RAID 2, RAID3 & RAID 4
RAID levels 2, 3 and 4 were proposed when RAID was first postulated as a method, but have been either little used or else have fallen out of favour. RAID levels 0, 1 and 5 seem to "cover all of the bases" for most general purpose RAID applications, and "nested" RAID levels based upon combination of the "standard" RAID levels, e.g. RAID50 and RAID01, have developed for more sophisticated systems.
About RAID 2
RAID 2 requires little or no consideration as it never became established in the commercial arena. Perhaps it was experimented with but the overheads in terms of the number of disk required for the error correction information in order to get the required performance meant that it fell between the business area that became occupied by mirroring versus RAID 5. For most applications or or other sufficed and a high performance expensive option such as RAID 2 was never going to become established.
RAID 2 uses Hamming code parity and synchronized hard disks with very small data stripes, typically single bytes or words. Error correction is calculated across corresponding bits on the hard disks, and is stored on multiple parity disks.
The lack of commercial implementations means that RAID 2 data recovery has not been a requirement in recent years.
About RAID 3
RAID 3 introduced striped data with a single dedicated parity disk. Data written to the drive was spanned across the data disks in the set and then parity was calculated across the sectors in the stripe and written to the dedicated parity drive.
The level of redundancy provided by RAID 3 is identical to that of RAID 5, though the use of a dedicated parity disk has an adverse impact upon performance as one disk is receiving all parity information rather than the load being balanced across all of the hard disk drives as is the case with distributed parity organisations such as RAID 5.
RAID 3 is not in significant use, we are not aware of any hardware implementations of RAID 3, consequently the requirement to recovery data from RAID 3 is never a requirement.
About RAID 4
RAID 4 is similar to RAID 3 in that data is striped across a set of disks and then parity (ECC) information is calculated from it and written to a dedicated parity disk. The difference being that rather than a data block being spanned across the sequence of disks in the array, a block is written to the first disk in the set, the next block to the next disk and so on
The level of redundancy provided by RAID 4 is identical to that of RAID 3 and RAID 5, though the use of a dedicated parity disk has an adverse impact upon performance as one disk is receiving all parity information rather than the load being balanced across all of the hard disk drives as is the case with distributed parity organisations such as RAID 5.
As with RAID 3 and RAID 2 the absence of any significant applications of RAID 4 means that data recovery from RAID 4 has not been a requirement in recent memory.
Last Updated (Thursday, 18 June 2009 15:39)