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RAID - Redundant Array of Inexpensive Disks

Redundancy (or mirroring) means that the data you write to the hard drives is stored with a backup so that if you suffer the loss of one drive in the array, you can reconstruct all of your data from the remaining drives.

Array means more than one drive working together and appearing to your machine as one storage device

Inexpensive related originally the difference in price between a group of 3 to 10 small hard drives and the expense of a single gigantic drive used in servers. This distinction is largely academic now.

Disks usually mean hard drives, but can be other media. One intrepid soul made a RAID out of five USB floppy drives.

The RAID function is controlled either by software or by a dedicated RAID controller card. Generally, a hardware RAID controller is faster but more expensive. Software RAID relies on your machine's CPU to do the calculations, so can slow perfomance down somewhat.

There are two components of a RAID array; striping and redundancy (or mirroring).

Striping takes your data and divides the bytes across two or more hard drives. The theory is that the writing and reading speed will be faster because as drive A is dealing with one byte, drive B is already getting a head start finding the next byte to read or write. This improves oveall speed because transferring the data takes only 20% of the time needed, the other 80% of the time the drive needs to move the heads and wait for the data on the disk to rotate around to the head.

The simplest RAIDs are called RAID 0 (striping) and RAID 1 (mirroring)

RAID 0 simply uses two identical hard drives, and stripes the data between them to create a single volume. There is no redundancy, therefore if one drive has a failure, ALL the data is lost on both drives. Cheapest but risky. 2 x 100 Gb drives in RAID 0 = 200 Gb space

RAID 1 (Mirroring) just means two drives, where data is written simultaneously to both. Drive B is a mirror of drive A. This provides good redunancy, at the expense of half or the drive space. 2 x 100 Gb in RAID 1 = 100 Mb space.

RAID 2 through RAID 5 are various arrangements of 3 or more drives, where data is striped across the drives, but "parity" data is also stored, to be able to reconstruct any single drive. Typically one drive is consumed by the parity data, so
3 x 100 Gb drives in RAID = 200 Gb space
5 x 100 Gb drives in RAID = 400 Gb space

You can also combine RAIDs, such as a striped pair (RAID 0) mirrored by another identical striped pair (RAID 1), which is sometimes called RAID10 or RAID 1+0 or RAID 0+1
4 x 100 Gb drives in RAID 0+1 - 200 Gb space.

Because there is no redundancy, and you are combining the risk of two drive mechanisms failing, RAID 0 is risky unless you maintain a fulltime backup onto another drive. RAID 1 - 5 provide additional security but have significant costs for drives, controller cards, and drive enclosures.

Remember that RAID 1 - 5 only protect against drive failure. If you delete a file, have a crash or get a virus, the damage is done instantaneously to the mirror as well as the primary data, so RAID gives you no protection against those events.

When not to use RAID:
For desktop computer use, RAIDs are almost never worth it, unless you are doing something that needs massive data streaming, like video production.There is an overhead to RAID in calculating where each byte of data is going to go and in typical desktop usage patterns, the overhead exceeds the savings. This means that RAIDs are slower than a single drive for everyday use, which is characterized by many small files being accessed one at a time.

When to use RAID:
RAID really shines when put under the loads of a file server where many users are demanding file access simultaneously, or if you need a single volume that exceeds the size of available single drives. For desktop machines, consider RAID if you have an application that demands high bandwidth streaming of large files; the classic example is digitizing high resolution video files. RAID 1 is a useful security measure if you have high value data that you want to secure against drive failure.

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