RAID stands for Redundant Array of Inexpensive (or sometimes “Independent”) Disks. RAID is a 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.
- 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 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
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.
Advantages:
- No parity generation
- Easy to implement
- Extremely fault tolerant
- Utilises full disk capacity
Disadvantages:
- 2 drives minimum
- Inefficient use of disk space
- High disk overhead
- Doubles number of writes
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. RAID 4 is identical to RAID 3, but does block-level striping instead of byte-level striping. In this setup, files can be distributed between multiple disks. Each disk operates independently which allows I/O requests to be performed in parallel, though data transfer speeds can suffer due to the type of parity. The error detection is achieved through dedicated parity and is stored in a separate, single disk unit.
Advantages:
- Single dedicated parity disk
- High read data rate
- High write data rate
Disadvantages:
- 4 drives minimum
- No performance degradation if drive fails
- Best and worst case performance similar
- Inefficient with small file transfer
RAID 5
Provides redundancy by writing data and parity information across three or more drives, thus increasing performance. The RAID Level 5 provides the best combination of disk array technology. The most popular RAID level supplied. RAID 6 is similiar to RAID 5, except it provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems. This becomes increasingly important because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if a (single) additional drive fails before the rebuild is complete.
Advantages:
- Most flexible of all disk arrays
- 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:
- 3 drives minimum
- Inefficient with large file transfer
- Disk failure has an impact on performance
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
RAID 0+1
Provides redundancy by writing all data to four or more drives. RAID 0+1 combines the benefits of RAID level 0 and RAID level 1. This offers both striping and mirroring with no parity generation. The RAID 0+1 also provides high performance and resilience to RAID 5.
Advantages:
- No parity generation
- Easy to implement
- Utilises full disk capacity
- 4 drives minimum
- Higher performance than RAID 5
Disadvantages:
- Inefficient use of disk space
- High disk overhead / Expensive
- Costly to deploy
A RAID system used as a main drive is not a replacement for backing up data. Data may become damaged or destroyed without harm to the drive(s) on which they are stored. For example, some of the data may be overwritten by a system malfunction; a file may be damaged or deleted by user error or malice and not noticed for days or weeks. RAID can also be overwhelmed by catastrophic failure that exceeds its recovery capacity and, of course, the entire array is at risk of physical damage by fire, natural disaster, or human forces.
RAID is also vulnerable to controller failure since it is not always possible migrate a RAID to a new controller without data loss. RAID drives can make excellent backup drives, when employed as backup devices to main storage, and particularly when located offsite from the main systems. However, the use of RAID as the main storage solution cannot replace backups.