2016-09-28 12:17:03 -04:00
+++
title = "So why do I need a backup?"
description = "Because RAID protects you against one and only one specific thing: disk failures."
weight = 2
type = "post"
+++
2016-09-29 13:31:21 -04:00
Having a number of disks in RAID may *seem* like a backup, especially if you're using a mirror mode. But this is *wrong* !
2016-09-28 12:17:03 -04:00
2016-09-29 13:17:31 -04:00
RAID protects you against one and only one thing: a disk failure.
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It does *not* protect you against any of the following things:
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2016-09-28 12:26:35 -04:00
* Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6).
2016-09-28 19:09:54 -04:00
* Failure of the RAID controller itself (especially when using hardware RAID), the computer running the RAID, or the environment (a flood, fire, theft, etc.).
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* Data corruption on-disk from filesystem bugs, cosmic rays, or minor hardware or firmware failures.
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* Malicious or accidental deletion or modification of files by yourself or another party, including viruses, bad application writes, or administrative mistakes (e.g. `rm` -ing the wrong file or `mkfs` on an existing filesystem).
2016-09-28 18:58:14 -04:00
Even ZFS, designed specifically to prevent the third point, is still susceptable to the others.
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2016-09-29 13:31:21 -04:00
The adage is simple: "RAID replicates *everything* , instantly, even the stuff you don't want." Like the deletion of a file or corruption.
2016-09-28 12:17:03 -04:00
For these reasons and more, RAID IS NOT A BACKUP!
