RAID stands for Redundant Array of Inexpensive (or sometimes "Independent") Disks. RAID isa method of combining multiple hard disks in a single logical unit to offer high availability, performance or a combination of both. This provides better resilience and performance than a single disk drive.

The benefits of RAID explained

• Provides real-time data recovery with uninterrupted access when a hard drive fails
• Increases system uptime and network availability
• Protects against data loss
• Multiple drives working in parallel increases system performance
• Software RAID

Many operating systems provide functionality for implementing software based RAID systems. The software RAID systems generate the RAID algorithms using the server CPU ,this can severely limit the RAID performance. Should a server fail the whole RAID system is lost. Cheap to implement and only need a single SCSI controller.

Hardware RAID

All RAID algorithms are generated on the RAID controller board, thus freeing the server CPU. Allows full benefits and data protection of RAID. More robust and fault tolerant than software RAID. Requires dedicated RAID controller to work

RAID levels

Various RAID levels exist these are 0, 1, 2, 3, 4, 5, 6, 7, 10, & 0+1. The levels of RAID protection varies with the RAID level selected RAID levels 0 & 1 are not technically RAID as they have no redundancy in the event of drive failure. The most common RAID levels are shown below. RAID levels 2, 4, 6, 7 & 0+1 are a combination of the other RAID levels shown.

RAID 0

Offers no redundancy or fault tolerance, hence does not truly fit the "RAID" acronym. In level 0, data is striped across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss. This level is commonly referred to as striping.

Advantages

• No parity generation
• Easy to implement
• Cost effective
• Utilises full disk capacity

Disadvantages

• Not a true RAID
• No redundancy / fault tolerance
• Drive failure will result in data loss
• Not suitable in mission critical environments

Applcations

• Pre-Press
• Video editing and production
• Image manipulation/editing

RAID 1

Provides redundancy by writing all data to two or more drives. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required; however, since one drive is used to store a duplicate of the data, This level is commonly referred to as mirroring.

Advantages

• No parity generation
• Easy to implement
• Extremely fault tolerant
• Utilises full disk capacity
• 2 drives minimum

Disadvantages

• Inefficient use of disk space
• High disk overhead
• Doubles number of writes

Applcations

• Pre-Press
• Video editing and production
• Image manipulation/editing

RAID 3

Provides redundancy by writing all data to three or more drives. This RAID 3 disk array provides excellent storage for videoimaging, streaming, publishing applications or any system that requires large file block transfers.

Advantages

• Single dedicated parity disk
• High read data rate
• High write data rate
• 4 drives minimum
• No performance degradation if drive fails
• Best and worst case performance similar

Disadvantages

• Inefficient with small file transfer

Applcations

• Video Streaming
• Video Publishing
• Video Editing
• Pre Press
• Image editing
• Any application that needs heavy updating and large file usage

RAID 5

Provides redundancy by writing data and parity in formation across three or more drives, thus increasing performance. The RAID Level 5provides the best combination of disk array technology. The most popular RAID level supplied.

Advantages

• Most flexible of all disk arrays
• Best balance cost / performance / protection of any RAID system
• Allows multiple simultaneous writes
• High read data rate
• 3 drives minimum
• Ideal for small write applications
• Highly efficient

Disadvantages

• Inefficient with large file transfer
• Disk failure has an impact on performance

Applcations

• Transaction processing
• Relational Databases
• File & Print Servers
• WWW, E-mail, and News servers
• Intranet Servers

RAID 10

Provides very high performance and redundancy. Data is simultaneously mirrored and striped. Can under circumstances support multiple drive failures.

Advantages

• Highly fault tolerant
• High data availability
• Very good read / write performance

Disadvantages

• Very expensive
• Drive spindles must be synchronised
• Not very scaleable

Applcations

• Where high performance and redundancy are critical