HP Array Configuration Utility User Guide (416146-001, January 2006)

Drive arrays and fault-tolerance methods 62
logical drive, the term array is often used as a synonym for logical drive. However, an array can contain
several logical drives, each of a different size.
Each logical drive in an array is distributed across all of the physical drives within the array. A logical
drive can also extend across more than one port on the same controller, but it cannot extend across more
than one controller.
Drive failure, although rare, is potentially catastrophic. For arrays that are configured as shown in the
previous figure, failure of any physical drive in the array causes every logical drive in the array to suffer
irretrievable data loss. To protect against data loss due to physical drive failure, logical drives are
configured with fault tolerance ("Fault-tolerance methods" on page 62).
For any configuration except RAID 0, further protection against data loss can be achieved by assigning a
drive as an online spare (or hot spare). This drive contains no data and is connected to the same
controller as the array. When any other physical drive in the array fails, the controller automatically
rebuilds information that was originally on the failed drive to the online spare. The system is thus restored
to full RAID-level data protection, although it now no longer has an online spare. (However, in the unlikely
event that another drive in the array fails while data is being rewritten to the spare, the logical drive will
still fail.)
When you configure an online spare, it is automatically assigned to all logical drives in the same array.
Additionally, you do not need to assign a separate online spare to each array. Instead, you can configure
one hard drive to be the online spare for several arrays if the arrays are all on the same controller.
Fault-tolerance methods
Several fault-tolerance methods exist. Those most often used with Smart Array controllers are hardware-
based RAID methods.
Two alternative fault-tolerance methods that are sometimes used are also described ("Alternative fault-
tolerance methods" on page 67). However, hardware-based RAID methods provide a much more robust
and controlled fault-tolerance environment, so these alternative methods are seldom used.
Hardware-based fault-tolerance methods
HP recommends the following hardware-based methods for use with Smart Array controllers:
• RAID 0—Data Striping only (no fault tolerance)