Compaq Smart Array 5i Plus Controller and Battery Backed Writer Cache Enabler User Guide
E-1
COMPAQ CONFIDENTIAL
Writer: Amy L. Laffitte File Name: n-appe Probability of Logical Drive Failure
Codename: Executor Part Number: 266328-001 Last Saved On: 4/18/02 2:50 PM
E
Probability of Logical 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 drive fails.
•
For a RAID 1+0 logical drive, the failure situation is complex.
—
The
maximum
number of physical drives that can fail without causing
failure of the logical drive is
n
/2, where
n
is the number of hard drives in the
array. This maximum is reached only if no failed drive is mirrored to any
other failed drive. In practice, a logical drive usually fails before this
maximum is reached. As the number of failed drives increases, it becomes
increasingly unlikely that a newly failed drive is not mirrored to a previously
failed drive.
—
The failure of
only two
physical drives is enough to cause a logical drive to
fail
if
the two drives 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 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.
The graph in Figure E-1 provides more quantitative information. The data for this
graph is calculated from the mean time between failure (MTBF) value for a typical
physical drive, assuming that no online spares are present. Adding an online spare to
any of the fault-tolerant RAID configurations further decreases the probability of
logical drive failure by a factor of about a thousand.
