User's Manual

Drive Arrays and Fault Tolerance

HP Smart Array 641/642 Controller User Guide D-9

HP CONFIDENTIAL

Writer: John Turner File Name: l-appd drive arrays & fault tolerance

Codename: darkmatter -antimatter Part Number: 309311-001 Last Saved On: 11/6/02 4:34 PM

RAID ADG—Advanced Data Guarding

NOTE: Not all controllers support RAID ADG.

RAID ADG is similar to RAID 5, because both methods generate and store parity

information to protect against data loss caused by drive failure. With RAID ADG,

however, two different sets of parity data are used, allowing data to still be preserved

if two drives fail. Each set of parity data uses up a capacity equivalent to that of one

of the constituent drives, as shown in Figure D-8.

P5,6

Q7,8

P3,4

Q5,6

P1,2

Q3,4

P7,8

Q1,2

Figure D-8: Advanced data guarding (RAID ADG)

This method is most useful when data loss is unacceptable, but cost is also an

important factor. 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 details,

refer to Appendix F).

Advantages

• High read performance

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

• More drive capacity usable than with RAID 1+0—parity information requires

only the storage space equivalent to two physical drives