We use XOR operation to calculate the Parity bit:

For the first column of the disks, to the left we have 1, 1 and 0:

1XOR 1 XOR 0 = Parity bit

Broken down: 1 XOR 1 = 0 and then 0 XOR 0 = The parity bit is 0.

This means first 1 XOR 1

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If any of Disk number 1, 2 or 3 break the parity information on Disk 4 is to be used to recreate the missing data. If we assume that disk number 2 unexpectedly goes down we have lost all read and write access to the real Disk 2, however with the help of the already recorded parity we calculate the information which is missing.

The primary feature in a RAID 5 disk set is to be able to access the data on a missing disk. This is done by using the exact same XOR operation over the remaining disks and the parity information. Let us look at the first column again. 1 XOR 0 = 1 (for disk 1 and disk 3) and then 1 XOR 0 (the parity) = 1. This means that there must have been a binary digit of 1 on the missing disk. If we do the same operation on the other columns we will end up with 1100, which is exactly the same data that was on the failed