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More RAID Levels and RAID Data Recovery


RAID 3 has a lot in common with its younger brother RAID 2 in that it also uses byte level striping and a dedicated parity disk. Where the siblings part company however, is in their error correcting methods. While RAID 2 uses Hamming code ECC, RAID 3 uses the more effective XOR algorithm to generate parity.

Unlike the previous levels we've seen, RAID 3 is a practical solution that delivers good performance and fault tolerance. The dedicated parity disk does slow down write speeds though, because the parity information has to be written to the parity drive whenever a write occurs. RAID data recovery however, is not as big an issue with this implementation.

RAID 3 requires at least 3 hard drives.

Figure 3. Under RAID 3, data is striped at the byte level, across multiple disks. The parity information is sent to a dedicated parity disk, but the failure of any disk in the array can be tolerated.



RAID 4 is very similar to RAID 3. In fact, it's so similar that people often confuse the two. There is one major difference between them however: RAID 4 uses block level striping. The advantage of block level striping is that you can change the stripe size to suit your application needs.

RAID 4 requires at least three hard drives. Like RAID 3, it offers good performance and fault tolerance, and RAID data recovery isn't as much of a concern. The dedicated parity disk however, remains the bottleneck.

Diagram 4. RAID 4 improves performance by striping data across many disks in blocks. It provides fault tolerance through a dedicated parity disk.



The most popular member of the RAID family, RAID 5 combines block level striping with distributed parity for good performance, fault tolerance and storage efficiency. This level minimizes the write bottlenecks of RAID levels 3 and 4, by distributing parity stripes over a series of hard drives. In doing so, it provides relief to the concentration of write activity on a single drive, which in turn enhances overall system performance.

RAID 5 is often used as an all-purpose RAID solution, but it is also used for database and file server applications.

RAID 5 requires a minimum of three hard drives, but often costs less to implement than RAID 3 or 4. RAID recovery may be necessary if more than one disk fails.

Diagram 5. In RAID 5, data and parity information are striped in blocks across all the drives in the array. Fault tolerance is maintained by ensuring that the parity information for any given block of data is placed on a separate drive from those used to store the data itself.


Below is a summary of some of the features of the RAID levels we've discussed. There are of course, many more RAID levels that exist, which aren't listed.

Table 1. RAID Level Summary

Level Techniques Description Min. Drives Failure Conditions Pros/Cons Uses
RAID O Disk striping (no fault tolerance) Data is broken into stripes which
are sent to each disk in the array.
2 When one drive fails, the entire array is compromised. Offers Best performance

No fault tolerance.
Video editing and production
RAID 1 Disk mirroring Data on one drive is mirrored on another. 2 If one drive fails, data is not lost. If both drives fail, the data is lost. 100% redundancy of data/Slower performance and 50% loss of storage space. Accounting, payroll, financial
RAID 2 Byte level striping with Hamming code ECC Data is split at the bit level over a number of data and ECC disks. Up to 14+ Only one drive may fail and still be recoverable "on the fly". On the fly data error correction/Extremely high cost. No commercial uses
RAID 3 Byte level striping with dedicated parity Data is striped at the byte-level, across multiple disks. 3 When more then one drive fails, the array is compromised. High read/write data transfer rates/Complex controller design Image and video editing
RAID 4 Block level striping with dedicated parity Data is striped in blocks across data disk, with parity store on a separate disk. 3 When more then one drive fails, the array is compromised. High Read/Low Write data transaction rates. General purpose
RAID 5 Block level striping with distributed parity Data and parity are striped in blocks across all disks. 3 When more then one drive fails, the array is compromised. High Read data transaction rates/ Complex controller design Web, database or file servers

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