Data is distributed in extremely small increments across all disks and adds one or more disks that contain a Hamming code for redundancy.This is not considered commercially viable due to the added disk requirements (10 to 20 percent must be added to allow for the Hamming disk.
The use of the Hamming(7,4) code (four data bits plus three parity bits) permits using 7 disks in RAID 2, with 4 being used for data storage and 3 being used for error correction.
RAID 2 is the only standard RAID level, other than some implementations of RAID 6, which can automatically recover accurate data from single-bit corruption in data. Other RAID levels can detect single-bit corruption in data, or can sometimes reconstruct missing data, but cannot reliably resolve contradictions between parity bits and data bits without human intervention.
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Characteristics & Advantages
"On the fly" data error correction
Extremely high data transfer rates possible
The higher the data transfer rate required, the better the ratio of data disks to ECC disks
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Disadvantages
Very high ratio of ECC disks to data disks with smaller word sizes - inefficient
Entry level cost very high - requires very high transfer rate requirement to justify.
Transaction rate is equal to that of a single disk at best .
No commercial implementations exist / not commercially viable.
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RAID 3
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RAID 3 - Is rarely used . So here it is not discusiing
One of the side effects of RAID 3 is that it generally cannot service multiple requests simultaneously.
In our example, a request for block "A" consisting of bytes A1-A6 would require all three data disks to seek to the beginning (A1) and reply with their contents.A simultaneous request for block B would have to wait.
RAID 3. also distributes data in small increments but adds only one parity disk. This results in good performance for large transfers, but small transfers show poor performance.
RAID Level 3 requires a minimum of 3 drives to implement
Characteristics & Advantages
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Very high Read,write data transfer rate
Disadvantages
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Transaction rate equal to that of a single disk drive at best (if spindles are synchronized)
Controller design is fairly complex
Very difficult and resource intensive to do as a "software" RAID
RAID 4
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