ITPEC FE Morning April 2023 Question 16
When system availabilities of a cold standby system, a simplex system, and a dual system are compared, which of the following shows the order arranged from highest to lowest? Here a computer or computers that compose each system are all the same.
Explanation
Dual system, cold standby system, simplex system — the shorter the outage after a failure, the higher the availability.
All three use the same computer, so failures are equally frequent; what differs is how long the service stays down after one.
- •Dual system: two computers run the same processing in parallel and compare the results. When one fails, the other carries on at once, and the service stops only if both are down.
- •Cold standby system: the backup computer is kept stopped. After a failure it must be started, loaded and switched in, and the service is down for that time.
- •Simplex system: a single computer; the service is down until it is repaired.
Answer (c)
Why not others:
- •(a) — puts cold standby above dual, but cold standby adds a switchover outage that a dual system does not have
- •(b) — puts simplex above dual, although a dual system keeps a second computer running
- •(d) — puts simplex first, but a system with no spare cannot beat one with a spare
Key rule: Availability is MTBF ÷ (MTBF + MTTR); a spare that is already running cuts the recovery time most, a cold spare less, and no spare not at all.
System Component · AI-written, may contain errors
This question comes from an official ITPEC past paper. ITPEC Practice is an independent study tool and is not affiliated with ITPEC. See the official FE past-paper collection or Report an issue.