HP A7143A RAID160 SA Controller Support Guide, February 2007

RAID Technology Overview

Smart Array Controller Supported RAID Configurations

Chapter 1

RAID ADG

RAID Advanced Data Guarding (ADG) is similar to RAID 5 in that parity data is generated and stored to

protect against data loss caused by physical disk failure. However, with RAID ADG two different sets of

parity data are generated for each data block on a stripe. The two parity data blocks are stored on different

physical disks, allowing data to be preserved even if two physical disks fail simultaneously. Figure 1-8

illustrates the two sets of parity data that require as much storage capacity as the data blocks they

correspond to on each stripe in a logical drive.

RAID ADG is most useful when data loss is unacceptable but cost must also be minimized. The probability

that data loss will occur when arrays are configured with RAID ADG is less than when they are configured

with RAID 5. For more information, see Appendix A, “Probability of Logical Drive Failure,” on page 93.

Figure 1-8 Advanced Data Guarding, Showing Parity Information (Px,y and Qx,y)

The advantages of RAID ADG are as follows:

• High read performance.

• High data availability—any two disks can fail without loss of critical data.

• More disk drive capacity usable than with RAID 1+0; parity information requires only the storage space

equivalent to two physical disks.

The only significant disadvantage of RAID ADG is a relatively low write performance (lower than RAID 5),

due to the need for two sets of parity data.

P5,6

Q7,8

P3,4

Q5,6

P1,2

Q3,4

P7,8

Q1,2