Introduction to RAID

Terminology

Raid: Redundant array of independent disks.
Redundancy: An array to survive a disk failure.
Mirroring: Two types of redundancy are applied to arrays. Mirroring replicates data onto every disk in the array. Read speeds are increased in a mirror array environment. The draw back is the write speed, all data has to be written twice or how ever many mirror disks you may have.
Parity: The second method of redundancy is parity method. Parity data is recovery data that is written to a separate drive called the parity drive.
Degraded: An array hat supports redundancy but has one or more failed disks.
Reconstruction, resynchronization, recovery and rebuild: All these terms mean that there is a hard drive failure and recovery process has been initiated.
Striping: This is a process by which data is spread across all the disks in the array. A fixed amount of data is written to each disk.

Raid 0: this is called stripping because data is interleaved across all drives in the array. Technically speaking, this is not a RAID at all and provides no data redundancy.
Raid 1: Mirroring stores an exact replica of all data on a separate disk or disks. Raid-1 provides complete data redundancy, however write performance is reduced because same data has to be written on multiple disks.
Raid 2: RAID 2 is the black sheep of the RAID family in that it doesn't use one or more of the standard striping, mirroring, or parity techniques. It does however, use something similar to striping with parity, which we'll read when we cover RAID level 3. Because of its high cost and complexity, RAID 2 never really caught on. In fact, it isn't even used commercially today. RAID 2 uses byte level striping with a form of error correcting code (ECC) known as Hamming code. The number of hard drives required for a RAID 2 configuration may vary, but a typical setup may use as many as 14 disk drives: 10 data disks and 4 ECC disks.
Raid 3: Striped set with dedicated parity/Bit interleaved parity.
Raid 4: This works the same way at stripping but with a dedicated parity drive.
Raid 5: This works the same as Raid-4 but the parity information is also stripped across all the drives in the array.
Raid 6: Striped set with dual distributed Parity.

Raid 0+1: RAID 0+1: striped sets in a mirrored set (minimum four disks; even number of disks)
Raid 1+0: mirrored sets in a striped set (minimum four disks; even number of disks)
JBOD: Just a bunch of disks is not really a raid.