raidisnotabackup/content/2.md

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2016-09-28 12:17:03 -04:00
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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"
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2022-10-13 09:39:59 -04:00
Having a number of disks in RAID may _seem_ like a backup, especially if you're using a mirrored RAID mode like RAID-1 or RAID-10. But this is _wrong_!
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RAID protects you against one and only one thing: a disk failure. It does _not_ protect you against any of the following things:
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* Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6), including possible [UREs](https://holtstrom.com/michael/blog/post/588/RAID-5-URE-Failures.html).
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* Failure of the RAID controller itself (if applicable), the computer running the RAID, or the environment containing the servers (e.g. a flood, fire, or theft).
* Data corruption from filesystem bugs, cosmic rays, or minor hardware or firmware failures, which can and do happen all the time - you usually just don't notice and software works around it.
* Malicious or accidental deletion or modification of files, including by 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
2017-02-22 01:03:24 -05:00
The adage is simple: "RAID replicates _everything_, instantly, even the stuff you don't want it to."