HP A7143A RAID160 SA Controller Support Guide, February 2007

Probability of Logical Drive Failure
RAID Level and Probability of Drive Failure
Appendix A
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RAID Level and Probability of Drive Failure
The probability that a logical drive will fail depends on the RAID level setting.
• A RAID 0 logical drive fails if only one physical disk fails.
• For a RAID 1+0 logical drive, the failure situation is complex.
— The maximum number of physical disks that can fail without causing failure of the logical drive is
n/2, where n is the number of physical disks in the array. This maximum is reached only if no failed
disk is mirrored to any other failed disk. In practice, a logical drive usually fails before this maximum
is reached. As the number of failed disks increases, it becomes increasingly unlikely that a newly
failed disk is not mirrored to a previously failed disk.
— The failure of only two physical disks is enough to cause a logical drive to fail if the two disks happen
to be mirrored to each other. The risk of this occurring decreases as the number of mirrored pairs in
the array increases.
• A RAID 5 logical drive (with no online spare) fails if two physical disks fail.
• A RAID ADG logical drive (with no online spare) fails when three physical disks fail.
At any given RAID level, the probability of logical drive failure increases as the number of physical disks in
the logical drive increases.
The graph in Figure A-1, “Relative Probability of Logical Drive Failure,” provides more quantitative
information. The data for this graph is calculated from the mean time between failure (MTBF) value for a
typical physical disk, 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.