Drive Arrays and Fault Tolerance

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 MethodsRAID 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

HP Smart Array 641/642 Controller User Guide

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