Sさんは, 現PCに保管した画像ファイルに色調補正などの加工をすることにした。加工作業のミスなどに備えて, バックアップツールを利用して, 毎年4月1日に完全バックアップを実施し, 毎月の1日に差分バックアップを実施する。差分バックアップは3回分を保管している。
ある年において, 画像ファイル X に対して表2のとおりに加工作業を行ったとき, 加工後の各画像ファイル X のうち, この年の6月3日に, バックアップから復旧できないものは, どの作業で加工したものか。
表2 加工作業
加工作業
作業日
作業a
3月20日
作業b
4月3日
作業c
4月10日
作業d
5月5日
出典 平成26年度 秋期 ITパスポート試験 問92
解説
問 92
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×Incorrectア
Operation a was performed on March 20, and a full backup was then taken on April 1. That full backup stores the entire state after operation a was applied, and on June 3 it is still kept as the latest full backup. So file X after operation a can be recovered.
○イ
正解The key point is which later backup captures the state right after each editing operation. On June 3, what is on hand is the full backup from April 1 plus the differential backups taken on the 1st of each month (only three are kept, so the most recent ones, from May 1 and June 1, are certainly still there). Operation b was performed on April 3, so the only chance for the state right after it to be saved was the next backup date, the differential backup of May 1. However, operation c was performed on April 10, before May 1, and file X was overwritten with the content produced by operation c. As a result, what goes into the May 1 differential backup is the state after operation c, not the state after operation b. The state right after operation b was never recorded in any backup, so it cannot be recovered on June 3. Therefore the correct answer is イ (Operation b).
×Incorrectウ
Operation c was performed on April 10, and no other editing was done between then and the differential backup of May 1. Therefore the May 1 differential backup records the state after operation c was applied. It can be restored using the April 1 full backup plus the May 1 differential backup, and the May 1 backup is still within the range of the three differential backups that are kept.
×Incorrectエ
Operation d was performed on May 5, and no other editing was done between then and the differential backup of June 1. The June 1 differential backup records the state after operation d was applied, and since it is the newest differential backup it is of course still kept. It can be restored using the April 1 full backup plus the June 1 differential backup.
ここがポイント
A full backup stores all of the data, while a differential backup stores only the changes made since the last full backup. To restore, you use the full backup plus one differential backup taken at the point in time you want to restore to.
A backup can only restore the state as it was at the moment the backup was taken. If the data is updated several times between two backups, the intermediate state (here, the state right after operation b) is overwritten and is lost for good.