+++
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"
+++

Having a number of disks in RAID may _seem_ like a backup, especially if you're using a mirror mode. But this is *wrong*!

RAID protects you against one and only one thing: a disk failure. It does *not* protect you against any of the following things:

  1. Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6).
  2. Failure of the RAID controller itself (especially when using hardware RAID), the computer itself, or the environment (a flood, or fire, perhaps).
  3. Data corruption on-disk (except for ZFS, and especially for BTRFS) from cosmic rays, or minor hardware or firmware failures.
  4. File corruption from bad writes or bit rot (except for ZFS), including whole-volume corruption from administrative mistakes (e.g. =mkfs= on an existing filesystem).
  5. Malicious or accidental deletion or modification of data by yourself or another party, including viruses.

The adage is simple: "RAID replicates _everything_, even the stuff you don't want, like the deletion of that file you needed."

For these reasons and more, RAID IS NOT A BACKUP!