what is RAID 5 Specifications ?
RAID Levels 0+5 (05) and 5+0 (50)
Common Name(s): RAID 0+5 or 05; RAID 5+0 or 50. As with the other multiple RAID levels, verify the exact implementation instead of relying on the label.
Technique(s) Used: Block striping with distributed parity combined with block striping.
Description: RAID 05 and 50 form large arrays by combining the block striping and parity of RAID 5 with the straight block striping of RAID 0. RAID 05 is a RAID 5 array comprised of a number of striped RAID 0 arrays; it is less commonly seen than RAID 50, which is a RAID 0 array striped across RAID 5 elements.RAID 50 and 05 improve upon the performance of RAID 5 through the addition of RAID 0, particularly during writes. It also provides better fault tolerance than the single RAID level does, especially if configured as RAID 50.
Most of the characteristics of RAID 05 and 50 are similar to those of RAID 03 and 30. RAID 50 and 05 tend to be preferable for transactional environments with smaller files than 03 and 30.
This illustration shows how files of different sizes are distributed between the drives on an eight-disk RAID 5+0 array using a 16 kiB stripe size. As with the RAID 0 illustration, the red file is 4 kiB in size; the blue is 20 kiB; the green is 100 kiB; and the magenta is 500 kiB, with each vertical pixel representing 1 kiB of space.
Each of the large, patterned rectangles represents a four-drive RAID 5 array. The data is evenly striped between these two RAID 5 arrays using RAID 0. Then within each RAID 5 array, the data is stored using striping with parity. So the first small file, and 12 kiB of the second file, were sent to the top RAID 5 array; the remaining 8 kiB of the second file and the first 8 kiB of the 100 kiB file went to the bottom RAID 5 array; then the next 16 kiB of the 100 kiB went to the top array, and so on.
Within each RAID 5 array the data is striped and parity calculated just like a regular RAID 5 array; each array just does this with half the number of blocks it normally would. Contrast this diagram to the ones forRAID 0 and RAID 5.
RAID 5 Controller Requirements:
Generally requires a high-end hardware controller.
RAID 5 Hard Disk Requirements:
Number of drives must be able to be factored into two integers, one of which must be 2 or higher and the other 3 or higher (you can make a RAID 30 array from 6 drives but not 7). Minimum number of drives is six, with the maximum set by the controller.
RAID 5 Array Capacity:
Same as RAID 03 and 30. For RAID 05: (Size of Smallest Drive) * (Number of Drives In Each RAID 0 Set) * (Number of RAID 0 Sets – 1). For RAID 50: (Size of Smallest Drive) * (Number of Drives In Each RAID 5Set – 1) * (Number of RAID 5 Sets).
For example, the capacity of a RAID 05 array made of 15 18 GB drives arranged as three five-drive RAID 0sets would be 18 GB * 5 * (3-1) = 180 GB. The capacity of a RAID 50 array made of 21 18 GB drives arranged as three seven-drive RAID 5 sets would be 18 GB * (7-1) * 3 = 324 GB. The same 21 drives arranged as seven three-drive RAID 5 sets would have a capacity of 18 GB * (3-1) * 7 = 252 GB.
RAID 5 Storage Efficiency:
Same as for RAID 03 and 30. For RAID 05: ( (Number of RAID 0 Sets – 1) / Number of RAID 0 Sets). ForRAID 50: ( (Number of Drives In Each RAID 5 Set – 1) / Number of Drives In Each RAID 5 Set).
Taking the same examples as above, the 15-drive RAID 05 array would have a storage efficiency of (3-1)/3 = 67%. The first RAID 50 array, configured as three seven-drive RAID 5 sets, would have a storage efficiency of (7-1)/7 = 86%, while the other RAID 50 array would have a storage efficiency of (3-1)/3 = 67%.
RAID 5 Fault Tolerance:
Same as for RAID 03 and 30. Good to very good, depending on whether it is RAID 05 or 50, and the number of parity drives relative to the total number. RAID 50 will provide better fault tolerance than RAID 05.
Consider the two different 21-drive RAID 50 arrays mentioned above: the first one (three seven-drive RAID 5sets) has higher capacity and storage efficiency, but can only tolerate three maximum potential drive failures; the one with lower capacity and storage efficiency (seven three-drive RAID 5 sets) can handle as many as seven , if they are in different RAID 5 sets. Of course few applications really require tolerance for seven independent drive failures! And of course, if those 21 drives were in a RAID 05 array instead, failure of a second drive after one had failed and taken down one of the RAID 0 sub-arrays would crash the entire array.
RAID 5 Availability:
Very good to excellent.
RAID 5 Degradation and Rebuilding:
Moderate for RAID 50; worse for RAID 05.
RAID 5 Random Read Performance:
Very good to excellent.
RAID 5 Random Write Performance:
RAID 5 Sequential Read Performance:
RAID 5 Sequential Write Performance:
RAID 5 Cost:
Relatively high due to requirements for a hardware controller and a large number of drives; storage efficiency is better than RAID 10 however and no worse than any other RAID levels that include redundancy.
RAID 5 Special Considerations:
Complex and expensive to implement.
RAID 5 Recommended Uses:
Applications that require high fault tolerance, capacity and random positioning performance. Not as widely used as many other RAID levels. Sometimes used instead of RAID 5 to increase capacity. Sometimes used for large databases.