User's Manual

HP Smart Array 641/642 Controller User Guide F-1

HP CONFIDENTIAL

Writer: John Turner File Name: n-appf probability of logical drive failure

Codename: darkmatter - antimatter Part Number: 309311-001 Last Saved On: 10/7/02 4:30 PM

Probability of Logical Drive Failure

The probability that a logical drive will fail depends on the RAID level setting. If the

logical drive does not have an online spare, the following results apply.

• A RAID 0 logical drive fails if only one physical drive fails.

• A RAID 1+0 logical drive fails if any two failed physical drives are mirrored to

each other.

— 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. 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 minimum number of physical drive failures that can cause the logical

drive to fail is two, if the two drives happen to be mirrored to each other. The

probability that this will happen decreases as the total number of hard drives

in the array increases.

• A RAID 5 logical drive fails if two physical drives fail.

• A RAID ADG logical drive fails when three 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 F-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. If an online spare is added

to any of the fault-tolerant RAID configurations, the probability of logical drive

failure is further decreased.