HP 166207-B21 Advanced Data Guarding Option Software Reference Guide - Page 10

Overview of RAID ADG Probability of Logical Drive Failure The probability that a logical drive will fail depends on the RAID level setting. • A RAID 0 logical drive will fail 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 physical 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 such mutually mirrored drives failing 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. • A RAID ADG logical drive (with no online spare) 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 1-2 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. Compaq Advanced Data Guarding Option Software Reference Guide 1-3