Parity Block

Part 1:

Suppose we are given 3 data disks:

disk 1: 1111 0000
disk 2: 1010 1010
disk 3: 0011 1000

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To find compute the redundant disk, we perform mod-2 sum:

disk 4: 0110 0010

 

Part 2:

Suppose we updated the value for disk 2:

disk 2 old value: 1010 1010
disk 2 new value: 1100 1100

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Mod-2 Sum: 0110 0110
*1’s are the position to change (2,3,6,7)

disk 4 original: 0110 0010
disk 4 updated: 0000 0100

Result:
disk 1: 1111 0000
disk 2: 1100 1100
disk 3: 0011 1000
disk 4: 0000 0100

 

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