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RAID Level 2

Common Name(s): RAID 2.

Technique(s) Used: Bit-level striping with Hamming code ECC.

Description: Level 2 is the "black sheep" of the RAID family, because it is the only RAID level that does not use one or more of the "standard" techniques of mirroring, striping and/or parity. RAID 2 uses something similar to striping with parity, but not the same as what is used by RAID levels 3 to 7. It is implemented by splitting data at the bit level and spreading it over a number of data disks and a number of redundancy disks. The redundant bits are calculated using Hamming codes, a form of error correcting code (ECC). Each time something is to be written to the array these codes are calculated and written along side the data to dedicated ECC disks; when the data is read back these ECC codes are read as well to confirm that no errors have occurred since the data was written. If a single-bit error occurs, it can be corrected "on the fly". If this sounds similar to the way that ECC is used within hard disks today, that's for a good reason: it's pretty much exactly the same. It's also the same concept used for ECC protection of system memory.

Level 2 is the only RAID level of the ones defined by the original Berkeley document that is not used today, for a variety of reasons. It is expensive and often requires many drives--see below for some surprisingly large numbers. The controller required was complex, specialized and expensive. The performance of RAID 2 is also rather substandard in transactional environments due to the bit-level striping. But most of all, level 2 was obviated by the use of ECC within a hard disk; essentially, much of what RAID 2 provides you now get for "free" within each hard disk, with other RAID levels providing protection above and beyond ECC.

Due to its cost and complexity, level 2 never really "caught on". Therefore, much of the information below is based upon theoretical analysis, not empirical evidence.

Controller Requirements: Specialized controller hardware required.

Hard Disk Requirements: Depends on exact implementation, but a typical setup required 10 data disks and 4 ECC disks for a total of 14, or 32 data disks and 7 ECC disks for a total of 39! The disks were spindle-synchronized to run in tandem.

Array Capacity: Depends on exact implementation but would be rather large if built today using modern drives.

Storage Efficiency: Depends on the number of data and ECC disks; for the 10+4 configuration, about 71%; for the 32+7 setup, about 82%.

Fault Tolerance: Only fair; for all the redundant drives included, you don't get much tolerance: only one drive can fail in this setup and be recoverable "on the fly".

Availability: Very good, due to "on the fly" error correction.

Degradation and Rebuilding: In theory, there would be little degradation due to failure of a single drive.

Random Read Performance: Fair. Bit-level striping makes multiple accesses impossible.

Random Write Performance: Poor, due to bit-level striping and ECC calculation overhead.

Sequential Read Performance: Very good, due to parallelism of many drives.

Sequential Write Performance: Fair to good.

Cost: Very expensive.

Special Considerations: Not used in modern systems.

Recommended Uses: Not used in modern systems.

Next: RAID Level 3

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