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Redundant Array of Independent Disks (RAID) is a storage technology
used to improve the processing capability of storage systems. This technology
is designed to provide reliability in disk array systems and to take advantage
of the performance gains offered by an array of multiple disks over single-disk
storage. RAID's two primary underlying concepts are:
In the event of a disk failure, disk access continues normally and the
failure is transparent to the host system. RAID Levels There are several different ways to implement a RAID array, using some
combination of mirroring, striping, duplexing, and parity technologies.
These different techniques are referred to as RAID levels. Each level
offers various mixes of performance, reliability, and cost. Each level
uses a different algorithm to implement fault tolerance. There are eight RAID level choices: RAID 0, 1, 3, 5, 1+0, 3+0 (30), and
5+0 (50). RAID levels 1,3, and 5 are the most commonly used. The following table provides a brief overview of the RAID levels. Capacity refers to the total number (N) of physical drives available
for data storage. For example, if the capacity = N-1, and the total number
of disk drives in the logical drive is six 36-Mbyte drives, the disk space
available for storage is equal to five disk drives -- (5 x 36 MB (180
Mbyte). The -1 refers to the amount of striping across six drives, which
provides redundancy of data and is equal to the size of one of the disk
drives. For RAID 3+0 (30) and 5+0 (50), capacity refers to the total number
of physical drives (N) minus one physical drive (#) for each logical drive
in the volume. For example, if the total number of disk drives in the
logical drive is twenty 36-Mbyte drives, and the total number of logical
drives is 2, the disk space available for storage is equal to 18 disk
drives -- 18 x 36 MB (648 Mbyte).
The RAID levels are described below. RAID 0 RAID 0 implements block striping, where data is broken into logical
blocks and striped across several drives. Unlike other RAID levels, there
is no facility for redundancy. In the event of a disk failure, data is
lost. In block striping, the total disk capacity is equivalent to the sum of
the capacities of all drives in the array. This combination of drives
appears to the system as a single logical drive. RAID 0 provides the highest performance. It is fast because data can
be simultaneously transferred to/from every disk in the array. Furthermore,
read/writes to different drives can be processed concurrently. RAID 1 RAID 1 implements disk mirroring, where a copy of the same data is recorded
onto two drives. By keeping two copies of data on separate disks, data
is protected against a disk failure. If, at any time, a disk in the RAID
1 array fails, the remaining good disk (copy) can provide all of the data
needed, thus preventing downtime. In disk mirroring, the total usable capacity is equivalent to the capacity
of one drive in the RAID 1 array. Thus, combining two 1-Gbyte drives,
for example, creates a single logical drive with a total usable capacity
of 1 Gbyte. This combination of drives appears to the system as a single
logical drive. NOTE: RAID 1 does not allow expansion. RAID levels 3 and 5 permit
expansion by adding drives to an existing array. RAID 1+0 RAID 1+0 combines RAID 0 and RAID 1 to offer mirroring and disk striping.
Using RAID 1+0 is a time-saving feature that enables you to configure
a large number of disks for mirroring in one step. It is not a standard
RAID level option that you can select; it does not appear in the list
of RAID level options supported by the controller. If four or more disk
drives are chosen for a RAID 1 logical drive, RAID 1+0 is performed automatically.
RAID 3 RAID 3 implements block striping with dedicated parity. This RAID
level breaks data into logical blocks, the size of a disk block, and then
stripes these blocks across several drives. One drive is dedicated to
parity. In the event a disk fails, the original data can be reconstructed
using the parity information and the information on the remaining disks. In RAID 3, the total disk capacity is equivalent to the sum of the capacities
of all drives in the combination, excluding the parity drive. Thus, combining
four 1-Gbyte drives, for example, creates a single logical drive with
a total usable capacity of 3-Gbyte. This combination appears to the system
as a single logical drive. RAID 3 provides increased data transfer rates when data is being read
in small chunks or sequentially. However, in write operations that do
not span every drive, performance is reduced because the information stored
in the parity drives needs to be recalculated and rewritten every time
new data is written, limiting simultaneous I/O. RAID 5 RAID 5 implements multiple-block striping with distributed parity.
This RAID level offers redundancy with the parity information distributed
across all disks in the array. Data and its parity are never stored on
the same disk. In the event a disk fails, original data can be reconstructed
using the parity information and the information on the remaining disks. RAID 5 offers increased data transfer rates when data is accessed in
large chunks or randomly and reduced data access time during many simultaneous
I/Os. Advanced RAID Levels The following advanced RAID levels require the use of the array's built-in
volume manager. These combination RAID levels provide the protection benefits
of RAID 1, 3, or 5 with the performance of RAID 1. To use advanced RAID,
first create two or more RAID 1, 3, or 5 arrays, and then join them. RAID 3+0 (30) RAID 3 logical drives that have been joined together using the array's
built-in volume manager. RAID 5+0 (50) RAID 5 logical drives that have been joined together using the array's
volume manager. |