HP 273914-B21 Smart Array 6400 Series Controllers User Guide - Page 45

Replacing, moving, or adding hard drives 45 − 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 • 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