HP A7143A RAID160 SA Controller Support Guide

Probability of Logical Drive Failure
RAID Level and Probability of Drive Failure
Appendix A134
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.