Data Backup and Redundancy

接觸 IT 之後才發現很多人對 redundancy 跟 backup 之間的分野是混淆的，這種疑問就像前幾年 high availability 很熱門的時候，不少人都認為都已經有 data redundancy 了，還需要作備份嗎？

這種疑問通常忽略了萬一系統寫入異常或中毒的狀況，單純的 data redundancy 並沒辦法取回之前的資料狀態。

這篇文章寫的還不錯，提出了幾點：

• Data redundancy and backups are both intended to prevent data loss, but the two technologies are quite different.
• Data redundancy often takes the form of a synchronized copy of the organization’s data.
• Data redundancy can help to prevent service outages.
• Backups, on the other hand, are copies of data and other resources.
• Backups are also a good choice for granular recovery (recovering a single file, email message and so on). In contrast, redundant systems are better suited to situations in which the organization needs to keep critical systems online and cannot tolerate a long recovery period.

相關於 backup 與 redundancy 的字眼分別有：

• Backup: recovery, point-in-time, versioning, mean time to recovery (MTTR), revert, offline
• Redundancy: failover, hot spare, online synchronizing, performance, reliability

Availability 是從使用者的角度出發，無論系統是忙碌或是失效，對使用者都是 not availability，所以 availability 這字眼會同時涵蓋 backup 與 redundancy。