Probability of logical drive failure
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Factors involved in logical drive failure | 56 |
Factors involved in logical drive failure
The probability that a logical drive will fail depends on the
•A RAID 0 logical drive fails if only one physical drive fails.
•A RAID 1+0 logical drive fails if any two failed physical drives are mirrored to each other.
•The maximum number of physical drives that can fail without causing failure of the logical drive is n/2, where n is the number of hard drives in the array. In practice, a logical drive usually fails before this maximum is reached. As the number of failed physical drives increases, it becomes increasingly likely that the newly failed drive is mirrored to a previously failed drive.
•The minimum number of physical drive failures that can cause the logical drive to fail is two. This situation occurs when the two failed drives are mirrored to each other. As the total number of drives in the array increases, the probability that the only two failed drives in an array are mirrored to each other decreases.
•A RAID 5 logical drive fails if two physical drives fail.
•A RAID 6 (ADG) logical drive fails when three physical drives fail.
At any given RAID level, the probability of logical drive failure increases as the number of physical drives in the logical drive increases. This principle is illustrated more quantitatively in the graph ("Probability of logical drive failure vs. number of drives in array" on page 57). The data for this graph is calculated from the MTBF value for a typical physical drive, assuming that no online spares are present. If an online spare is added to any of the
Probability of logical drive failure 56
