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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" +++
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:
- Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6).
- Failure of the RAID controller itself (especially when using hardware RAID), the computer itself, or the environment (a flood, or fire, perhaps).
- Data corruption on-disk from filesystem bugs, cosmic rays, or minor hardware or firmware failures.
- Malicious or accidental deletion or modification of files by yourself or another party, including viruses, bad application writes, or administrative mistakes (e.g.
mkfson an existing filesystem).
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!
ZFS is an interesting case: while it does protect from corruption, it is still susceptable to the other failure modes and hence is still NOT A BACKUP!