平成27年度秋期 ITパスポート試験問82

This question's explanation and your answers so far.

問82

頻出度★★★

2系統の装置から成るシステム構成方式 a～c に関して, 片方の系に故障が発生したときのサービス停止時間が短い順に左から並べたものはどれか。 a デュアルシステム b デュプレックスシステム (コールドスタンバイ方式) c デュプレックスシステム (ホットスタンバイ方式)

出典　平成27年度秋期 ITパスポート試験問82

解説

問 82

ア
正解 The correct answer is ア. Arranging the service stop time when one system fails from shortest to longest gives a→c→b. In a, the dual system runs the same processing on two systems in parallel and cross-checks the results; even if one system fails, the remaining system keeps processing as is, so the stop time is nearly 0 and the shortest. In c, the hot-standby duplex system has its standby system already powered on and ready to take over at any time, so the switchover when the active system fails is fast and the next shortest. In b, the cold-standby method keeps the standby system stopped, so when the active system fails it must be started up and load its programs, taking the longest to switch over. Therefore ア, ordered a, c, b, is correct.
Incorrectイ
This option places b (cold standby) first, treating it as the shortest, but cold standby—which requires starting up the standby system—takes the longest to switch over and has the longest stop time. The order is wrong.
Incorrectウ
This option places c (hot standby) before a (dual system), treating it as shorter, but the dual system—in which both systems run in parallel and almost no stop occurs—is shorter. The first two positions are swapped, so it is wrong.
Incorrectエ
This option places a (dual system) last, treating it as the longest, but the dual system is nearly non-stop even when one system fails and is actually the shortest. It confuses the shortest with the longest, so it is wrong.

ここがポイント

Dual system = two systems process in parallel, nearly non-stop even when one system fails (shortest); a duplex system switches over to the standby system when the active system fails.
A duplex system's switchover time follows the relation: hot standby (standby system is ready to run = fast) < cold standby (the standby system must be started up = slow).

用語

解答問82ア