RAIDS. Definitions.
RAID (redundant array of independent disks; originally redundant array of
inexpensive disks) is a way of storing the same data in different places (thus,
redundantly) on multiple hard disks. By placing data on multiple disks, I/O
operations can overlap in a balanced way, improving performance. Since multiple
disks increases the mean time between failure (MTBF), storing data redundantly
also increases fault-tolerance.
There are at least nine types of RAID plus a non-redundant array (RAID-0):
- RAID-0
- This technique has striping but no redundancy of data. It offers the best
performance but no 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
information. The embedded error checking (ECC) information is used to detect
errors. Data recovery is accomplished by calculating the exclusive OR (XOR) of
the information recorded on the other drives. Since an I/O operation addresses
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.
See Also
Dynamic Disks and Volumes
