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Probability of logical drive failure
Factors involved in logical drive failure
The probability that a logical drive will fail depends on the RAID-level setting and on the number and type
of physical drives in the array. 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
o
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
o
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 failure for different logical
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 fault-tolerant RAID configurations, the probability of logical drive failure is further
decreased.
drives" on page 93). The data for this graph is calculated from
("Relative
Probability of logical drive failure 92
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