RAID Terminology

 

RAID
Redundant Array of Independent Disks

RAID 0 (Striping)
Speed: Very High
Safety: Low
Minimum Number of Drives: 2

RAID 0 is a performance-oriented, non-redundant data mapping technique. Striping splits data evenly across two or more disks simultaneously, dramatically increasing performance. However, if any drive in the array is lost, all data will be lost.

The capacity of a RAID 0 array is equal to the sum of all drives. For example, a RAID 0 array consisting of two 500 GB drives, would store 1.0TB of data.

RAID 1 (Mirroring)
Speed: Moderate
Safety: High
Minimum number of drives: 2

RAID 1 consists of at least two drives storing duplicate copies of the same data. The data is simultaneously written to all drives. This does provide redundancy, but the performance is slower than most other RAID setups.

The capacity of a RAID 1 array is equal to half the size of the disks in the array. For Example, a RAID 1 array consisting of two 500 GB drives would store 500 GB of data.

RAID 5
Speed: High
Safety: High
Minimum number of drives: 3

A RAID 5 array consists of at least 3 drives. This type of array uses block level striping and parity data such that one drive in the array can fail, and the array can still be accessed with no data loss. However, a second drive failure results in the loss of all data. Speed is much better than a RAID 1 array.

The capacity of a RAID five array is equal to the sum of the disks in the array minus 1 disk. For example, a RAID 5 array consisting of four 500GB disks would store 1.5 TB of data.

RAID 5+Hot Spare
Speed: High
Safety: Very High
Minimum number of drives: 4

A RAID 5+Hotspare array consists of at least 4 drives. Three drives form a conventional RAID 5 array with the fourth being reserved for automatic rebuilding of the array in the event of a drive failure. This is done to mitigate the risk of a second drive failure before user intervention occurs, with an obvious impact on total storage capacity.

The capacity of a RAID 5+ Hotspare array is equal to the sum of three disks in the array minus 1 disk. The Hotspare drive is not used until one of the other drives fails. For example, a RAID 5+1 Hotspare array consisting of four 500GB disks would store 1.0 TB of data.

RAID 0+1
Speed: Very High
Safety: Very High
Minimum number of drives: 4

RAID 0+1 is also called “Hybrid” or “Nested” RAID. It is essentially at least two RAID 0 arrays, that are then mirrored into a RAID 1 array. Most of the time, it is possible to lose up to two drives in a RAID 0+1 array without data loss.

The capacity of a RAID 0+1 array is equal to half of the drives in the array. For Example, a RAID 0+1 array consisting of four 500GB drives would store up to 1.0TB of data.

JBOD (Just a Bunch Of Disks)
Speed: Moderate
Safety: Moderate
Just like it sounds, this mode allows the system to be configured as a multi-disk storage enclosure. The number of volumes and their sizes will be the same as the disks that are installed, but there is no redundancy, and no protection against data loss.