問61
頻出度★★★
あるシステムは5,000時間の運用において，故障回数は20回，合計故障時間は2,000時間であった。おおよそのMTBF，MTTR，稼働率の組合せのうち，適切なものはどれか。
平成21年度 春期 ITパスポート試験 問61
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 61
Incorrectア
MTBF 100 and MTTR 150 have the values of MTBF and MTTR swapped. MTBF is the 3,000 hours of “time it was running” divided by 20 failures, giving 150 hours, and MTTR is the 2,000 hours of “time it was stopped” divided by 20 failures, giving 100 hours. The availability of 40% is also wrong: 40% corresponds to the proportion of time it was stopped (2,000 ÷ 5,000). It is a doubly wrong combination.
Incorrectイ
The availability of 60% is a correct value, but MTBF 100 and MTTR 150 are swapped. The total failure time of 2,000 hours (÷ 20 failures = 100 hours) is the MTTR, and the uptime of 3,000 hours (÷ 20 failures = 150 hours) is the MTBF. The values make it look as though the time stopped by failures was the longer one, which is also inconsistent with the availability of 60%.
Incorrectウ
MTBF 150 hours and MTTR 100 hours are correct, but the availability of 40% is wrong. 40% is MTTR ÷ (MTBF + MTTR) = 100 ÷ 250, that is, the proportion of the total operating time during which it was stopped, while availability is the remaining 60%. This choice mistakes availability for the proportion of time it was stopped.
- エ
正解 Of the 5,000 hours of total operating time, 2,000 hours were stopped by failures, so the uptime during which the system was actually running is 5,000 - 2,000 = 3,000 hours. MTBF (mean time between failures) is the uptime divided by the number of failures, 3,000 ÷ 20 = 150 hours, and MTTR (mean time to repair) is the total failure time divided by the number of failures, 2,000 ÷ 20 = 100 hours. Availability is MTBF ÷ (MTBF + MTTR) = 150 ÷ (150 + 100) = 150 ÷ 250 = 0.6, that is 60% (calculating it as uptime ÷ total operating time = 3,000 ÷ 5,000 = 60% gives the same result). Therefore エ, the combination of MTBF 150 hours, MTTR 100 hours, and availability 60%, is appropriate.
ここがポイント
- MTBF = the total uptime (time in normal operation) ÷ the number of failures; MTTR = the total failure (repair) time ÷ the number of failures. Do not use the total operating time as it is as the numerator of MTBF. The first move is to obtain the uptime as “total operating time - total failure time”.
- Availability = MTBF ÷ (MTBF + MTTR) = uptime ÷ total operating time. Note also that 1 - availability = MTTR ÷ (MTBF + MTTR) = 100 ÷ 250 = 40% is the “proportion of time it was stopped”, not the availability, so be careful not to choose this 40% as the answer.
用語