Fault Tolerance of Logical Drive(s)

For links and interfaces common to all Express Configuration links, please see Express Overview in ACU help.

In the following screens, you will be asked a few simple questions that will allow you to set the RAID type for the logical drives on your controller. You will have a chance at the end to review your choices and make changes before they are saved.

On this screen you will be asked what fault tolerance you would like. You have various RAID options:

RAID 0 - No Fault Tolerance
This radio button offers the greatest capacity and performance without data protection. RAID 0 provides data striping but no fault tolerance. If you select this option for any of your logical drives, you will experience data loss for that logical drive if one physical drive fails.

However, because no logical drive capacity is used for redundant data, this method offers the best processing speed and capacity. You may consider assigning RAID 0 to drives that require large capacity and high speed, but pose no safety risk.

RAID 1+0 - Drive Mirroring
This radio button offers the best combination of data protection and performance. RAID 1+0 (drive mirroring) creates fault tolerance by storing duplicate sets of data on a pair of disk drives. There must be an even number of drives for RAID 1+0. This is the most costly fault tolerance method because it requires 50 percent of the drive capacity to store the redundant data. RAID 1+0 first stripes your data across half of the disks, then mirrors this data to the other half.

If a physical drive fails, the mirror drive provides a backup copy of the files and normal system operations are not interrupted. The mirroring feature requires a minimum of two drives and, in a multiple drive configuration (four or more drives), mirroring can withstand multiple simultaneous drive failures as long as the failed drives are not mirrored to each other.

RAID 5 - Distributed Data Guarding
This radio button offers the best combination of data protection and usable capacity. RAID 5 stores parity data across all the physical drives in the array and allows more simultaneous read operations and higher performance than data guarding (RAID 4). If a drive fails, the controller uses the parity data and the data on the remaining drives to reconstruct data from the failed drive. The system continues operating with a slightly reduced performance until you replace the failed drive.

RAID 5 requires an array with a minimum of 3 physical drives. The capacity of the logical drive used for fault tolerance depends on the number of physical drives in the array. For example, in an array containing 3 physical drives, 33 percent of the total logical drive storage capacity is used for parity data; a 14-drive configuration uses only 7 percent.

RAID ADG - Advanced Data Guarding
This radio button will only be available if the controller has an enabler. RAID ADG is a fault-tolerance method that provides the highest level of data protection. It is similar to RAID 5 in that parity data is distributed across all drives in the array, except that multiple separate sets of parity data are used in RAID ADG, and the capacity of multiple drives is used to store the parity data. Simultaneous failure of several drives is thereby tolerated in RAID ADG, whereas RAID 4 and RAID 5 can only sustain failure of a single drive. The fault-tolerance of RAID ADG configurations is actually higher than that of RAID 1+0 configurations, since in RAID 1+0 configurations there is a chance that two drives mirrored to each other will fail simultaneously.

RAID ADG read performance is similar to that of RAID 5, since all drives can service read operations, but the write performance is lower than that of RAID 5 because the parity data must be updated on multiple drives. Performance is reduced further in a degraded state.

RAID ADG requires an array with a minimum of 2+P physical drives, where P is the number of drives used to store parity data; normally, P= 2. The percentage of the total drive capacity used for fault tolerance is equal to the number of drives used for parity data divided by the total number of physical drives. For example, in an array of five physical drives that has two parity drives, 40 percent of the total logical drive storage capacity is used for fault tolerance. A 14-drive configuration that also has two parity drives uses only 14 percent of storage capacity for fault tolerance.

Note: Some controllers may not support this option. In this case, the Advanced Data Guarding (RAID ADG) option will not be available on this screen.

RAID 4 - Data Guarding
RAID 4 is a method that assures data reliability while using only a small percent of the logical drive storage capacity. A designated, single physical drive contains parity data. If a drive fails, the controller uses the data on the parity drive and the data on the remaining drives to reconstruct data from the failed drive. This allows the system to continue operating with slightly reduced performance until you replace the drive.

RAID 4 requires a minimum of 3 physical drives (2 data drives and 1 parity drive) in an array. The capacity of the logical drive used for fault tolerance depends on the number of physical drives in the array. For example, in an array containing 3 physical drives, only 33 percent of the total logical drive storage capacity is used for fault tolerance; while a 14-drive configuration uses only 7 percent.

Some new controllers or firmware versions may no longer support this option. In this case, the Data Guarding (RAID 4) option will not be available.