What is raid?
(Redundant Array of Independent Disks) is an acronym first used in a 1988 paper by Berkeley researchers Patterson, Gibson and Katz. It described array configuration and applications for multiple independent hard disks, providing fault tolerance (redundancy) and improved access rates.

RAID provides a method of accessing multiple individual disks as if the array were one large disk, spreading data access out over these multiple disks, thereby reducing the risk of losing all data if one drive fails, and improving access time.

Typically the RAID is used in large file servers, transaction of application servers, where data accessibility is critical, and fault tolerance is required. RAID is an emerging storage technology with the potential to revolutionize the data storage technology. A typical RAID unit contains a set of disk drives, which appear to the user to be equivalent to a single large capacity disk drive.

The remarkable benefit of disk array is that if any single disk in the RAID fails, the system and array still continues to function without loss of data. This is possible because the redundancy data is stored on separate disk drives and the RAID can reconstruct the data that was stored on the failed disk drive. As the effective seek time for finding data on a disk can potentially be reduced by allowing multiple simultaneous access of different data on different disks. Utilizing parallel reads and writes of the data spread across the disks in the array, the data transfer rate can be increased significantly over that of a single disk.

Below is an overview of the most popular RAID levels:


RAID level 0: "Disk Striping"
The advantages of RAID 0 is it’s high level of I/O performance because the I/Os are spread across multiple channels and drives.
Because RAID 0 offers no fault tolerance, if one drive should fail, this will result a loss of all your data. This should never be used in a environment where data is mission critical.

* RAID 0 requires a minimum of two drives to initiate.



RAID level 1: "Disk Mirroring"
RAID 1: Known as "Disk Mirroring" provides redundancy by fully duplicating drive data to all other drives in the array. If one drive fails, the others contain exact duplicate of the data and the RAID can switch to using the mirror drive with no lapse in user accessibility. The disadvantages of mirroring are no improvement in data access speed, and capacity is low. However, it provides the best protection of data since the array management software will simply direct all application requests to the surviving disk members when a member of disk fails.

RAID level 1 provides both very high data reliability and continued data availability in the event of a failure of any array member. When a RAID level 1 member disk fails, array management software simply directs all application requests to the surviving member.

RAID level 1 is most suitable for data which reliability requirements are extremely high, or for data to which high performance access is required. RAID 1 offers 100% data redundancy and requires no rebuild time.



RAID level 3: "Parallel transfer disk w/ Parity"
RAID level 3 stripes data across multiple drives, with an additional drive dedicated to parity, for error correction & recovery.
The technology used for a dedicated parity disk is to store redundant information about the data on several data disks. RAID level 3 is an excellent choice for applicators which require single stream I/O with a high data transfer rate.
RAID level 3 is optimal for applications in which large blocks of sequential data must be transferred quickly, these applications usually are:
- Graphical imaging processors, CAD/CAM files, and others.
- Non-interactive applications that process large data sequentially.


* RAID level 3 requires a minimum of 3 drives to initiate.




RAID level 5: "Independent access array w/rotating Parity"
When RAID level 5 technology is combined with cache memory to improve it’s write performance, the result can be used in any applications where general purpose disks would be suitable.
For read only or read mostly application I/O loads, RAID level 5 performance should approximate that of a RAID level 0 array. In fact, for a given user capacity, RAID level 5 read performance should normally be slightly better because requests are spread across one more members than they would be in a RAID level 0 array of equivalent usable capacity.
RAID level performs best in applications where data and I/O load characteristics match their capabilities. Those of which whose data availability is worth protecting, but the value of a full disk is questionable.

* RAID level 5 requires a minimum of 3 drives to initiate.



RAID level 6: "Disk Striping with Double Distributed Parity"
RAID 6 stripes blocks of data and parity across an array similar to RAID 5. Instead of a single parity, RAID 6 calculates two sets of parity information for each parcel data. This results in improving the arrays fault tolerance. RAID 6 can be slower than RAID 5 in terms of writes due to the added overhead of more parity calculations, but may be slightly faster in random reads due to the spreading of data over one more disk. RAID 6 requires a minimum of four drives to implement."
Data is striped on a block level across a set of drives in a similar fashion as RAID 5 but a second set of parity is also calculated and written across all the drives. RAID 6 offers fault tolerance and drive-failure tolerance that can sustain multiple simultaneous drive failures making it ideal for mission critical applications.

RAID 6 requires a complex controller to implement and compensate for the high overhead of dual parity computations. Requires N+2 drives to implement because of the two dimensional parity scheme. Rebuilding significantly effects performance.
• Enterprise Storage
• Disk to Disk Backup
• Performance Driven HD video streaming applications
• Fixed content backup or archiving
• Regulation Compliance Storage
• Disaster recovery Storage

* RAID level 6 requires a minimum of 4 drives.



RAID level TP: "Disk Striping with Triple Distributed Parity"
Similar to Raid 5 and 6, Raid TP stripes data across drives, but calculates for three parities that are written to three individual disks.

RAID TP uses three indepdendent equations to calculate each individual parity that enable reconstruction of data when three disks and/or blocks fail at the same time.
RAID TP adds an extra level of redundancy to help protect your data.

Recommended Applications
• Enterprise Storage
• Disk to Disk Backup
• Performance Driven HD video streaming applications
• Fixed content backup or archiving
• Regulation Compliance Storage
• Disaster Recovery Storage

* RAID level TP requires a minimum of 4 drives.

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