Contact Information
500D Huntmar Park Drive
Herndon, VA 20170
Phone: (703) 471-8108
Fax: (703) 471-8195
techsupport@risingedge.com
|
RAID Levels
In 1987, professors Garth Gibson, Randy Katz, and David Patterson at the University of
California outlined 6 RAID levels. These formally defined RAID levels are a family of
techniques for managing a suite of disk drives so that reliability, data availability,
and performance are all maximized for a wide variety of applications. Since
then, additional levels have been defined and implemented by various suppliers. Each RAID level
defines how the RAID will function. The following Table summarizes the basic RAID levels 0 though
6.
Raid
Levels
|
Description
|
RAID
Level 0
|
Block Striping: RAID level 0 implements a data striping approach across
all drives in the system. Blocks of data are stored sequentially on each drive in the
system. RAID level 0 has no redundancy. If one drive fails, all data stored on the
system becomes unusable. Due to its lack of fault tolerance, RAID level 0 is best suited
for high performance applications that do not require data protection.
|
| RAID Level 1 |
Disk Mirroring: RAID level 1 implements a mirror disk for each drive in
the system. Use of a mirror drive virtually eliminates system interruption due to drive
failure. Data is duplicated (in pairs) for storage on each drive. The use of dual drives
more than doubles the cost of the storage unit. RAID level 1 provides fair read
performance but much slower write performance since it must write to two disks. |
| RAID Level 2 |
Bit Interleaving: RAID level 2 interleaves bits across all drives in the array. Additional
drives are inserted in the system for error correcting code or parity data. RAID 2 is
typically used for large computer systems due to the robustness of its design.
|
RAID
Level 3
|
Synchronized Spindles: RAID level 3 is similar to RAID level 2 but better
suited to microcomputers. In a typical RAID level 3 system only one parity drive is
used for all data drives. Interleaving of data occurs at a predefined logical size
(bit, byte, block, or other unit). Each drive holds a portion of the data. The
synchronized spindles yield high data transfer rates. This makes RAID level 3 ideal
for high performance systems, such as image processing, where speed and accuracy are
important.
|
RAID
Level 4
|
Block Stripping with Parity Drive: RAID level 4 is RAID level 0 with a parity drive.
Typically write operations are slow due to the parity read to a single drive that occurs
before each write.
|
RAID
Level 5
|
Block Striping with Distributed Parity: RAID level 5 contains both data and parity blocks
on each disk, thus eliminating parity drives. Eliminating the parity drive allows the
system to perform multiple read and write functions at the same time. This increases
the virtual transfer rate by one-half the number of drives in the array, thus increasing
performance as more drives are added.
|
RAID
Level 6
|
RAID level 6 is an extension of RAID
level 5 that improves fault tolerance by supporting dual parity. Data is striped on a block level across a set of drives, just like in RAID 5,
and a second set of parity is calculated and written across all the drives; RAID
6 provides for an extremely high data fault tolerance and can sustain multiple
simultaneous drive failures. |
|
While the RAID levels are as different as the user communities that they are designed to serve,
they do share some common characteristics. With the exception of RAID level 0, a simple
non-redundant disk array, these RAID levels all provide no loss of data due to individual
disk failure and continuous operation during the downtime of a failed disk. In the event
of a failed disk drive, RAID systems notify the operator of the failure and continue to
operate by using the redundantly stored information to respond to user data requests.
Upon replacement of the failed disk, the RAID system will automatically and transparently
use the redundant data stored on the array to reconstruct the data previously stored on
the failed disk.
|
|
|
