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RAID Level

RAID Level
RAID level is a number indicating the type of organisation of disks in the RAID array.

The different RAID levels are:

RAID 0 Striping. No redundancy. Fastest. Provides maximum storage. No fault tolerance. Requires 2 or more drives.
Storage = number of drives x drive capacity
RAID 1 Disk mirroring. Good performance. Fault tolerant. Only fault tolerant option if using 2 drives.
Storage = number of drives x drive capacity / 2
RAID 2 Error correction data written to separate disk.  
RAID 3 Striping (small stripe size) with one parity disk.  
RAID 4 Striping (large stripe size) with one parity disk.  
RAID 5 Striping with parity. Parity information is distributed across all drives. Good performance. Reads can be faster than Raid 1 or individual drives, but not as good as Raid 0. Writes are slower than Raid 1 because a write may require reads in order to correctly update the parity information.

Fault tolerant. Slowest to rebuild (if one disk is replaced). Better storage than RAID 1. Requires at least 3 disks. Tolerant of a single disk failure.

Storage = (number of drives -1) x drive capacity

In general the current recommendation is to use RAID 10 in preference to RAID 5.

RAID 6 RAID 5 with an extra parity disk. Tolerant of two disks failing.
RAID 0+1 Mirrored array (RAID 1) whose segments are RAID 0 arrays, i.e. mirroring of striped sets.  
RAID 10 Striped array (RAID 0) of mirrored sets (RAID 1).  
RAID 53 Combination of RAID 0 and RAID 3. Each stripped set (of RAID 0) are RAID 3 sets.  

RAID 1 (disk mirroring) and RAID 5 (striping with parity) are the most common forms of RAID.

For more information see: