ITPEC IP October 2018 Question 26
A hard disk failure occurred on a mail server, and e-mails of many users were lost. Recovery of data was attempted, but e-mails sent and received during the two-week period after the last backup could not be restored because backups were only created every two weeks. Taking a lesson from this incident, a measure is to be implemented to allow restoration up to the state of the day before such a failure. Which of the following is an appropriate measure?
Explanation
Daily incremental backup in addition to the biweekly full backup — a backup taken at the end of every day lets the mail be restored up to the day before a failure.
The mail lost was everything received since the last backup, so how often a backup is taken decides how far back a restore goes, and the copy must sit on media other than the disk that fails. With a full backup every two weeks and an incremental backup every day, restore the last full backup and then apply each daily incremental in order: the data returns to its state at the last daily backup, the day before the failure. An incremental backup copies only that day's changes, so the daily run stays short.
Answer (a)
Why not others:
- •(b) — spreading the mail over several disks limits how much one disk failure destroys, but the mail on the failed disk still has only the two-week-old backup behind it
- •(c) — mirroring keeps a second live copy, so one disk failing no longer loses mail, but it is not a backup: a deletion or corruption is written to both disks at once, and the only restorable copy is still up to two weeks old
- •(d) — the daily backup is kept on the same hard disk media, so the failure that destroys the data destroys the backup with it, leaving only the biweekly copy
Key rule: The backup interval sets how much data can be lost, and a backup protects only against the failure of media it is not stored on; mirroring adds availability, not restore points.
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