The acronym RAID stands for Redundant Array of Inexpensive (or Independent) Disks. In a RAID, two or more physical disks are combined in any of numerous ways to behave as a single disk. RAIDs come in many flavors, each designed to meet certain needs.
For example, in a striped, or RAID 0, array, data is divided evenly among all the disks; because portions of each file can be written to and read from multiple disks in parallel, overall performance increases. However, if any disk in a RAID 0 array were to fail, all the data would be lost because no single disk contains a complete copy of any file.
Because RAID 0 arrays don't provide any data redundancy, I don't discuss them further in this book.
In a mirrored, or RAID 1, array, identical data is written to two or more disks at once. That means that if any disk in the array were to fail, all the data would still be intact; if the faulty disk were replaced, it could rebuild its mirror from the other disk(s) still in the array.
In a concatenated RAID, also known as JBOD (Just a Bunch of Disks), two or more disks appear as a single large volume whose capacity is the total of all member disks. (For example, two 1 TB disks function as a single 2 TB disk.) If one of the disks fails, its files become unavailable, but the files on the other disk(s) could still be accessed.
Many other sorts of RAIDs exist, including several varieties that combine the virtues of striping, mirroring, and/or concatenation, ...