capacity of a single drive. Figure 2–6 illustrates a RAID 1 write.

Figure 2–6 Diagram of a RAID 1 Write

Host Data
0010	Controller writes
	data from cache
1011
	to all drives in
0110
	the array

SHR-1055

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.

2.6.3 RAID 0+1

RAID 0+1 (see Figure 2–7) combines RAID 0 (striping) with RAID 1 (mirror- ing). 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.

EK–SMCPQ–UG. C01 2–11