Reliability Data - Compaq 166207-B21 - Smart Array 5302/32 RAID Controller Reference Manual

C
Appendix

Reliability Data

Figure C-1 shows the relative probability of logical drive failure for different
RAID settings and different logical drive sizes, assuming no online spares are
present. With RAID 0, the logical drive will fail if only one physical drive
fails; with RAID 5, two physical drives must fail; and with RAID ADG, three
hard drives must fail.
The situation is more complex for RAID 0+1. The maximum number of hard
drives that can fail without failure of the logical drive is n/2. This will happen
only if none of the failed drives are mirrored to each other. In practice, logical
drive failure will usually occur before this maximum number is reached, since
the probability of a newly failed drive not being mirrored to a previously failed
drive becomes increasingly small as the number of failed drives increases.
Also note that a RAID 0+1 logical drive may fail if only two hard drives fail,
if they are mirrored to each other; the probability of this happening decreases
as the number of mirrored pairs in the array increases.
An online spare can be added to any of the fault-tolerant RAID levels to
further decrease the probability of logical drive failure to about a thousandth of
the previous level.
For RAID 0+1 and RAID 5, Compaq recommends that no more than 14
physical drives be used per logical drive. However, logical drive failure is
much less likely with RAID ADG, and Compaq supports the use of up to 56
physical drives per logical drive when running this fault-tolerance method.