[ Contents ] [ Index ]

RAID Levels

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:

  • Distributing data over multiple hard drives improves performance.
  • Using multiple drives properly enables any one drive to fail without loss of data and without system downtime.

In the event of a disk failure, disk access continues normally and the failure is transparent to the host system.

There are several ways to implement a RAID array, using a combination of mirroring, striping, duplexing, and parity technologies. These various techniques are referred to as RAID levels. Each level offers a mix of performance, reliability, and cost. Each level uses a distinct algorithm to implement fault tolerance.

There are several RAID level choices: RAID 0, 1, 1+0, 3, 5, 3+0, and 5+0. RAID levels 1,3, and 5 are the most commonly used.

The following table provides a brief overview of the RAID levels.

RAID Level Description Number of Drives Supported Capacity Redundancy
0 Striping 2-36 N No
1 Mirroring 2 N/2 Yes
1+0 Mirroring and Striping 4-36 (even number only) N/2 Yes
3 Striping with dedicated parity 3-31 N-1 Yes
5 Striping with distributed parity 3-31 N-1 Yes
3+0 Striping of RAID 3 logical drives 2-8 logical drives N-number of logical drives Yes
5+0 Striping of RAID 5 logical drives 2-8 logical drives N-number of logical drives Yes

Capacity refers to the total number (N) of physical drives available for data storage. For example, if the capacity is 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 Mbyte or 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 and 5+0, 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 Mbyte (648 Mbyte).

The RAID levels are described below.

RAID 0

RAID 0 implements block striping, where data is broken into logical blocks and is 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 or from every disk in the array. Furthermore, read/writes to various 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.

In addition to the data protection that RAID 1 provides, this RAID level also improves performance. In cases where multiple concurrent I/O is occurring, that I/O can be distributed between disk copies, thus reducing total effective data access time.

RAID 1+0

RAID 1+0 combines RAID 0 and RAID 1 - 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 that 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 that 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/O cycles.

Advanced RAID Levels

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. See the following for a description of the advanced RAID levels.

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.

[ Contents ] [ Index ]