Using RAID Redundancy
For servers and other critical uses where you need information available all the time, a major enemy is disk failure. If something goes wrong in the disk itself, you can lose all the data stored on it. Using a backup utility such as OS X's Time Machine (see Chapter 32) can help you get back up and running, but you lose any data saved after the last backup.
RAID, which stands for redundant array of independent disks, is a technology meant to better protect your data in case of failure. Basically, RAID writes the data to multiple disks, so if one fails, the information is still available on the other disks. That also means there's no downtime, because the data remains accessible. (You would repair or replace the broken disk, but you can usually do so without taking the Mac offline.)
RAID is a technology with two distinct uses. First, it can use multiple disks to create a single volume. For example, you can install four 512GB hard disks to create a single volume providing 2TB of space. This is particularly useful for server space and for users who use large files. (For example, high-definition video editing takes up a significant amount of space.)
You have two ways to use RAID to create shared disks. One is called a concatenated disk set, which stores the files on whatever disk is available but presents the array of disks to the Finder as if it were one disk so the Mac can put the file on the most accessible disk at the moment. The other is ...