RAID Technology Overview - September 2007

A Logical Drive Failure Probability
This appendix discusses the probability of logical drive failure.
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.
• A RAID 1+0 logical drive fails under the following conditions:
— 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 likely that a newly failed disk is mirrored to a
previously failed disk.
— The failure of only two physical disks can 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.
Figure A-1: “Relative Probability of Logical Drive Failure”, provides quantitative information
about logical drive failure. 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 a fault-tolerant RAID configurations, the probability of logical drive
failure decreases.
RAID Level and Probability of Drive Failure 21