HP A7143A RAID160 SA Controller Support Guide, February 2007

Hard Drive Installation and Replacement
Drive Failure During Rebuild
Appendix C
107
Drive Failure During Rebuild
If another drive in the array fails while fault tolerance is unavailable during rebuild, a fatal system error may
occur. If this happens, all data on the array is lost. In exceptional cases, however, failure of another drive need
not lead to a fatal system error. These exceptions include:
• Failure after activation of a spare drive
• Failure of a drive that is not mirrored to any other failed drives (in a RAID 1+0 configuration)
• Failure of a second drive in a RAID ADG configuration
Minimizing Fatal System Errors During Rebuild
When a hard drive is replaced, the controller gathers fault-tolerance data from the remaining drives in the
array. This data is then used to rebuild the missing data (originally on the failed drive) onto the replacement
drive. If more than one drive is removed at a time, the fault-tolerance data is incomplete. The missing data
cannot then be reconstructed and is likely to be permanently lost.
To minimize the likelihood of fatal system errors, take these precautions when removing failed drives:
• Do not remove a degraded drive if any other member of the array is offline (the Online LED is off). In this
condition, no other drive in the array can be removed without data loss.
• There are some exceptions:
• 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, as long as no two
failed drives belong to the same mirrored pair.
• When RAID ADG is used, two drives can fail simultaneously (and be replaced simultaneously)
without data loss.
• If an online spare has an unlit Online LED (it is offline), the degraded drive can still be replaced.
• Do not remove a second drive from an array until the first failed or missing drive has been replaced and
the rebuild process is complete. (When the rebuild is complete, the Online LED on the front of the drive
stops blinking.)
• There are some exceptions:
• In RAID ADG configurations, any two drives in the array can be replaced simultaneously.
• 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.