Disk duplexing is similar to RAID 1 except the two disks are attached to different disk controllers. The advantage here is a further gain of fault tolerance. The system or data can now withstand the loss of a disk controller and a disk. In some circumstances a reboot of the system is required, to switch to the other disk controller and to bring the system back online.

5.2.5.3 Enhanced RAID 1

Enhanced RAID 1 is a combination of RAID 0 and RAID 1. To put it simply, it is the "mirroring" of a RAID 0 array, within a single array (shown in Figure 13):

RAID-1

 

 

 

 

Enhanced

data1

data2

data3

data4

Fault Tolerant

data4

data1

data2

data3

 

 

 

 

 

5842\RAID1

Figure 13. Example of Enhanced RAID 1

This combination of RAID 0 and RAID 1 has advantages of increased performance when reading data but has an overhead of writing data twice on all writes. However, this can be used to an advantage, by placing the Enhanced RAID 1 in a mostly read environment.

Loss of a single disk (as in RAID 1) does not create data loss. In most circumstances this will provide very good data protection, while providing enhanced performance.

5.2.5.4 RAID Five (RAID 5)

RAID 5 is the striping of data and parity (or checksum) across all disks (shown in Figure 14):

RAID-5

Fault Tolerant

CS=6

data1

data2

data3

 

 

data4

CS=19

data5

data6

 

data7

data8

CS=32

data9

 

data10

data11

data12

CS=42

Based upon the single equation single unknown technique.

For example, data1 + data2 + data3 = checksum

5842\RAID5

Figure 14. Example of RAID 5

Data is striped with a parity or checksum on each write of data. The total amount of data that can be stored in a RAID 5 array is N-1, where N is the number of disks within the single array. For example, 5 x 9.1 GB in an array will give 4 x 9.1 GB of data storage capacity. If a disk should fail, the system will keep providing data to end users, creating read data by using the checksum as required. There

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IBM B73.3 manual Enhanced RAID, RAID Five RAID