This article is taken from http://www.raidweb.com/whatis.html
RAID (Redundant Array of Independent Disks) is an acronym first used in a 1988. RAID boxes provide the user a way to access multiple individual hard disks as if they were one larger disk, spreading data access out over the multiple disks, which reduces the risk of losing all data if one drive fails. This process improves disk access time.
In simpler terms, a RAID unit with eight bays, populated with 200Gb disks, can appear to a server as a single, 1.6Tb disk, or can be configured to recover data if a disk goes bad.
NAS (Network Attached Storage) also provides mass data storage, but interfaces to a network utilizing an IP address and an Ethernet interface. While NAS units can utilize RAID technology, including data redundancy, they are not RAID devices. NAS units often contain an internal O/S element which allows network interaction.
Why use RAID?
Typically RAID is used in large file servers, transaction of application servers, where data accessibility is critical, and fault tolerance is required. Nowadays, RAID is also being used in desktop systems for CAD, multimedia editing and playback where higher transfer rates are needed.
RAID 0: Also known as "Disk Striping", this is technically not a RAID level since it provides no fault tolerance. Data is written in blocks across multiple drives, so one drive can be writing or reading a block while the next is seeking the next block.
The advantages of striping are the higher access rate, and full utilization of the array capacity. The disadvantage is there is no fault tolerance - if one drive fails, the entire contents of the array become inaccessible.
RAID 1: Known as "Disk Mirroring" provides redundancy by writing twice - once to each drive. If one drive fails, the other contains an exact duplicate of the data and the RAID can switch to using the mirror drive with no lapse in...