Parity Calculations and RAID 5
RAID level 5 offers a fault tolerance of 1 drive failing from any RAID 5 array. The array controller calculates error correction data, known as parity by comparing the data on the other drives in the array.
The way in which parity is recorded differs depending upon the type of RAID array:
In RAID 3 & 4 - A dedicated parity disk is used for the storage of error correction data, this is an uncommon method of providing RAID redundancy, due to the fact that performance suffers as the entire array can only be as fast as the parity disk plus the overhead for the actual calculation performed by the controller. This can be seen below:
DISK 1 DISK 2 DISK 3
0 0 0
0 1 1
1 0 1
1 1 0
In RAID 5 - A more efficient process whereby the calculated parity is spread amongst the member drives of the array is used. For each striped block of data there is a block of parity data written to one of the drives. This block moves sequentially through the hard drives so that no single drive proves to be a bottleneck. This is shown below:
DISK 1 DISK 2 DISK 3
0 0 0
0 1 1
1 0 1
1 1 0
So What Is Parity?
Parity data is calculated using the XOR operation, also known as Exclusive OR (EOR) or Exclusive Disjunction. This simple mathematical principal has the following truth table:
1 2 3
0 0 0
0 1 1
1 0 1
1 1 0
The table above shows that when a single value in item 1 and item 2 is equal to 1 the resulting calculation (parity) is equal to one. When both values are equal to 1 or both are equal to zero the resultant parity calculation is equal to zero.
The way in which parity is recorded differs depending upon the type of RAID array:
In RAID 3 & 4 - A dedicated parity disk is used for the storage of error correction data, this is an uncommon method of providing RAID redundancy, due to the fact that performance suffers as the entire array can only be as fast as the parity disk plus the overhead for the actual calculation performed by the controller. This can be seen below:
DISK 1 DISK 2 DISK 3
0 0 0
0 1 1
1 0 1
1 1 0
In RAID 5 - A more efficient process whereby the calculated parity is spread amongst the member drives of the array is used. For each striped block of data there is a block of parity data written to one of the drives. This block moves sequentially through the hard drives so that no single drive proves to be a bottleneck. This is shown below:
DISK 1 DISK 2 DISK 3
0 0 0
0 1 1
1 0 1
1 1 0
So What Is Parity?
Parity data is calculated using the XOR operation, also known as Exclusive OR (EOR) or Exclusive Disjunction. This simple mathematical principal has the following truth table:
1 2 3
0 0 0
0 1 1
1 0 1
1 1 0
The table above shows that when a single value in item 1 and item 2 is equal to 1 the resulting calculation (parity) is equal to one. When both values are equal to 1 or both are equal to zero the resultant parity calculation is equal to zero.