User's Manual

Drive Arrays and Fault Tolerance
HP Smart Array 641/642 Controller User Guide D-5
HP CONFIDENTIAL
Writer: John Turner File Name: l-appd drive arrays & fault tolerance
Codename: darkmatter -antimatter Part Number: 309311-001 Last Saved On: 11/6/02 4:34 PM
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 quickly restored to
full RAID-level data protection. (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; you can configure one hard drive to be the online spare for several arrays,
as long as the arrays are all on the same controller.
Fault-Tolerance Methods
RAID 0—No Fault Tolerance
This configuration (refer to Figure D-3) provides data striping, but there is no
protection against data loss when a drive fails. However, it is useful for rapid storage
of large amounts of non-critical data (for printing or image editing, for example), or
when cost is the most important consideration.
Advantages
• Highest performance method for writes
• Lowest cost per unit of stored data
• All drive capacity is used to store data (none needed for fault tolerance)
Disadvantages
• All data on the logical drive is lost if a physical drive fails
• Cannot use an online spare
• Can only preserve data by backing it up to external drives