First and foremost, RAID can be used to enhance availability, but it also can be used to enhance performance for...
I/O, for throughput and things like that. So real quickly. . . .
- RAID 0: data spread across multiple drives, gives you performance but no availability;
- RAID 1: data mirroring. Mirroring data across two or more disk drives gives you availability and can also enhance read operation;
- RAID 01 or RAID 10: combine striping and mirroring or mirroring and striping for a compromise of good performance and good availability;
- RAID 3: striping with dedicated parity. Good for large sequential bandwidth operations but not as good for concurrent access.
- This is where RAID 5 steps in, which stripes the data across multiple drives with rotating parity. This gives you a nice balance of good read performance and reduced cost, because you don't need to have as many drives for mirroring. It's a compromise between performance and availability at the expense of writes, because you have parity updates.
Now, you are starting to see dual-parity schemes, such as RAID 6 -- multiple drives striped together with at least two parity drives.
Check out the entire RAID FAQ guide.