RAID Levels Explained

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Redundant Array of Independent Disks (RAID)

Below is a table showing the different RAID levels and common nested RAID levels. This is not a comprehensive list and RAID levels 2 and 4 are not explained since they are rarely used today.

RAID	Capacity	Large Data Transfers	High I/O Rate	Data Availability
Single Disk	Fixed 100%	Good	Good	Single drive MTBF
RAID 0	Excellent 100%	Very Good	Very Good	Poor * MTBF of drive
RAID 1	Moderate 50%	Good	Good	Good
RAID 3	Good - Very Good*	Very Good	Good	Good
RAID 5	Good - Very Good*	Good - Very Good*	Good	Good
RAID 6	Moderate - Good**	Very Good**	Good	Very Good
RAID 0+1	Moderate 50%	Good	Very Good	Good
RAID 10 (1+0)	Moderate 50%	Very Good	Very Good	Very Good
RAID 30 (3+0)	Good - Very Good*	Very Good	Excellent	Excellent
RAID 50 (5+0)	Good - Very Good*	Good - Very Good*	Excellent	Excellent

MTBF = Mean Time Between Failures
* dependant on the number of drives in the RAID (3 to 16).
** dependant on the number of drives in the RAID (5 to 16).

Raid 0

Striped Disk Array without Fault Tolerance
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. Requires a minimum of 2 drives to implement.

RAID 0 offers a very high level of performance for both read and write operations
No parity bit to generate
Maximum storage capacity used, no disk used
Easy to implement

Disadvantages

Not fault tolerant as no it is not a "true RAID"
A single drive failure will result in all data lost
Not recommend for mission critical systems

Application

Video Production and Editing
Image Editing
Pre-Press Applications
Ideal as a starter solution where high I/O performance is required more than file redundancy

Raid 1

Disk Mirroring & Duplexing
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, the cost per megabyte is high. This level is commonly referred to as mirroring. Requires a minimum of 2 drives to implement.

Extremely fault tolerant
No parity bit to generate
Utilises full disk capacity
Easy to implement

Disadvantages

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

Application

Accounting
Payroll
Financial
Any application requiring very high availability

Raid 3

Parallel Transfer with Parity
RAID 3 provides redundancy by writing all data to three or more drives. This RAID 3 disk array provides excellent storage for video imaging, streaming, publishing applications or any system that requires large file block transfers. Requires a minimum of 3 drives to implement.

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

Disadvantages

Inefficient with small file transfer

Application

Video Production and live streaming
Image Editing
Video Editing
Prepress Applications
Ideal for video streaming/processing that process large blocks and files sequentially.

Raid 5

Striping with dedicated parity drive
RAID 5 provides redundancy by writing data and parity information across three or more drives, thus increasing performance. The RAID 5 provides the best combination of disk array technology. Requires a minimum of 3 drives to implement.

Best balance cost / performance / protection of any RAID system
Allows multiple simultaneous writes
High read data rate
Medium write data rate
Ideal for small write applications
Highly efficient

Disadvantages

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

Application

File and Application servers
Database servers
Web, E-mail, and News servers
Intranet servers

Raid 6

Striping with double parity drive
RAID 6 is an evolution of RAID 5. RAID 6 uses double parity for additional fault tolerance. Like in RAID 5, data is striped at a block level across the disk sets while parity information is generated and written across the array. Now it's possible for more than one drive to fail simultaneously, and the RAID will still operate. Requires a minimum of 4 drives to implement.

Perfect solution for mission critical applications as it can sustain multiple drive failures

Disadvantages

Uses 2 drives for parity

Application

File and Application servers
Database servers
Web, E-mail, and News servers
Intranet servers
Transaction processing

Raid 0+1

Striping and Mirroring across multiple RAID levels
RAID 0+1 provides redundancy by writing all data to four or more drives. RAID 0+1 combines the benefits of RAID 0 and RAID 1. This offers both striping and mirroring with no parity generation. The RAID 0+1 also provides high performance and resilience to RAID 5. Requires a minimum of 4 drives to implement.

No parity bit to generate
Easy to implement
Utilises full disk capacity
Higher performance than RAID 5

Disadvantages

Inefficient use of disk space
High disk overhead / Expensive
Expensive to deploy

Application

General File servers
Imaging applications
Medium size database applications

Raid 10 (1+0)

Mirroring and Striping across multiple RAID levels
RAID 10 provides very high performance and redundancy. Data is simultaneously mirrored and striped. Can under circumstances support multiple drive failures. Requires a minimum of 4 drives to implement

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

Disadvantages

Very costly to implement
Drive spindles must be synchronised
Not very scaleable

Application

Where high performance and redundancy are critical

Raid 30 (3+0)

Striping across dedicated parity RAID systems
RAID 30 is also known as striping of dedicated parity arrays. It is a combination of RAID level 3 and RAID level 0. RAID 30 provides high data transfer rates, combined with high data reliability. RAID 30 is best implemented on two RAID 3 disk arrays with data striped across both disk arrays. RAID 30 breaks up data into smaller blocks, and then stripes the blocks of data to each RAID 3 raid set. RAID 3 breaks up data into smaller blocks, calculates parity by performing an Exclusive OR on the blocks, and then writes the blocks to all but one drive in the array. The parity bit is created using the Exclusive OR is then written to the last drive in each RAID 3 array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created.

Can sustain one to four drive failures while maintaining data integrity if each failed disk is in a different RAID 3 array.
Offers highest level of redundancy and performance

Disadvantages

Very costly to implement

Application

Data warehousing

Raid 50 (5+0)

Mirroring and Striping across multiple RAID levels
RAID 50 is a combination of RAID level 5 and RAID level 0. RAID 50 includes both parity and disk striping across multiple drives. RAID 50 is best implemented across two RAID 5 arrays with data striped across both disk arrays. RAID 50 breaks the data into smaller blocks, and then stripes the blocks to each RAID 5 raid set. RAID 5 breaks up data into smaller blocks, calculates parity by performing and Exclusive OR on the blocks, and then writes the blocks of data and parity to each drive in the array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created.
Here is an example of RAID 51 which is a combination of RAID level 5 and RAID level 1.

Can sustain one to four drive failures while maintaining data integrity if each failed disk is in a different RAID 5 array.
Offers highest level of redundancy and performance

Disadvantages

Very costly to implement
Drive spindles must be synchronised
Failure of two drives in one of the RAID 5 segments renders the whole array unusable

Application

Data warehousing

RAIDCalc - iBeast - Raid Disk Space Utilization Calculator
Broadberry
Wikipedia: RAID

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