RAID (Redundant Array of Independent Disks) is being specified by an increasing number of libraries. The principle is to bind two or more drives together to improve performance and/or reliability. Now that disk storage represents only a small percentage of the purchase price of an automated library system, RAID is within reach of most libraries. However, the key to specifying RAID is understanding the difference among the "flavors."
RAID level 0 is the simplest. It is popularly called striping. Used with a minimum of two drives, RAID 0 writes sequential blocks of data to separate drives. While one drive is seeking; another can be writing. Therefore, performance is increased. There is no redundancy in RAID 0.
RAID 1 is another name for what is often called mirroring: the same data is read to two separate drives. If either drive in a mirrored pair fails, the other drive keeps going with virtually no deterioration in performance. Redundancy is the sole benefit of RAID 1.
RAID 5 offers both striping and mirroring. Performance is better than one can realize with a single drive or with RAID 1, and the failure of one drive does not result in loss of access to data. One must have a minimum of three disk drives for RAID 5. Performance will deteriorate when not all drives in a RAID 5 configuration are available.
Because most libraries can realize the response times they require with a single low-cost 4.0 or 9.1 GB disk drive, RAID 0 is not necessary. For libraries, the choices will be between RAID 1 or 5.
If redundancy (i.e., data protection) is all that is needed, RAID 1 is the better choice because performance is good and does not deteriorate if one drive is lost.
There often is a need to improve performance when a great deal of Java is used. With Java, the clients have to load applications from the server, thus requiring a boost in performance to avoid unsatisfactory response times. RAID 5 offers that boost in performance for only a little more than RAID 1, and includes redundancy as a low-cost extra.
A library should ask vendors to quote RAID 1 and 5 as options and discuss the merits of each, given its system's design.
