RAID is NOT a backup

So you’ve built a server. It has multiple disks in a RAID configuration. Do you need a backup? YES!

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), including possible UREs - on that later subject, RAID-5 should be considered harmful these days for any disks larger than 1TB.
• Failure of the RAID controller itself (especially when using hardware RAID), the computer running the RAID, or the environment containing the servers (a flood, fire, theft, etc.).
• Data corruption on-disk 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 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), which any seasoned sysadmin has done at least once (and hopefully not to production data)!

The adage is simple: “RAID replicates everything, instantly, even the stuff you don’t want it to.”