HP StorageWorks 500 G2 Modular Smart Array - Manual (Page 82)

82 HP StorageWorks MSA500 G2 Storage System User Guide
• 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. (The rebuild is
complete when the Online LED on the front of the drive stops blinking.)
These cases are the 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.
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:
• 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
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