問73
頻出度★★☆
同じ容量の2台のハードディスクを使う記録方式を考える。2台をストライピングする方式と比較して, ミラーリングする方式では, 記録できる情報量は何倍になるか。
平成25年度 春期 ITパスポート試験 問73
テクノロジ系システム構成要素 › システムの構成
This question's explanation and your answers so far.
解説
問 73
- ア
正解 The correct answer is ア, “0.5”. Suppose two hard disks of the same capacity are used, and let C be the capacity of one disk. Striping (RAID 0) splits the data into blocks and writes them across the two disks in parallel, keeping no duplicate copy of the same data. The capacity of both disks is therefore usable as is, so the amount of information that can be recorded is 2C (reading and writing get faster, but in exchange, if either one of the disks fails all of the data is lost). Mirroring (RAID 1), on the other hand, writes exactly the same content to both disks at the same time to duplicate it, so the amount of information a user can actually record is only C, the capacity of a single disk (in return, operation can continue on the remaining disk alone even if one fails). Taking striping as the baseline, therefore, the amount mirroring can record is C ÷ 2C = 0.5 times. The relationship that mirroring buys reliability at the cost of capacity shows up directly in this number.
Incorrectイ
A factor of 1 would mean that both methods can record the same amount of information, but that is wrong. Mirroring writes the same data to two disks in duplicate, so half of the 2C of physical capacity across the two disks is spent on the copy and only C is usable. Striping, by contrast, keeps no copy, so all 2C is usable. The usable amounts are clearly different, so the factor is not 1.
Incorrectウ
A factor of 2 is a wrong answer that reverses the direction of the comparison. The factor of 2 holds on the other side — striping measured against mirroring as the baseline (2C ÷ C = 2). This question asks about mirroring measured against striping as the baseline, so the reciprocal, 0.5, is correct. You have to read the question through to the end to identify what serves as the baseline and what the factor is being taken of.
Incorrectエ
A factor of 4 has no basis. Only two disks are used, so the amount of information that can be recorded falls in the range from a minimum of C (mirroring) to a maximum of 2C (striping), and the ratio between the two can only be 0.5 or 2, whichever direction it is taken in. A value of 4 cannot be derived either from the number of disks or from how each method works.
ここがポイント
- The two representative RAID methods and their effective capacity: striping (RAID 0) spreads data across several disks and accesses them in parallel, so it is strong in both speed and capacity but has no redundancy; with n disks the effective capacity is that of all n. Mirroring (RAID 1) writes the same data to two disks in duplicate and can withstand the failure of one disk, but its effective capacity is halved to that of a single disk.
- The question “by what factor is B compared with A” is computed as B ÷ A. If you mistake which side is the baseline (the thing being compared against = A), 2 and 0.5 swap places, so always confirm the direction of the comparison given in the question.
用語