There are at least nine types of RAID plus a non-redundant array
- RAID-0. This technique has striping but no redundancy
of data. It offers the best performance but is not fault-tolerance.
- RAID-1. This type is also known as disk mirroring
and consists of at least two drives that duplicate the storage of data.
There is no striping. Read performance is improved since either disk
can be read at the same time. Write performance is the same as for single
disk storage. RAID-1 provides the best performance and the best fault-tolerance
in a multi-user system.
- RAID-2. This type uses striping across disks with
some disks storing error checking and correcting (ECC) information.
It has no advantage over RAID-3.
- RAID-3. This type uses striping and dedicates one
drive to storing parity (an error checking technique) information. The
embedded error checking (ECC) information is used to detect errors and
data loss. Data recovery is accomplished by calculating the exclusive
OR (XOR) of the information recorded on the other drives. Since an Input
or Output operation an address all drives at the same time, RAID-3 cannot
overlap I/O. For this reason, RAID-3 is best for single-user systems
with long record applications.
- RAID-4. This type uses large stripes, which means
you can read records from any single drive. This allows you to take
advantage of overlapped I/O for read operations. Since all write operations
have to update the parity drive, no I/O overlapping is possible. RAID-4
offers no advantage over RAID-5.
- RAID-5. This type includes a rotating parity array,
thus addressing the write limitation in RAID-4. Thus, all read and write
operations can be overlapped. RAID-5 stores parity information but not
redundant data (but parity information can be used to reconstruct data).
RAID-5 requires at least three and usually five disks for the array.
It's best for multi-user systems in which performance is not critical
or which do few write operations.
- RAID-6. This type is similar to RAID-5 but includes
a second parity scheme that is distributed across different drives and
thus offers extremely high fault- and drive-failure tolerance. There
are few or no commercial examples currently.
- RAID-7. This type includes a real-time embedded operating
system as a controller, caching via a high-speed bus, and other characteristics
of a stand-alone computer. One vendor offers this system.
- RAID-10. This type offers an array of stripes in
which each stripe is a RAID-1 array of drives. This offers higher performance
than RAID-1 but at much higher cost.
- RAID-53. This type offers an array
of stripes in which each stripe is a RAID-3 array of disks. This offers
higher performance than RAID-3 but at much higher cost.