AMCC 3WARE 720-0138-00 - Manual (Page 73)

About Degraded Units
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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.
• RAID 1 and 10 units each use mirroring to achieve fault tolerance.
Identical data is stored on two or more drives to protect against drive
failure.
• RAID 5 units achieve fault tolerance by using a simple (exclusive OR)
function to generate the parity data that is distributed on all drives.
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 3DM pages. On the 3ware Sidecar,
the LED for failed drives turns 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” on page 73.
Rebuilding Units
About Inoperable Units
Units become inoperable when there are no longer enough drives in the unit
for it to function. For example, a RAID 5 created from 4 drives becomes
degraded if one drive fails or is removed, but becomes inoperable if two
drives fail or are removed.
Data on an inoperable unit cannot be accessed.
If you have data on a unit that is currently “inoperable,” contact technical
support.