When speaking in terms of data storage, RAID stands for one of the two phrases:
Based on the title of the original paper authored by the founders of RAID, the i stands for inexpensive. Regardless, RAID storage takes multiple disks and combines them to function as a one or more logical disk or disks. The result is redundancy, efficiency (speed) or a combination of both. Redundancy is achieved by increasing the mean time between failures. Efficiency is achived as data can be read or written to or from multiple drives at the same time. RAID is also commonly referred to as a RAID Array or RAID Storage.
The two main techniques used by RAID are disk mirroring and disk striping.
There are several levels of RAID. The most frequently used are RAID 0, RAID 1, RAID 5 and RAID 6. These are all known as standard RAID levels.
There is a cue to how nested RAID levels work right in its name. Nested RAID is a combination of two or more standard RAID levels. Nested RAID levels include RAID 10 (or RAID 1+0), RAID 01 (or RAID 0+1), RAID 03 (RAID 0+3, RAID 53 or RAID 5+3) and RAID 50 (RAID 5+0).
Conceptually, RAID 10, or RAID 1+0, is the easiest to envision. Simply stated, two RAID 1 levels are placed in a RAID 0 configuration. RAID 1 is striping, and RAID 0 is mirroring. For that reason, RAID 10 or RAID 1+0 is known as stripe + mirror.
The math for a computing the capacity and usable capacity of a small RAID setup isn't too bad. But, as you add drives and compare the different RAID levels available things can get complex. We have two tools to help with these calculations.
In addtion to the standard and nested RAID levels, there are also non-standard RAID levels, JBOD (Just a Bunch of Disks) and ZFS RAID. We've also created a ZFS RAID (RAIDZ) calculator to explore the various levels available.
No. While RAID does appear to many as a way of backing up data, that is not its intended purpose.