What do you mean by Parity ?
a different way of protecting data is provided as an alternate to mirroring. It involves the use of parity information, which is redundancy information calculated from the actual data values.
The principle behind parity is simple: take "N" pieces of data, and from them, compute an extra piece of data. Take the "N+1" pieces of data and store them on "N+1" drives. If you lose any one of the "N+1" pieces of data, you can recreate it from the "N" that remain, regardless of which piece is lost. Parity protection is used with striping, and the "N" pieces of data are typically the blocks or bytes distributed across the drives in the array. The parity information can either be stored on a separate, dedicated drive, or be mixed with the data across all the drives in the array.
The parity calculation is typically performed using a logical operation called "exclusive OR" or "XOR".
Let's take an example to show how this works; you can do this yourself easily on a sheet of paper. Suppose we have the following four bytes of data: D1=10100101, D2=11110000, D3=00111100, and D4=10111001. We can "XOR" them together as follows, one step at a time:
D1 XOR D2 XOR D3 XOR D4
= ( (D1 XOR D2) XOR D3) XOR D4
= ( (10100101 XOR 11110000) XOR 00111100) XOR 10111001
= (01010101.XOR 00111100) XOR 10111001
= 01101001 XOR 10111001
= 11010000
So "11010000" becomes the parity byte, DP. Now let's say we store these five values on five hard disks, and hard disk #3, containing value "00111100", goes el-muncho. We can retrieve the missing byte simply by XOR'ing together the other three original data pieces, and the parity byte we calculated earlier, as so:
D1 XOR D2 XOR D4 XOR DP
= ( (D1 XOR D2) XOR D4) XOR DP
= ( (10100101 XOR 11110000) XOR 10111001) XOR 11010000
= (01010101 XOR 10111001) XOR 11010000
= 11101100 XOR 11010000
= 00111100
Which is D3, the missing value. Pretty neat, huh? :^) This operation can be done on any number of bits, incidentally; I just used eight bits for simplicity. It's also a very simple binary calculation--which is a good thing, because it has to be done for every bit stored in a parity-enabled RAID array.
Advantages
The most obvious advantage is that parity protects data against any single drive in the array failing without requiring the 50% "waste" of mirroring; only one of the "N+1" drives contains redundancy information. (The overhead of parity is equal to (100/N)% where N is the total number of drives in the array.) Striping with parity also allows you to take advantage of the performance advantages of striping.
Disadvantages:-
The chief disadvantages of striping with parity relate to complexity: all those parity bytes have to be computed--millions of them per second!--and that takes computing power. This means a hardware controller that performs these calculations is required for high performance--if you do software RAID with striping and parity the system CPU will be dragged down doing all these computations. Also, while you can recover from a lost drive under parity, the missing data all has to be rebuilt, which has its own complications; recovering from a lost mirrored drive is comparatively simple.
see:- http://aarklonlinuxinfo.blogspot.com/2011/08/raid-example.html
