A cluster in an active-active configuration shares processing evenly among five servers. Utilization is proportional to the amount of processing. The goal is to keep each server's utilization at 80% or less even when one server fails and the remaining four take over its processing evenly. Which is the upper limit of each server's utilization in normal operation (all five running)?

Answer: B. 64%

Let u be each server's utilization in normal operation; taking one server's capacity as 1, the total amount of processing is 5u. When one server fails, the remaining four share it, so each server's utilization becomes 5u ÷ 4, and keeping this at 80% or less requires u ≤ 80% × 4 ÷ 5 = 64%. In an active-active configuration every server does processing, so enough spare capacity to take on a failed server's share must be kept even in normal operation.

• A. This is the value when two servers fail and three take over (80% × 3 ÷ 5).
• B. Correct. 80% × 4 ÷ 5 = 64%.
• C. This value does not consider taking over the failed server's share.