Drive Arrays and Fault Tolerance

In each mirrored pair, the physical drive that is not busy answering other requests answers any read request sent to the array. (This behavior is called load balancing.) If a physical drive fails, the remaining drive in the mirrored pair can still provide all the necessary data. Several drives in the array can fail without incurring data loss, as long as no two failed drives belong to the same mirrored pair.

This fault-tolerance method is useful when high performance and data protection are more important than the cost of physical drives.

NOTE: When there are only two physical drives in the array, this fault-tolerance method is often referred to as RAID 1.

Advantages

•Highest read and write performance of any fault-tolerant configuration

•No loss of data as long as no failed drive is mirrored to another failed drive (up to half of the physical drives in the array can fail)

Disadvantages

•Expensive (many drives needed for fault tolerance)

•Only half of total drive capacity usable for data storage

RAID 5—Distributed Data Guarding

By this method, a block of parity data is calculated for each stripe from the data that is in all other blocks within that stripe. The blocks of parity data are distributed over every physical drive within the logical drive (refer to Figure D-7). When a physical drive fails, data that was on the failed drive can be calculated from the user data on the remaining drives and the parity data. This recovered data is usually written to an online spare in a process called a rebuild.

HP Smart Array 641/642 Controller User Guide

D-7