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Maintaining Units > About Degraded Units
About Degraded Units
Fault-tolerant RAID units provide data redundancy by duplicating information on multiple drives. These RAID units make it possible to continue use even if one of the drives in the unit has failed.
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RAID 1 and RAID 10 units each use mirroring to achieve fault tolerance. Identical data is stored on two or more drives to protect against drive failure.
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RAID 5, RAID 6, and RAID 50 units achieve fault tolerance by using a simple (exclusive OR) function to generate the parity data that is distributed on all drives.
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RAID 6 adds an extra level of protection over RAID 5 by generating a second parity when data is written. This allows two drives to fail without compromising data integrity, especially on larger units.
When one of the drives in a fault-tolerant unit fails or is removed or unplugged, the unit is said to be degraded.
You can still read and write data from a degraded unit, but the unit will not be fault tolerant until it is rebuilt using the Rebuild feature.
When a RAID unit becomes degraded, it is marked as such, and the drive(s) that failed are marked as Degraded in the 3DM2 pages. If supported by your enclosure, the LED for failed drives may turn red.
You should replace the failed drive and rebuild the unit as soon as it is convenient to do so. The unit will not be fault tolerant until it has been rebuilt. Rebuilding can occur automatically, depending on your settings. For more information, see Rebuilding Units.
Related Topics
Checking Unit and Drive Status
Enclosure Drive LED Status Indicators
Unit Statuses
Drive Statuses
Viewing Alarms, Errors, and Other Events
Downloading an Error Log
Viewing SMART Data About a Drive
About Inoperable Units
Rebuilding Units
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