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Combined RAID Levels
Basically, these are combinations of classic RAID levels. The syntax is a little unclear, but one can generally assume that the first number indicates the level RAID "clusters" while the second number indicates the overall RAID level. In absolute terms, there is nothing to prevent implementation of RAID arrays using 3 or more RAID levels but such arrays remain in the realm of theory and experimentation for the moment.
The large (and growing!) number of possible permutations prevents us from listing them all, but we can present the advantages and weaknesses of the most common combinations.
RAID 01 (RAID 0+1) |
It can mirror faster since it is based on cluster stripping. Each cluster containing at least 2 elements, and because a minimum of 2 clusters is necessary, you will need at least 4 hard drives to create a RAID 0 +1 array. Reliability is average, as a failed drive causes the failure of an entire cluster. In addition, it takes a much longer time to reconstruct the lost data and degrades the performance of the array during the reconstruction. The main interest is that in the case of a mirror with 3 or more clusters, the voluntary withdrawal of a whole cluster allows for an ''instant'' backup without losing redundancy. |
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RAID 10 (RAID 1+0) |
RAID 10 ensures the reliability of the aggregate volume strip since it is based on replicated clusters. Each cluster contains at least 2 elements and a minimum of 2 clusters are necessary, so you will need at least 4 hard drives to create a RAID 10 array. Its reliability is good enough, because all hard drives must fail for the RAID array to fail. Its data reconstruction process is quite efficient since it uses only 1 cluster and not the entire array. |
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RAID 05 (RAID 0+5) |
RAID 05 uses the same approach as the RAID 0 +1 but employing RAID 5 instead. Each cluster contains at least 2 drives, and a minimum of 3 clusters is necessary. Therefore, it takes at least 6 hard drives to create a RAID 05 array. |
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RAID 15 (RAID 1+5) |
RAID 15 provides an array with efficent data redundency since it is based on cluster replicating mirror. Each cluster contains at least 2 drives, with a minimum of 3 clusters. Therefore, it also requires at least 6 hard drives to create a RAID 15 array. This method is very reliable because it requires all the drives to be defective to bring down the RAID array. It is, however, an expensive solution for the amount of storage capacity. |
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RAID 50 (RAID 5+0) |
RAID 50 provides an aggregate volume by bands based on RAID 5. Each cluster contains at least 3 drives, and a minimum of 2 clusters is necessary. Therefore, it takes at least 6 hard drives to create a RAID 50 array. This method provides one of the best compromise when it comes to improving performance without degrading reliability. Indeed, the aggregate band (which is relatively fragile) is based on redundant clusters. With this array, 2 drives from a single cluster must die to put the RAID array out of commission. |
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RAID 51 (RAID 5+1) |
RAID 51 provides a replicated volume based on RAID 5 clusters. Each cluster contains at least 3 drives, and a minimum of 2 clusters is necessary. Therefore, it takes at least 6 hard drives to create a RAID 51 array. It is an expensive method delivering a low storage capacity for the number of hard drives used. |
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