22 lines
1.3 KiB
Markdown
22 lines
1.3 KiB
Markdown
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title = "So why do I need a backup?"
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description = "Because RAID protects you against one and only one specific thing: disk failures."
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weight = 2
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type = "post"
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Having a number of disks in RAID may _seem_ like a backup, especially if you're using a mirror mode. But this is _wrong_!
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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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* 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 (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).
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The adage is simple: "RAID replicates _everything_, instantly, even the stuff you don't want." Like the deletion of a file or corruption.
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For these reasons and more, RAID IS NOT A BACKUP!
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