RAID LEVELS Level

This level is also known as disk striping because of its use of a disk file system called a stripe set. Data is divided into blocks and spread in a fixed order among all disks in an array. RAID 0 improves read/write performance by spreading operations across multiple disks, so that operations can be performed independently and simultaneously. RAID 0 is similar to RAID 5, except RAID 5 also provides fault tolerance. The following illustration shows RAID 0.

A0 A1 A2 A3

B0 B1 B2 B3

C0 C1 C2 C3

D0 D1 D2 D3

Disk 1 Disk 2 Disk 3 Disk 4

Level 1

This level is also known as disk mirroring because it uses a disk file system called a mirror set. Disk mirroring provides a redundant, identical copy of a selected disk. All data written to the primary disk is written to the mirror disk. RAID 1 provides fault tolerance and generally improves read performance (but may degrade write performance). The following illustration shows RAID 1.

A A B B C C D D Disk 1 Disk 2 Mirror Fig. 8.2 RAID level 1 Level 2

This level adds redundancy by using an error correction method that spreads parity across all disks. It also employs a disk-striping strategy that breaks a file into bytes and spreads it across multiple disks. This strategy offers only a marginal improvement in disk utilization and read/write performance over mirroring (RAID 1). RAID 2 is not as efficient as other RAID levels and is not generally used.

A0 B0 C0 D0

A1 B1 C1 1 party

A2 B2 2 parity D2

A3 3 parity C3 D3

Disk 1 Disk 2 Disk 3 Disk 4

0 party E1 E2 E3 Disk 5 4 parity B4 C4 D4 E4

Fig. 8.3 RAID level 2

Level 3

This level uses the same striping method as RAID 2, but the error correction method requires only one disk for parity data. Use of disk space varies with the number of data disks. RAID 3 provides some read/write performance improvement. RAID 3 also is rarely used.

Level 4

This level employs striped data in much larger blocks or segments than RAID 2 or RAID 3. Like RAID 3, the error correction method requires only one disk for parity data. It keeps user data separate from error-correction data. RAID 4 is not as efficient as other RAID levels and is not generally used.

Level 5

Also known as striping with parity, this level is the most popular strategy for new designs. It is similar to RAID 4 because it stripes the data in large blocks across the disks in an array. It differs in how it writes the parity across all the disks. Data redundancy is provided by the parity information. The data and parity information are arranged on the disk array so the two are always on different disks. Striping with parity offers better performance than disk mirroring (RAID 1). However, when a stripe member is missing, read performance degrades (for example, when a disk fails). RAID 5 is one of the most commonly used RAID configurations. The following illustration shows RAID 5.

A0 A1 A2 A3

B0 B1 B2 B3

C0 C1 C2 C3

D0 D1 D2 D3

Disk 1 Disk 2 Disk 3 Disk 4

A0 A1 A2 A3

B0 B1 B2 B3

C0 C1 C2 C3

D0 D1 D2 D3

Disk 1 Disk 2 Disk 3 Disk 4

Fig. 8.4 RAID level 5 Level 10 (1 + 0)

This level is also known as mirroring with striping. This level uses a striped array of disks, which are then mirrored to another identical set of striped disks. For example, a striped array can be created using four disks. The striped array of disks is then mirrored using another set of four striped disks. RAID 10 provides the performance benefits of disk striping with the disk redundancy of mirroring. RAID 10 provides the highest read/write performance of any of the RAID levels at the expense of using twice as many disks. The following illustration shows RAID 10.

As mentioned above, RAID 1 and RAID 0+1 offer the best data protection and best performance among RAID levels, but cost more in terms of disks required. When cost of hard disks is not a limiting factor, RAID 1 or RAID 0+1 are the best choices in terms of both performance and fault tolerance.

RAID 5 costs less than RAID 1 or RAID 0+1 but provides less fault tolerance and less write performance. The write performance of RAID 5 is only about half that of RAID 1 or RAID 0+1 because of the additional I/O needed to read and write parity information.

The best disk I/O performance is achieved with RAID 0 (disk striping with no fault tolerance protection). Because RAID 0 provides no fault tolerance protection, it should never be used in a production environment, and it is not recommended for development environments. RAID 0 is typically used only for benchmarking or testing.

RAID 0

RAID 1

RAID 5

RAID 0 + 1

Data

Fault tolerance information

Fig. 8.5 Comparison of different RAID levels

Many RAID array controllers provide the option of RAID 0+1 (also referred to as RAID 1/ 0 and RAID 10) over physical hard drives. RAID 0+1 is a hybrid RAID solution. On the lower level, it mirrors all data just like normal RAID 1. On the upper level, the controller stripes data across all of the drives (like RAID 0). Thus, RAID 0+1 provides maximum protection (mirroring) with high performance (striping). These striping and mirroring operations are transparent to Windows and RDBMS because they are managed by the RAID controller. The difference between RAID 1 and RAID 0+1 is on the hardware controller level. RAID 1 and RAID 0+1 require the same number of drives for a given amount of storage. For more information on RAID 0+1 implementation of specific RAID controllers, contact the hardware vendor that produced the controller.

The Fig. 8.5 shows differences between RAID 0, RAID 1, RAID 5, and RAID 0+1. Note: In the illustration above, in order to hold four disks worth of data, RAID 1 (and RAID 0+1) need eight disks, whereas Raid 5 only requires five disks. Be sure to involve your storage vendor to learn more about their specific RAID implementation.