HP StorageWorks D2D100 - Backup System User Manual page 28

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
can 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:
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 6 (ADG) configuration
Time required for a rebuild
The time required for a rebuild varies considerably, depending on several factors:
The priority that the rebuild is given over normal I/O operations (you can change the priority setting
by using ACU)
The amount of I/O activity during the rebuild operation
The rotational speed of the hard drives
The availability of drive cache
The brand, model, and age of the drives
The amount of unused capacity on the drives
For RAID 5 and RAID 6 (ADG), the number of drives in the array
Allow approximately 15 minutes per gigabyte for the rebuild process to be completed. This figure is
conservative; the actual time required is usually less than this.
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.
When automatic data recovery has finished, the Online/Activity LED of the replacement drive stops
blinking steadily at 1 Hz and begins to either glow steadily (if the drive is inactive) or flash irregularly (if
the drive is active).
CAUTION: If the Online/Activity LED on the replacement drive does not light up while the corresponding
LEDs on other drives in the array are active, the rebuild process has abnormally terminated. The amber Fault
LED of one or more drives might also be illuminated. Refer to "Abnormal termination of a rebuild (on page
28)" to determine what action you must take.
Abnormal termination of a rebuild
If the Online/Activity LED on the replacement drive permanently ceases to be illuminated even while other
drives in the array are active, the rebuild process has abnormally terminated. The following table
indicates the three possible causes of abnormal termination of a rebuild.
Observation
None of the drives in the array have
an illuminated amber Fault LED.
The replacement drive has an
illuminated amber Fault LED.
One of the other drives in the array
has an illuminated amber Fault LED.
Each of these situations requires a different remedial action.
Case 1: An uncorrectable read error has occurred.
Cause of rebuild termination
One of the drives in the array has
experienced an uncorrectable read error.
The replacement drive has failed.
The drive with the illuminated Fault LED has
now failed.
Replacing, moving, or adding hard drives 28