Troubleshooting 31
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When RAID 1+0 is used, drives are mirrored in pairs. Several drives can be in a failed condition
simultaneously (and they can all be replaced simultaneously) without data loss, if no two failed
drives belong to the same mirrored pair.
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When RAID 6 with ADG is used, two drives can fail simultaneously (and be replaced
simultaneously) without data loss.
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If the offline drive is a spare, the degraded drive can be replaced.
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Do not remove a second drive from an array until the first failed or missing drive has been replaced
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the rebuild process is complete. (The rebuild is complete when the online LED on the front of the
drive stops blinking.)
Exceptions:
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In RAID 1+0 configurations, any drives that are not mirrored to other removed or failed drives
can be simultaneously replaced offline without data loss.
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In RAID 6 with ADG configurations, any two drives in the array can be replaced simultaneously.
Automatic data recovery (rebuild)
When you replace a hard drive in an array, the controller uses the fault-tolerance information on the
remaining drives in the array to reconstruct the missing data (the data that was originally on the replaced
drive) and write it to the replacement drive. This process is called automatic data recovery, or rebuild. If
fault tolerance is compromised, this data cannot be reconstructed and is likely to be permanently lost.
If another drive in the array fails while fault tolerance is unavailable during rebuild, a fatal system error
may occur, and all data on the array is then lost. In exceptional cases, however, failure of another drive
need not lead to a fatal system error. These exceptions include:
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Failure after activation of a spare drive
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Failure of a drive that is not mirrored to any other failed drives (in a RAID 1+0 configuration)
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Failure of a second drive in a RAID 6 with ADG configuration
Time required for a rebuild
The time required for a rebuild varies considerably, depending on several factors:
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The priority that the rebuild is given over normal I/O operations (you can change the priority setting
by using ACU)
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The amount of I/O activity during the rebuild operation
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The rotational speed of the hard drives
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The availability of drive cache
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The brand, model, and age of the drives
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The amount of unused capacity on the drives
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The number of drives in the array (for RAID 5 and RAID 6 with ADG)
Allow approximately 1 minute per gigabyte for the rebuild process to be completed.
System performance is affected during the rebuild, and the system is unprotected against further drive
failure until the rebuild has finished. Therefore, replace drives during periods of low activity when
possible.