HP Microsoft Windows Server 2003 for Itanium-based Systems manual Fault-tolerance methods

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 60).

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 65). 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)

Drive arrays and fault-tolerance methods 60