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Probability of Logical Drive Failure

The probability that a logical drive will fail depends on the RAID level setting. If the logical drive does not have an online spare, the following results apply.

•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 drives increases, it becomes increasingly unlikely that a newly failed drive is not mirrored to a previously failed drive.

—The minimum number of physical drive failures that can cause the logical drive to fail is two, if the two drives happen to be mirrored to each other. The probability that this will happen decreases as the total number of hard drives in the array increases.

•A RAID 5 logical drive fails if two physical drives fail.

•A RAID 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.

The graph in Figure F-1provides more quantitative information. The data for this graph is calculated from the mean time between failure (MTBF) value for a typical physical drive, assuming that no online spares are present. If an online spare is added to any of the fault-tolerant RAID configurations, the probability of logical drive failure is further decreased.

HP Smart Array 641/642 Controller User Guide

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