Changes to 16-SEP-2012: RAID Allocation Guide between r4 and r5
%| A | B | Redundancy | Space Overhead |%
| 0 | 0 | 1 | 0 % |
| 0 | 1 | ''N'' (7) | (''N' - 1) / ''N'' (86%) |
| 0 | 5 | 2 | 1 / ''N'' (14%) | --> Given several trays (T) of disks, each tray with the same number of disks (N) per tray we can compute the "redundancy" (R) of an array built out of various standard and nested configurations, as well as how much space is lost to parity/mirroring.
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Redundancy (R) is defined as the minimum number of disks that can be lost to cause integrity failure (position is worst-case)
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Standard RAID levels
%| RAID Level | Redundancy (R) | Space Overhead |%
| 0 | 1 | 0 % |
| 1 | ''N'' x ''T'' | (''N'' x ''T'' - 1) / (''N' x ''T'') (95%) |
| 5 | 2 | 1 / (''N'' x ''T'') (5%) |
| 6 | 3 | 2 / (''N'' x ''T'') (10%) |
| 10 | 2 | - |
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Nested RAID Levels (RAID ''A' of RAID ''B''s, or RAID''B''+''A''):
%| A | B | Redundancy (R) | Space Overhead |%
| 0 | 0 | 1 | 0 % |
| 0 | 1 | ''N'' (7) | (''N'' - 1) / ''N'' (86%) |
| 0 | 5 | 2 | 1 / ''N'' (14%) |
| 0 | 6 | 3 | 2 / ''N'' (28%) |
| 1 | 0 | ''T'' (3) | (''T'' - 1) / ''T'' (66%) |
| 1 | 1 | ''N'' x ''T'' (21)| (''N'' x ''T'' - 1) / (''N'' x ''T'') (95%) |
| 1 | 5 | 2 x ''T'' (6) | (''N'' x (''T'' - 1) + 1) / (''N'' x ''T'') (71%) |
| 1 | 6 | 3 x ''T'' (9) | (''N'' x (''T'' - 2) + 1) / (''N'' x ''T'') (76%) |
| 5 | 0 | 2 | 1 / ''T'' (33%) |
| 5 | 1 | ''N'' x 2 (14) | 1 - ((1 - ((''N'' - 1) / ''N'')) x ((''T'' - 1) / ''T'')) (91%) |
| 5 | 5 | 4 | (''N'' + ''T'' - 1) / (''N'' x ''T'') (43%) |
| 5 | 6 | 6 | (''N'' + (''T'' - 1) x 2) / (''N'' x ''T'') (52%) |
| 6 | 0 | 3 | 2 / ''T'' (66%) |
| 6 | 1 | ''N'' x 3 (21) | (''N'' x 3 - 1) / (''N'' x ''T'') (95%) |
| 6 | 5 | 6 | (''N'' x 2 + 1) / (''N'' x ''T''') (71%) |
| 6 | 6 | 9 | (''N'' x 2 + 2) / (''N'' x ''T''') (76%) |
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Given this we can also come up with an aggregate "mean time between failure" (MTBF) based on the MTBF of each disk and the amount of redundancy.
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For all RAID levels (except possibly RAID10) this would be: ''MTBF_AGG'' = ''MTBF_DISK'' / (''N'' x ''T'' - ''R'' - 1)
Only in r4
Only in r5 --> Modified slightly between r4 and r5