# Understanding RAID level-5 Enhanced

Note: This feature is not supported on the ServeRAID-5i controller.

RAID level-5E is the same as RAID level-5 with a built-in spare drive. Like RAID level-5, this RAID level stripes data and parity across all of the drives in the array.

RAID level-5E offers both data protection and increased throughput. When an array is assigned RAID level-5E, the capacity of the logical drive is reduced by the capacity of two physical drives in the array (one for parity and one for the spare).

Reading from and writing to four physical drives is more efficient than reading from and writing to three physical drives and an idle hot spare. Therefore, RAID level-5E provides a higher level of performance than RAID level-5.

The spare drive is actually part of the RAID level-5E array, as shown in the following example. With such a configuration, you cannot share the spare drive with other arrays. If you want a spare drive for any other array, you must have another spare drive for those arrays.

RAID level-5E requires a minimum of four drives and, depending upon the level of firmware and the stripe-unit size, supports a maximum of 8 or 16 drives. RAID level-5E is also firmware-specific.

Note: For RAID level-5E, you can have only one logical drive in an array. When using RAID level-5E, you can have a maximum of seven logical drives on the controller.

The following illustration is an example of a RAID level-5E logical drive.

## RAID level-5 Enhanced example

Start with four physical drives. |
Create an array using all four physical drives. |
Then create a logical drive (labeled as 1) within the array. Notice that the distributed spare drive is the free space (labeled as 2) shown below the logical drive. |
The data is striped across the drives, creating blocks in the logical drive. The storage of the data parity (denoted by *) is striped, and it shifts from drive to drive as it does in RAID level-5. Notice that the spare drive is not striped. |
If a physical drive fails in the array, the data from the failed drive is reconstructed. The array undergoes compression, and the distributed spare drive becomes part of the array. The logical drive remains RAID level-5E. When you replace the failed drive, the data for the logical drive decompresses and returns to the original striping scheme. |

If you use a RAID level-5E logical drive in a failover or cluster configuration, the RAID level-5E logical drive will not failover while undergoing compression or decompression.

RAID level-5E offers the following advantages and disadvantages.

Advantages | Disadvantages
--- | ---
100% data protection Offers more physical drive storage capacity than RAID level-1 or level-1E Higher performance than RAID level-5 | Lower performance than RAID level-1 and level-1E Supports only one logical drive per array Cannot share a hot-spare drive with other arrays Not supported on the ServeRAID-5i controller

Note: The Configuration wizard does not default to RAID level-5E. If you have four physical drives, Express configuration defaults to RAID level-5 with a hot-spare drive.

## More information

- Understanding RAID technology
- Understanding stripe-unit size
- Selecting a RAID level
- RAID level-0
- RAID level-1
- RAID level-1 Enhanced
- RAID level-5
- RAID level-x0

Glossary | Contents

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