問70
頻出度★★★
MTBF が600時間, MTTR が12時間である場合, 稼働率はおおよそ幾らか。
平成24年度 春期 ITパスポート試験 問70
テクノロジ系システム構成要素 › システムの評価指標
This question's explanation and your answers so far.
解説
問 70
Incorrectア
0.02 is the value of MTTR / (MTBF + MTTR) = 12 / 612 = 0.0196..., which is not the availability but the proportion of time the system is not running (the unavailability). Availability and unavailability add up to 1, so the counterpart of 0.02, namely 0.98, is the availability being asked for. This is the classic mix-up of putting MTTR in the numerator.
Incorrectイ
0.20 does not come out of any of the formulas. It results from a calculation slip such as misreading 12 / 600 = 0.02 by one decimal place as 0.2, and since it is a value on the unavailability side to begin with, it cannot be the answer for the availability either.
Incorrectウ
0.88 corresponds to 1 - 0.12, an error that regards MTTR = 12 as 12 %, treating a unit of time as if it were a percentage. Correctly, MTTR has to be compared with the total time MTBF + MTTR; since 12 / 612 is about 2 %, the availability is 0.98, not 0.88.
- エ
正解 Availability is the proportion of time during which a system can be used. It is obtained from MTBF, the mean time the system keeps running, and MTTR, the mean time spent repairing it after a failure, as Availability = MTBF / (MTBF + MTTR). The denominator MTBF + MTTR represents the total time of one cycle, that is, one period of operation plus one repair. Substituting the values in this question gives 600 / (600 + 12) = 600 / 612 = 0.9803..., which is roughly 0.98. Therefore エ is correct. It also checks out intuitively: against 600 hours of running, the system is stopped for only 12 hours (about 2 % of the whole), so the availability is close to 1, namely 0.98.
ここがポイント
- Availability = MTBF / (MTBF + MTTR). MTBF is the mean time between failures (the average time the system runs without failing) and MTTR is the mean time to repair (the average time it takes to fix it); the denominator represents the total time of one cycle.
- Unavailability = MTTR / (MTBF + MTTR), and availability plus unavailability equals 1. If MTTR is much smaller than MTBF, the availability approaches 1; making that rough estimate first and then checking the calculation is a good habit.
用語