Standard RAID Levels
RAID 0: Striping: Data is stripped across multiple drives to increase performance and capacity. No protection is present, so data is lost in the event of a drive failure.
RAID 1: Mirroring: The same data is written to two drives in parallel. In this scenario, if one drive fails, data can still be accessed, but has the drawback of reducing the capacity in half.
RAID 1E: Mirroring: This is the same as RAID 1, but can be implemented with more than two drives.
RAID 5: Parity: Combines three or more disks in such a way that if one fails, data can still be accessed via the operational drives, although any further failures would result in data loss. Capacity is reduced by one drive.
RAID 6: Enhanced parity: Protects against two drives failing rather than just one but capacity is reduced by two drives.
Nested RAID
To gain performance and/or additional redundancy standard, RAID levels can be combined to create hybrid or “nested” RAID levels. The most common are:
RAID 10 (1+0): Stripe of mirrors: Performance of striping with the reliability of mirrored drives. Minimum of four drives and useable capacity is half of the number of drives.
RAID 50 (5+0) or 60 (6+0): Stripe of RAID five or six parity protected drive sets. Useable capacity reduced by the parity drives for each striped set.
RAID 51 (5+1) or 61 (6+1): Mirror of RAID five or six parity protected drive sets. Highest fault tolerance, but with lower capacity and expensive to implement.