HP StorageWorks 1510i HP Storage Management Utility user guide (383075-002, Ju - Page 89

RAID 6—advanced data guarding, Comparison of RAID Methods

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Table 5 RAID 5 features Advantages Disadvantages High read performance. Relatively low write performance. No loss of data if one physical drive fails. Loss of data if a second drive fails before data from the first failed drive is rebuilt. More usable drive capacity than RAID 1+0, because parity information requires the storage space equivalent to one physical drive. RAID 6-advanced data guarding RAID 6 (also called 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 6, however, two different sets of parity data are distributed across the physical drives, allowing data to be preserved even 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 22. B1 B2 P1,2 Q1,2 B3 P3,4 Q3,4 B4 P5,6 Q5,6 B5 B6 Q7,8 B7 B8 P7,8 D1 D2 D3 D4 15317 Figure 22 RAID 6 (ADG) array, with four physical hard drives (D1, D2, D3, D4) showing distributed parity information (Px,y)(Qx,y) 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 6 (ADG) is less than when they are configured with RAID 5. Table 6 RAID 6 (ADG) features Advantages Disadvantages High read performance. Relatively low write performance (lower than RAID 5), because of the need to create two sets of parity data. High data availability-Any two drives can fail without loss of critical data. More drive capacity is usable than with RAID 1+0-Parity information requires only the storage equivalent to two physical drives. Comparison of RAID Methods Table 7 summarizes important features of the different RAID levels. HP Storage Management Utility user guide 89

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Table 5 RAID 5 features
Advantages
Disadvantages
High read performance.
Relatively low write performance.
No loss of data if one physical drive fails.
Loss of data if a second drive fails before data from
the
fi
rst failed drive is rebuilt.
More usable drive capacity than RAID 1+0, because
parity information requires the storage space
equivalent to one physical drive.
RAID 6—advanced data guarding
RAID 6 (also called 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 6, however, two different
sets of parity data are distributed across the physical drives, allowing data to be preserved even 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 22
.
B1
B3
P5,6
Q7,8
D1
D2
D3
D4
B2
B7
P3,4
Q5,6
B5
B8
P1,2
Q3,4
B4
B6
P7,8
Q1,2
15317
Figure 22 RAID 6 (ADG) array, with four physical hard drives (D1, D2, D3, D4) showing
distributed parity information (Px,y)(Qx,y)
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 con
fi
gured with RAID 6 (ADG) is less than when they
are con
fi
gured with RAID 5.
Table 6 RAID 6 (ADG) features
Advantages
Disadvantages
High read performance.
Relatively low write performance (lower than RAID 5),
because of the need to create two sets of parity data.
High data availability—Any two drives can fail
without loss of critical data.
More drive capacity is usable than with RAID
1+0—Parity information requires only the storage
equivalent to two physical drives.
Comparison of RAID Methods
Table 7
summarizes important features of the different RAID levels.
HP Storage Management Utility user guide
89