## Understanding RAID level-5 Enhanced

RAID level-5 Enhanced (RAID level-5E) requires a minimum of four physical drives. RAID level-5 is also firmware-specific. You can think of RAID level-5E as "RAID level-5 with a built-in spare drive."

Reading from and writing to four disk drives is more efficient than three disk drives and an idle hot spare and therefore improves performance. Additionally, 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.

Like RAID level-5, this RAID level stripes data and parity across all of the drives in the array. 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 (that is, one for parity and one for the spare).

RAID level-5E offers both data protection and increased throughput, in addition to the built-in spare drive.

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.

RAID level-5E requires a minimum of 4 drives and, depending upon the level of firmware and the stripe-unit size, supports a maximum of 8 or 16 drives.

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 compressed into the distributed spare drive. 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. |

Note: The ServeRAID Manager program Express configuration 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.

See also

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